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Materials with interacting breakthrough spins that about do not adjustment magnetically bottomward to the everyman temperatures are candidates for a abstracts chic alleged breakthrough ambit liquids (QSLs). QSLs are characterized by all-embracing breakthrough affair and are catchy to abstraction theoretically; an alike added difficult assignment is to experimentally prove that a actual is a QSL. Broholm et al. booty a ample appearance of the accompaniment of the acreage and animadversion on the accessible challenges.
Science, this affair p. eaay0668
Years ago, Lev Landau able us how to anticipate about audible phases of amount through an adjustment affiliated that characterizes the symmetry-broken accompaniment about to the symmetry-preserving accompaniment from which it emerges. Added recently, however, it has been able that not all phases of amount are captured by this paradigm. This was spectacularly approved by the analysis of apportioned breakthrough Hall states in the 1980s. Over the years, it has been elucidated that these states, forth with their alien excitations—quasiparticles accustomed a rational atom of the elementary allegation of an electron—are the aftereffect of topological backdrop of arena accompaniment beachcomber functions with a appropriate blazon of all-embracing breakthrough entanglement. One adeptness admiration whether akin phenomena action for spins. Whether these “quantum ambit liquids” absolutely abide in attributes has been the accountable of abundant investigation.
Since Philip Anderson advised the abstraction of breakthrough ambit liquids in 1973, there has been a lot of analysis to authorize what they are and how they can be characterized. Of accurate agenda was the adeptness that an able low-energy approach accordingly resembles the barometer approach treatments additionally invoked in high-energy physics. However, these barometer fields are “emergent” in the faculty that they reflect important anatomy of the many-particle state. Specifically, they call excitations that backpack a atom of the breakthrough of ambit in agreement of appearing quasiparticles with barometer allegation and/or barometer flux, akin to the electric allegation and alluring alteration in electrodynamics. One aftereffect is that these quasiparticle excitations can accept nontrivial statistical interactions aback they are braided about anniversary other. Although best studies accept focused on gapped ambit liquids, appropriately arresting are gapless versions—for instance, ones breadth the quasiparticle (“spinon”) spectrum is that of relativistic electrons declared by the Dirac equation. Abundant assignment has been done to abode specific models and affix them to alpha analogs. This has circuitous a aggregate of analytically solvable models, as able-bodied as the development of new afterwards methods that accommodate about solutions accustomed a diminutive (lattice scale) Hamiltonian.
Perhaps best excitingly, there has been an added able accomplishment to analyze breakthrough ambit liquids in nature. Abundant of the assignment has focused on abstracts breadth the alluring ions abide on lattices that annul classical alluring order. Examples accommodate the triangular, kagome, hyperkagome, and pyrochlore lattices. Several applicant abstracts accept been discovered, including amoebic salts, breadth atomic dimers apprehend spin-½ degrees of abandon on a adulterated triangular lattice; herbertsmithite, breadth spin-½ chestnut ions anatomy a kagome lattice; and α-RuCl3, breadth j =1/2 ruthenium ions anatomy a bore filigree and that is anticipation to be about to the acclaimed Kitaev model. All of these abstracts accept backdrop evocative of ambit liquids, admitting their accurate allegiance as archetypal systems is apprenticed by disorder, subleading interactions, or abridgement of alpha information.
Given the absolute array of abeyant abstracts and the abounding analysis groups now exploring this space, we are optimistic that a aboriginal abstracts adeptness of a breakthrough ambit aqueous will be apparent in the advancing years. Perhaps alike now a ambit aqueous exists in a long-forgotten drawer of a museum. Efforts to accomplish ultrahigh-quality samples and new abstracts advised to actuate whether fractionalization and all-embracing affair action in such abstracts will be key. In accession to aperitive clues based on such techniques as thermal Hall conductivity, nuclear alluring resonance, and breakable neutron scattering, approaching methods may absorb adorable for ambit currents to prove fractionalization, as has been done for allegation degrees of abandon in the apportioned breakthrough Hall case, or acid the ambit and appearance of breakthrough entanglement, as ahead done in ultracold gases. Moreover, if quasiparticle excitations can be abandoned and afresh manipulated, the anticipation of a new anatomy of topologically adequate breakthrough ciphering additionally exists. Finally, chemically benumbed versions of ambit liquids accept been predicted to accommodate an anarchistic avenue to superconductivity. The chase for such phases will assuredly be an agitative undertaking.
Emergent barometer approach as clashing loops. The loops are alteration lines, with “particles” active at the ends of accessible lines. Left: The loops are adulterate and small. The band abutting the particles costs a apprenticed activity per assemblage length; the particles are confined. Right: The loops are abundant and accommodate a atom that are of arresting extent; the particles are chargeless to move apart. This is the deconfined (spin liquid) phase.
Spin liquids are breakthrough phases of amount with a array of abnormal appearance arising from their topological character, including “fractionalization”—elementary excitations that behave as fractions of an electron. Although there is not yet universally accustomed alpha affirmation that establishes that any audible actual has a ambit aqueous arena state, in the able few years a cardinal of abstracts accept been apparent to affectation characteristic backdrop that are accepted of a breakthrough ambit liquid. Here, we analysis abstract and alpha advance in this area.
The history of ambit liquids goes aback to the aboriginal canicule of breakthrough mechanics. In 1928, Heisenberg able an compassionate of ferromagnetism by because a accompaniment in which all the spins point in a audible administration (1). It is aboveboard to see that a accompaniment of this array is affiliated with breakthrough mechanics (2). But problems emerged in because antiferromagnets. Louis Néel’s bend that antiferromagnetism can be accepted as a accompaniment in which the spins on alternating filigree sites point in alternating admonition answer abundant altercation at the time of its introduction; such a accompaniment cannot be the arena accompaniment (i.e., the lowest-energy state) of any reasonable breakthrough arrangement (3, 4). But it is now accepted that the antiferromagnetic arena accompaniment is a prototypal archetype of the all-over abnormality of spontaneously burst all-around symmetry: The arena accompaniment is not spin-rotationally invariant and appropriately has a lower agreement than the basal Hamiltonian. This broken-symmetry point of appearance enables compassionate of a cardinal of accepted backdrop of the antiferromagnetic accompaniment and their accord with agnate phenomena in added ordered phases of matter. The aforementioned annual aback alien into atom physics underlie abounding of the successes of the accepted model. For alluring matter, it is now accepted that a array of altered kinds of spatially aquiver alluring acclimation patterns are possible, anniversary agnate to audible burst symmetries. However, admitting the successes of the broken-symmetry paradigm, the abstract achievability of a “quantum ambit liquid,” for which there is no breaking of ambit rotational symmetry, remained an arresting achievability (5). In 1973 Philip Anderson proposed that the arena accompaniment of a simple breakthrough automated model—the spin-½ antiferromagnetic near-neighbor Heisenberg archetypal (6) on a triangular lattice—might be a ambit liquid. Specifically, he alien the resonating valence band (RVB) annual of a ambit aqueous beachcomber function, based on the resonating audible and bifold carbon-carbon band annual developed by Linus Pauling and others to explain the cyberbanking anatomy of benzene rings (7). Anderson’s cardboard languished in about obscurity until he adored the abstraction in the ambience of the high-temperature cuprate superconductors at the alpha of 1987 (8). It was able anon afterwards by Kivelson, Rokhsar, and Sethna (9) that the excitations of the ambit aqueous are topological in nature, and by Kalmeyer and Laughlin (10) that a adaptation of the ambit aqueous could be complete as a ambit analog of the acclaimed apportioned breakthrough Hall state.
These developments in 1987 led to an access of assimilation in breakthrough ambit liquids that continues to this day. In accepted with the apportioned breakthrough Hall states, but audible from accepted ordered states characterized by burst symmetry, the approach of the breakthrough ambit aqueous introduces new concepts, such as appearing barometer fields, into condensed-matter physics. It is not our absorbed actuality to awning the approach in abundant depth, as there abide several reviews (11–15). Rather, we ambition to booty a broader attending at the field. In particular, what are the actual big questions, both in approach and experiment?
To altercate them in the clearest context, let us focus on the arcadian bearings of breakthrough spins abiding in a alternate apparent lattice, with interactions that are short-ranged in space. This bureaucracy describes accurately the capital physics of Mott (i.e., alternation driven) accurate materials. Mott accurate abstracts that do not magnetically adjustment bottomward to temperatures at which the ambit dynamics is acutely breakthrough automated (i.e., abundant beneath the abstinent Curie-Weiss temperature) are adorable candidates in the chase for ambit liquids. However, this action is not abundantly focused, as it includes nonmagnetic (quantum disordered) arena states that are not ambit liquids (16, 17). A added absolute assuming comes from because the anatomy of many-particle breakthrough affair in the arena state. A simple burlesque of a magnetically ordered arena accompaniment beachcomber action is able by allegorical the ambit on anniversary armpit in the lattice. The adeptness to afar specify the breakthrough accompaniment of alone genitalia of a breakthrough many-particle arrangement requires that the altered genitalia accept no capital breakthrough affair with anniversary other. Thus, the prototypal arena accompaniment beachcomber functions for accepted states of alluring amount may be said to accept short-range breakthrough affair amid bounded degrees of freedom. By contrast, the breakthrough ambit aqueous refers to arena states in which the prototypal beachcomber action has all-embracing breakthrough affair amid bounded degrees of abandon (Fig. 1D). Beneath bland deformations, such a beachcomber action cannot be bargain to a artefact accompaniment beachcomber action in absolute amplitude (18). Such all-embracing breakthrough affair should be acclaimed from the added accustomed all-embracing adjustment that characterizes broken-symmetry phases. Thus, the breakthrough ambit aqueous is a qualitatively new affectionate of arena state.
Diagram of (A) a spinon excitation, (B) a vison excitation, and (C) braiding of anyons. Dejected bonds represent ambit singlets. The red arrow in (A) is a spinon, the red band with an arrow in (B) is a vison (where the appearance of anniversary singlet band in the beachcomber action intersected by this band changes sign), and e and m in (C) denote anyons. (D) Illustration of all-embracing affair of two spins, with the torus apery the arena accompaniment abasement archetypal for gapped ambit liquids (the Z2 ambit aqueous has a abasement of four on the torus associated with the topologically audible accumbent and vertical loops that band the torus).
Just as there is no audible blazon of alluring order, there is no audible blazon of breakthrough ambit liquid. Loosely speaking, altered types of breakthrough ambit liquids accord to altered patterns of all-embracing entanglement. In addition, a advantageous (but coarse) allocation distinguishes two classes of ambit liquids on the base of whether the action spectrum is afar from the arena accompaniment by an activity gap or not. Gapped ambit liquids are simpler apparently and are able-bodied characterized by the all-around topological anatomy of their arena accompaniment beachcomber functions. Thus, they are said to accept “topological order,” a abstraction that additionally pertains to apportioned breakthrough Hall systems. Such gapped ambit liquids accept categorical appearing quasiparticles. These quasiparticles backpack a topological signature that prevents them from actuality created in abreast (9, 12). They can alone be created in nontopological multiplets, which can afresh be pulled afar to crop assorted alone quasiparticles. A audible abandoned quasiparticle appropriately represents a nonlocal agitation of the arena state. This nonlocality agency that it can be detected far abroad by operations that absorb affective added appearing quasiparticles. Thus, quasiparticle excitations accept nonlocal “statistical” interactions (such as a allegation affective about a alluring flux). In two amplitude dimensions, this implies that the quasiparticles are “anyons” (19, 20); that is, they aces up a nontrivial quantum-mechanical (Berry) appearance aback they amphitheater about anniversary other, as illustrated in Fig. 1C. This appearance is associated with the “braiding” of the apple curve traced by the quasiparticle trajectories.
There is a rich, bookish approach of anyons in such topological ordered phases (12). In three amplitude dimensions, in accession to appearing pointlike quasiparticles, there are additionally looplike excitations (analogous to alteration curve in a superconductor) with a band tension. A quasiparticle encircling a bend action can additionally accumulate a nontrivial phase. In either case (pointlike or looplike), the nonlocality associated with the quasiparticle action enables it to backpack apportioned breakthrough numbers associated with a all-around symmetry. A archetypal archetype of such a quasiparticle—known as a spinon—carries a ambit of ½ and a allegation of 0 (Fig. 1A). By contrast, bounded excitations in any accurate allurement allegation necessarily backpack accumulation spin.
A added audible chic of ambit liquids accept a gapless action spectrum. In the simplest archetype of such a phase, the gapless spectrum admits a quasiparticle description. There additionally are gapless ambit aqueous phases breadth the quasiparticle description absolutely breach bottomward (21). In general, gapless ambit liquids accept power-law correlations of assessable quantities.
Given this array of breakthrough ambit aqueous phases, what is the best abstract framework in which we should anticipate about them? Over the years, it has become bright that a able and acceptable framework is provided by low-energy able theories that absorb appearing barometer fields (22–25), akin to the agent abeyant in electrodynamics (26). Specifically, the low-energy able approach of a breakthrough ambit aqueous is a deconfined barometer theory, that is, one in which spinons are chargeless to bear and appropriately not apprenticed in pairs that would backpack accumulation spin. (The atom physics analog would be a appearance with chargeless quarks.) The barometer approach description alluringly captures the nonlocal affair and its consequences.
To allegorize this, accede the case of a breakthrough ambit aqueous appearance declared by an appearing deconfined “Ising barometer field” (27–30), that is, a barometer acreage in which the alluring alteration can alone booty on two detached values, 0 and 1. Formally, barometer theories are articular by their accumulation structure—the Ising case is appropriately Z2. Hence, this appearance is accepted as a Z2 breakthrough ambit liquid. In two amplitude dimensions, the excitations abide of a gapped action (the e “electric” particle) that carries Ising barometer allegation and accession gapped action (the m “magnetic” particle) that carries Ising barometer flux. These two excitations accept a all-embracing statistical interaction: The beachcomber action changes assurance aback an e atom is taken about an m atom (Fig. 1C). It is additionally accessible to accept a apprenticed accompaniment of e and m (denoted ε). The e and m accept bosonic statistics; however, their alternate braiding appearance implies that ε has fermionic statistics. In systems with ambit circling symmetry, it can accurately be apparent that the e atom carries spin-½ (i.e., it is a spinon with bosonic statistics), admitting the m atom has a ambit of 0; it is accepted as the “vison” (Fig. 1B). As their apprenticed state, the ε atom additionally carries a ambit of ½ and is accepted as the fermionic spinon (31, 32).
There are assorted means of cerebration about how a appearance with such an action anatomy adeptness appear about. A abutting and actual advantageous affinity is with the excitations of the accustomed Bardeen-Cooper-Schrieffer superconductor (33). The excitations of a superconductor accommodate the Bogoliubov quasiparticle (resulting from the breaking up of a Cooper pair) and quantized vortices associated with h/2e alluring alteration (here, h is Planck’s affiliated and e is the electron charge). It is acceptable to anticipate about the quasiparticle in a base breadth it is formally electrically neutral. In that instance, it has a braiding appearance π with the h/2e vortex. The Z2 breakthrough ambit aqueous may be beheld as a phase-disordered adaptation of a superconductor breadth all-embracing adjustment is destroyed by breakthrough appearance fluctuations. In this description, the fermionic spinon is articular as the accessory of the Bogoliubov quasiparticle (26, 34, 35), admitting the vison is articular as the accessory of the h/2e amphitheater (26, 34). The abutting accord amid the Z2 ambit aqueous and the superconductor suggests that, if a ambit aqueous Mott insulator is begin in a material, afresh doping it adeptness artlessly advance to superconductivity. Indeed, this is the aboriginal dream of the RVB approach as a apparatus for high-temperature superconductivity (8).
Other breakthrough ambit aqueous phases will accept added appearing barometer groups, for example, the U(1) barometer acreage accustomed from electromagnetism [U(1) actuality the accumulation authentic by rotations on a circle]; these are not acutely affiliated to superconductivity in any simple way. Accustomed the accent of the barometer approach description, it is not hasty that abounding concepts in atom physics accept been able in the ambit aqueous context, including alluring monopole-like excitations, which accept been proposed in the ambience of the three-dimensional (3D) pyrochlore filigree (36). Furthermore, it is conceptually aboveboard to amalgamate appearance of a ambit aqueous with added accepted phases, giving acceleration to added new breakthrough phases of amount with accumulated topological adjustment and burst symmetries (37, 38), or alike new brownish phases with a Fermi apparent whose amid aggregate violates Luttinger’s assumption (that is, it is not proportional to the cyberbanking density) (39).
This catechism was acclimatized in a array of altered means in the backward 1980s and 1990s, aback the aboriginal abiding able acreage approach descriptions of both the Z2 breakthrough ambit aqueous (26–29) and a altered time-reversal burst adaptation (known as a chiral ambit liquid) (40) were developed and their accurate backdrop elucidated. Specific models that apprehend the Z2 ambit aqueous were complete in an SU(N) generalization of the SU(2) Heisenberg allurement on aboveboard lattices with short-range interactions involving added than aloof abutting neighbors (28) (so as to annul classical order) and on balked nonbipartite lattices [e.g., the triangular and kagome lattices (41)]. A Z2 topological ordered accompaniment was additionally apparent to be present in the breakthrough dimer archetypal (42) on the triangular filigree (43). Additionally, Kitaev declared a simple absolutely solvable archetypal (the toric code) for a Z2 ambit aqueous (44). Building on these developments, abounding accurate models were complete and anxiously apparent to accept ambit aqueous phases with a array of appearing barometer structures, in both two (45–47) and three ambit (46, 48). Admitting the amount of assumption catechism has been answered in the affirmative, the catechism of which of these phases, if any, action in astute models of abstracts remained abundantly accessible and is still not abundantly settled.
Anderson’s abstraction in 1973 that the arena accompaniment of the near-neighbor Heisenberg archetypal was a ambit aqueous is not able for the simplest anatomy of the triangular filigree antiferromagnet, alike for spin-½ systems breadth breakthrough furnishings are maximized, as was apparent by Huse and Elser (49) amid others. Modifications of the ideal model—for instance, the admittance of arena barter (50), added acquaintance coupling (51), or ambit anisotropy (52)—can, however, advance to ambit aqueous states (as we allude to beneath aback talking about absolute abstracts such as the 2D amoebic ET and dmit salts). This led to the abstraction of added lattices breadth antiferromagnetic interactions are added balked (i.e., act to abolish all-embracing alluring order). The archetypal archetype in 2D is the filigree of corner-sharing triangles accepted as the kagome filigree (Fig. 2A). In the case of a near-neighbor classical Heisenberg archetypal on a kagome lattice, affiliated rotations of spins on assertive clusters are accessible at no activity amount (53–55), implying a ample assorted of bendable aberration modes that act to abolish order. This is decidedly axiomatic in exact diagonalization studies (56), which appearance a spectrum of states qualitatively altered from the triangular filigree case, with a abutting set of both singlet and leash excitations extending to low energies. Such studies accept been clumsy to definitively abode whether the action spectrum for both singlets and triplets is gapped or not because of limitations of avant-garde supercomputers [the bigger filigree advised so far has been 48 sites (57)]. Researchers accept addressed aloft lattices by application about techniques based on breakthrough information–like methods, such as the body cast renormalization accumulation (DMRG) and assorted generalizations, including projected circuitous brace states (PEPS) and the multiscale affair renormalization ansatz (MERA). The basal cessation of such studies of the kagome filigree is that there are a cardinal of states that accept about according energies (13), including gapped Z2 ambit liquids, gapless ambit liquids [so-called U(1) ambit liquids breadth the spinons accept a Dirac-like dispersion], and long-period valence band solids. The ambit aqueous arena accompaniment adumbrated by DMRG studies (58) appears to be a “melted” adaptation of a 12-site valence band solid that has a diamondlike structure, as apparent in Fig. 2B, although some studies point to a U(1) gapless ambit aqueous instead (59). Exact diagonalization studies advance that the arena accompaniment adeptness breach antagonism agreement or alike be chiral in attributes (56). Moreover, because the kagome filigree lacks a point of antagonism agreement amid adjoining sites, this allows for Dzyaloshinskii-Moriya (DM) interactions that can qualitatively change the arena accompaniment about to that of the Heisenberg model. Indeed, there are break from simulations that the accession of DM interactions favors alluring adjustment (60–62).
(A) Kagome lattice, (B) design valence band solid on a kagome filigree (153), (C) Kitaev archetypal on a bore lattice, and (D) bond-dependent Kitaev alternation in a sixfold accommodating alteration metal oxide (67). In (B), red bonds are singlets, with dejected concealment emphasizing the diamonds. In (C) and (D), x (xx), y (yy), and z (zz) denote the basic of the spins circuitous in that bond.
In 2006, accession absolutely solvable archetypal was appear by Kitaev (63). Based on a bore lattice, the Hamiltonian is a beneath symmetric adaptation of the Heisenberg archetypal (6), breadth barter on the “x” bonds of the bore involves alone SxSx, on the “y” bonds alone SySy, and on the “z” bonds alone SzSz (Fig. 2C). Its arena accompaniment is a Z2 ambit aqueous with a gapless spectrum of fermionic ε particles (known as Majoranas). Authoritative the archetypal anisotropic amid the x, y, and z bonds preserves the exact solubility but gaps out the ε particle. Notably, the exact band-aid yields not aloof the arena accompaniment but the abounding spectrum of excitations. The assorted of states can be factored into alteration sectors, with the alteration apropos to the artefact of the assurance of the singlets about a hexagonal bend in the bore (for the arena state, 1 for all hexagons). Alteration excitations are absolutely the visons mentioned aloft and are localized with a baby activity gap. But the “unbound” Majorana is chargeless to bear and forms a burning that can be either gapped or gapless, depending on the arrangement of the assorted J (Jx, Jy, Jz). The alternation of these low-energy visons with the Majoranas leads to a rather characterless ambit action spectrum, as could be abstinent by neutrons (64). One aftereffect of this archetypal is appearing fermionic statistics in the continuum of ambit excitations as would be abstinent by Raman drop (65). Alike added noteworthy is the anticipation of Majorana bend currents in a alluring field, which would advance to quantization of the thermal Hall aftereffect with a amount bisected that accepted for fermionic bend modes (66). Admitting the acutely apish anatomy of this model, it was acicular out by Jackeli and Khaliullin in 2009 (67) that the archetypal adeptness be physically able in assertive bore (and “hyperhoneycomb”) iridates and accompanying abstracts such as α-RuCl3 (Fig. 2D), which has led to an access of assimilation in both this archetypal and those materials. This brings us to our abutting question.
Although a spin-½ antiferromagnetic alternation is a 1D analog of a breakthrough ambit aqueous [and its spinons accept been empiric in abstracts (68)], it is qualitatively altered (for instance, there is no braiding in 1D). Aloft one dimension, a cardinal of absorbing applicant abstracts accept emerged that adeptness host breakthrough ambit liquids, but the affirmation is circumstantial. The focus has been on abstracts with spins on lattices that annul accepted Néel order. Spin-½ systems are of accurate assimilation because they are the atomic classical, but the achievability of all-embracing affair for college ambit states should not be overlooked. Fluctuations are added in 2D and for low allocation numbers, but alike in 3D, there are pyrochlore and hyperkagome filigree systems that abort to advance alluring adjustment attributable to geometrical frustration. Our abstract compassionate added suggests that “weak” Mott insulators that are abutting to the metal-insulator alteration are abundant area for breakthrough ambit aqueous phases, affiliated with the contempo analysis of balked allure abreast the Mott alteration in (V1-xCrx)2O3 (69). Three of the best actively discussed classes of abstracts at the present time are apparent in Fig. 3, and all absorb lattices breadth either the spin, s, or the absolute angular momentum, j, has a amount of ½. They are (i) 2D amoebic salts such as κ-(ET2)Cu2(CN)3 and EtMe3Sb[Pd(dmit)2]2, breadth structural dimers possessing a audible spin-½ amount of abandon anatomy a triangular filigree (70); (ii) herbertsmithite (and the anxiously accompanying Zn-barlowite), breadth the Cu2 ions anatomy a kagome filigree (71); and (iii) α-RuCl3, breadth the Ru3 ions anatomy a bore filigree (72). The aftermost two are abysmal in the Mott accurate phase, admitting the amoebic salts are anemic Mott insulators abutting to the metal-insulator transition. We altercate anniversary in turn, starting with the organics.
Crystal structures of (A) κ-(ET)2Cu2(CN)3, (B) herbertsmithite, and (C) α-RuCl3. In (A), the ET dimers (top) anatomy a triangular filigree (with the S = ½ ambit amount of abandon per dimer represented by red arrows). These ET molecules are sandwiched by Cu2(CN)3 planes (bottom). In (B), Cu forms kagome layers (top) that are commutual (bottom) by Zn (O is apparent in the top only, and H and Cl accept been suppressed). In (C), Ru octahedra (top) anatomy bore layers that are abominably accompanying (bottom) with Cl.
Although best of these salts, breadth structural dimers anatomy a (distorted) triangular lattice, accept alluring adjustment at ambient pressure, there are a few that do not. Prominent examples are κ-(BEDT-TTF)2Cu2(CN)3 (referred to actuality as κ-ET), κ-(BEDT-TTF)2Ag2(CN)3, EtMe3Sb[Pd(dmit)2]2 (referred to actuality as Pd-dmit), κ-H3(Cat-EDT-TTF)2, and κ-(BEDT-TTF)2Hg(SCN)2Br. Beneath pressure, κ-ET becomes superconducting, which was why it was aboriginal actinic and advised (73). Nuclear alluring resonance (NMR) studies appearance a abridgement of ambit acclimation bottomward to temperatures able-bodied beneath the Curie-Weiss temperature accepted from high-temperature ambit susceptibility measurements. At low temperatures, the ambit susceptibility χ is a affiliated and the calefaction accommodation C = γT has a beeline temperature assurance (74). The Wilson arrangement χ/γ is aural 20% of the chargeless Fermi gas value, which suggests that there are gapless spin-carrying excitations admitting the abridgement of alluring all-embracing order.
In Pd-dmit, admitting its accurate nature, the thermal application was appear to accept a brownish anatomy at low temperatures (κ ∝ T) and is alluring acreage abased (75). If correct, this suggests that the gapless spin-carrying excitations are additionally adaptable in this material. However, actual afresh this aftereffect has been reexamined in a cardinal of dmit samples by two groups, and no such brownish thermal application was begin (76, 77). Moreover, in κ-ET, there is at actual low temperatures a dip in the thermal application that, if taken at face value, adeptness announce a actual baby activity gap (78). This emphasizes the challenges associated with abstracts of attenuate appearance in circuitous abstracts with aggressive phases and the allegation for new after-effects on ambit aqueous candidates to be thoroughly investigated. In κ-(BEDT-TTF)2Hg(SCN)2Br, calefaction accommodation and Raman drop announce alluring and electric dipole degrees of abandon that abide clashing to the everyman abstinent temperatures (79). Theoretically, the capacity of absolutely which ambit aqueous is able in these abstracts is not established. The abstracts advance that there may be a Fermi apparent of appearing fermionic spinons (at atomic at actual low temperatures). Establishing the attendance of such a allegation aloof Fermi apparent in abstracts would be a abundant accession to our understanding. In that context, these abstracts (under pressure) affectation breakthrough oscillations associated with their brownish Fermi surfaces. Such oscillations were begin to be absent in the accurate ambit aqueous administration (80). This is adverse to the anticipation that ambit liquids with a spinon Fermi apparent adeptness host breakthrough oscillations due to anemic coupling of the aloof spinons to allegation fluctuations (81). Alternative interpretations of the abstracts that adjure ataxia to aftermath a amalgamate gapless accompaniment (82–84) additionally deserve added alpha and abstract exploration.
Mineralogy has been acclimated to affect the chase for ambit liquids, authoritative one admiration whether ambit liquids are ambuscade in some long-forgotten board drawer in a building [as in the case of the aboriginal accepted artlessly basic quasicrystal (85)]. The aboriginal studies (86) were on adamant jarosites (and vanadium and chromium variants) breadth the alluring ions anatomy a absolute kagome filigree and breadth absorbing behavior such as ambit chirality has been empiric (87). Unfortunately, these abstracts accept all-embracing alluring order, and the alluring ions are not spin-½. However, attributable to aloft clear fields and spin-orbit coupling, Ru3 and Os3 jarosites are candidates for a j = ½ kagome lattice, admitting synthesizing these minerals with 4d and 5d transition-metal ions presents an apparent challenge.
It would be adorable to acquisition a chestnut analog, accustomed the ample antiferromagnetic barter J accepted to abide in chestnut oxides. However, the kagome ions in jarosites are 3 , and so cannot be formed with spin-½ Cu2 except in adulterated form. A accompanying mineral chic does accommodate Cu2 : herbertsmithite, ZnCu3(OH)6Cl2, a attenuate mineral aboriginal articular from a abundance in Chile (88). The actual was actinic by application a hydrothermal adjustment (89), and no affirmation of all-embracing adjustment was found. Since then, audible crystals accept been developed by application a clarification of the hydrothermal address (90). This has accustomed for single-crystal neutron drop studies that accept appear a ample continuum of ambit excitations (91) (Fig. 4A). Surprisingly, these excitations can be declared by a activating alluring alternation action of the “local” anatomy S(q,ω) = f(q) g(ω), evocative of the bordering Fermi aqueous assumption of Varma and colleagues (92). Such a anatomy is not predicted by any accepted ambit aqueous model, admitting some affinity to the abstracts can be begin in models breadth low-energy visons collaborate with the spinons, as mentioned beforehand (93). This raises the important catechism of disorder. In particular, admitting it is claimed that the kagome ambit excitations are gapped [as accepted from NMR (94) and neutron studies (95)], in reality, the absolute low-energy spectrum is bedeviled by birthmark spins (often referred to as “orphan” spins). These spins arise from the transition-metal sites amid the kagome planes that are not absolutely inhabited by nonmagnetic Zn but additionally accommodate alluring Cu2 (96). Agnate issues abide aback Zn is replaced by added 2 ions such as Mg or Cd. Getting rid of these birthmark spins is a aloft challenge, not alone for herbertsmithite but for best ambit aqueous candidates breadth agnate furnishings occur. This is important because some of the backdrop apparent in herbertsmithite are evocative of accidental ambit singlet states breadth there is a administration of barter energies J (97), and it has been claimed that the breakable neutron drop (INS) abstracts can be accepted in this way, as able-bodied (98). Such accidental singlet states are not breakthrough ambit liquids (because their beachcomber functions accept a artefact form), alike admitting they do affectation quantum-critical–like scaling.
(A) Ambit continuum of herbertsmithite from breakable neutron drop [S(q,ω) at 1.6 K in the hk0 plane: upper, 6 meV; middle, 2 meV; lower, 0.75 meV] (91). (B) Acreage assurance of the ambit gap of Zn-barlowite from NMR [upper: 19F Knight about-face adjoin temperature for assorted alluring fields; lower: alluring acreage assurance of the ambit gap, Δ, with abject curve the accepted behavior for S = ½ and S = 1 excitations] (101), and (C) quantized plateau in the thermal Hall aftereffect of α-RuCl3 [κxy/T adjoin alluring field: upper, 3.7 K; average 4.3 K; lower, 4.9 K] (112).
These issues accept led to the abstraction of accompanying abstracts such as Zn-barlowite, which is agnate to herbertsmithite except that the kagome layers are ample abnormally (99, 100). One advantage of Zn-barlowite is that the fluorine NMR band is simple, accustomed its nuclear ambit of ½. Analysis of these NMR abstracts indicates a ambit gap whose acreage assurance is affiliated with a gas of spin-½ particles (i.e., spinons) (101) (Fig. 4B). This is added accurate by INS studies, which announce that the INS ambit gap is alert that accepted by NMR (consistent with the actuality that INS measures spin-1 excitations, i.e., pairs of spinons) (102), admitting as in herbertsmithite (93, 94), the low-energy backdrop of barlowite are bedeviled by birthmark spins. Best recently, attempts accept been fabricated to blockhead herbertsmithite to apprehend the long-sought “doped ambit liquid” affected by Anderson in 1987 (8). However, intercalating Li (103) or replacing Zn2 by Ga3 (104) leads to localized polarons [as accepted by body anatomic calculations (105)], and appropriately no adaptable carriers as in high-temperature superconducting cuprates (106). Alike if polarons were not to occur, DMRG simulations adumbrate Wigner condensate of the benumbed carriers (107).
The bend by Jackeli and Khaliullin (67) that assertive Mott-Hubbard systems with partially abounding t2 g-levels and able spin-orbit coupling adeptness apprehend the Kitaev archetypal led to an acute search. The aboriginal abstracts advised were those such as α-Na2IrO3 and α-Li2IrO3, breadth Ir4 ions (with able j = ½) anatomy a bore lattice. Although these abstracts affectation all-embracing alluring order, polarized beating x-ray abstracts appearance that bond-directional Kitaev interactions (Fig. 2D) absolutely action in this chic of abstracts (108). This demonstrates why the contempo analysis of a variant, H3LiIr2O6, that does not affectation continued ambit adjustment is important (109).
The adeptness that α-RuCl3 has backdrop agnate to those of the iridate bore abstracts led to a huge advance in these studies. In α-RuCl3, alluring Ru is begin on a bore filigree amid close-packed Cl planes (Fig. 3C). This actual is almost accessible to abound in single-crystalline anatomy and dispense (as the layers are van der Waals coupled, they can be exfoliated). Also, the thermal neutron assimilation cantankerous area for Ru is a agency of 170 beneath than for Ir, so α-RuCl3 is acquiescent to INS studies, which acknowledge a continuum of ambit excitations (110). However, there has been some agitation about which backdrop of this actual are attributable to the Kitaev model, as adjoin to added acceptable physics (stemming from the non-negligible Heisenberg interaction). In particular, questions accept been aloft whether magnon-like excitations could explain some (or all) of the abstracts (111), accustomed that the actual does adjustment at low temperatures. Nevertheless, the ambit continuum as detected in Raman abstracts seems to obey fermionic statistics (65). Best notably, alluring adjustment is suppressed aloft applying a alluring field, implying that a ambit aqueous appearance adeptness abide in a ambit of alluring fields. This led to a altitude of a thermal Hall arresting that plateaued in a baby ambit of temperature and alluring acreage (112) (Fig. 4C). The amount of this plateau is affiliated with Majorana bend modes, actuality one-half of the amount for fermionic bend modes (66). The ascertainment of such a quantized plateau is peculiar, accustomed that the thermal Hall bend is baby (the longitudinal thermal application is bedeviled by phonons), but this has been explained by two altered approach efforts (113, 114). As with best important abstracts in this field, this aftereffect has yet to be reproduced by added groups. In addition, affiliated with the organics and herbertsmithite, ataxia should comedy an important role as well, decidedly accustomed the attendance of stacking faults (115). Finally, based on the affirmation that α-RuCl3 exhibits ambit aqueous behavior, it is of abundant assimilation to abstraction the accurate backdrop of electron- and hole-doped variants (116, 117).
A big catechism looms for the honeycomb-based ambit aqueous candidates: Is the Kitaev archetypal absolutely accordant to these materials? The ambit aqueous in the exact band-aid may accept alone a tiny administration of adherence aloft the solvable limit, on the base of afterwards calculations of the Kitaev archetypal supplemented with Heisenberg barter interactions (118). Furthermore, in the exact solution, the vison gap is actual baby (only a few percent of the Kitaev exchange) and so thermally, the ambit aqueous accompaniment alone occurs at actual low temperatures. Contempo calculations advance that a assertive spin-anisotropic “symmetric exchange” enhances the adherence of the absolutely apparent ambit aqueous (119). Alternatively, the achievability that any ambit aqueous that occurs in α-RuCl3 or the iridates may not be calmly affiliated to the Kitaev ambit aqueous allegation be kept in apperception (120).
Space considerations avert a abundant annual of added ambit aqueous candidates. Of contempo assimilation has been YbMgGaO4, breadth the Yb ions anatomy a triangular lattice, admitting with ataxia on the nonmagnetic cation site. It is accessible to abound and study, and the baby activity scales associated with the 4f Yb ion accomplish it added acquiescent to assertive types of studies [extensive neutron drop studies accept been done (121)]. It, too, has been claimed to possibly accept a “spinon” Fermi apparent (122), but as with best ambit aqueous candidates, ataxia plays an important role (82, 123)—in this case, Mg and Ga interchanges that alter the Yb ambiance (124). Agnate considerations administer to Ba3CuSb2O9 (125), breadth Cu/Sb interchanges occur. Accession candidate, Ca10Cr7O28, can be declared as a triangular filigree of six Cr5 -based spin-½ clusters—each consisting of an antiferromagnetic and a ferromagnetic triangle interacting ferromagnetically with anniversary other. All-encompassing alpha and afterwards assignment on this bilayer kagome actual has accustomed its ambit Hamiltonian and the abridgement of changeless ambit acclimation at temperatures as low as 0.3 K (126). For both YbMaGaO4 (127) and Ca10Cr7O28 (128), however, the absence of a beeline appellation in the thermal application argues adjoin the actuality of a spinon Fermi surface. Moreover, a abridgement of all-embracing alluring adjustment has been appear in the triangular-based abstracts NiGa2S4 (129), Ba8CoNb6O24 (130), NaYbO2 (131), and Ba4NbIr3O12 (132), as able-bodied as in the honeycomb-based actual BaCo2As2O8 (133). Recently, a chestnut oxide, averievite [Cu5V2O10(CsCl)], was articular in which the chestnut ions anatomy a pyrochlore slab. Aboriginal apparent in a abundance in Kamchatka, the actual was actinic and afterwards languished in an bookish thesis, alone to be “rediscovered” (thanks to Google Scholar) (134). Substitution by zinc acceptable replaces the intersite chestnut ions (as in herbertsmithite), isolating the chestnut kagome layers, and the affiliated susceptibility and specific calefaction are evocative of herbertsmithite (132). Several abstracts are additionally accepted breadth alluring ions anatomy a “hyper-kagome” filigree (obtained by demography the kagome band and affairs it into the third dimension). Of accurate agenda are Na4Ir3O8 (135) and PbCuTe2O6 (136), but afresh both accept quenched ataxia (for the aloft material, acquired by apportioned activity of the Na sites) and distortions (for the closing material, there are abounding barter ambit associated with its adulterated hyperkagome lattice). As for added balked 3D lattices, all-encompassing studies on rare-earth and transition-metal pyrochlores are aloft the ambit of this article, and the clairvoyant is referred to a contempo analysis (137). An agitative contempo bend (138), which has accustomed some alpha abutment (139, 140), is that the layered transition-metal dichalcogenide 1T-TaS2 adeptness be a breakthrough ambit liquid.
This analysis of breakthrough ambit liquids may leave one to ask, “What abroad is out there?” About certainly, a lot. As for materials, abounding absorbing ones accepted in mineralogical anatomy accept yet to be fabricated in the lab and advised for their alluring properties. As an example, quetzalcoatlite (named afterwards an Aztec god) has chestnut ions on a absolute kagome filigree (141). But it, like abounding added minerals, is accepted alone by its anatomy and annihilation else. A analytical abstraction of potentially balked allure in mineral collections adeptness be a acceptable start, followed by attempts to accomplish cleaner constructed versions of the most-promising minerals. A alternate claiming with balked magnets is that actinic ataxia acts at the “ultraviolet” scale, giving acceleration to drop spins. Clearly, added absorption (and resources) needs to be adherent to synthesis, both in developing able new amalgam routes (high pressure, hydrothermal, atomic axle epitaxy, etc.) and award means to abate and ascendancy disorder. This is a difficult task, but it is advantageous to anamnesis that it took decades of abstracts analysis to advance basal silicon apple-pie abundant for applications and modulation-doped AlGaAs heterostructures that affectation the apportioned breakthrough Hall effect. Disorder—especially aback anxiously controlled—can additionally be illuminating. The aperitive achievability of replacing Fe by Ru or Os in jarosites has been mentioned above. Similarly, one wonders what the osmium analog of α-RuCl3 would be like (142). And, of course, accepting a benumbed ambit aqueous that is brownish would be the angelic beaker for abounding (8, 9, 22, 143). This could potentially be able by ionic aqueous gating to abstain actinic disorder.
Having addressed materials-based issues above, we about-face to theory. Although abundant strides accept been fabricated in afterwards techniques, we still do not know, for instance, what the arena accompaniment is of the near-neighbor Heisenberg archetypal on a kagome lattice, and beneath about abounding added balked lattices, or for “real” Hamiltonians that accommodate assorted barter ambit as able-bodied as anisotropic barter and DM terms. Still beneath is accepted about dynamical and nonequilibrium properties. Although neutron drop aback accumulated with abstract calculations of the alluring anatomy agency S(q,ω) can accommodate amplified affirmation for a ambit aqueous (144), methods to delving affair are bare to access model-independent evidence. As ambit liquids are ambit ancestors of the apportioned breakthrough Hall effect, it would accomplish faculty to administer methods accepted from spintronics to chase for ambit currents (145, 146), the ambit Hall effect, ambit noise, and added spin-related furnishings that adeptness betrayal the attributes of the spinons (if they absolutely exist). As for visons, a bend for their abstraction was fabricated abounding years ago (147) that involves adorable for trapped alluring alteration in a spin-liquid ring. This agreement was absolutely performed on a superconducting cuprate with a absent aftereffect (148), but acutely accomplishing this array of agreement on ambit aqueous applicant abstracts would be in order. Similarly, algae can be exploited not alone to allurement Majorana fermions but additionally to abet Friedel oscillations abreast defects (that could be detected by a scanning tunneling probe) that could acknowledge a spinon Fermi apparent should it abide (149). And tunneling has been advocated as a accessible delving of how electrons could potentially fractionalize aback injected into a ambit aqueous (150). Ultimately, if topological excitations were articular in a material, afresh one could anticipate about acid and extending their appearance adherence time and braiding them in accomplish against their appliance for “topological” breakthrough ciphering (151) (Fig. 1C). As for added abeyant applications, we can anticipate of no bigger way to end than with Michael Faraday’s declared acknowledgment to William Gladstone’s adjournment of a accurate discovery: “What use is it?” he quipped. “Why, sir, there is every anticipation that you will anon be able to tax it.”
E. Fradkin, Acreage Theories of Condensed Amount Systems (Addison-Wesley, 1991).
X. G. Wen, Breakthrough Acreage Approach of Many-Body Systems (Oxford Univ. Press, 2004).
S. Sachdev, Breakthrough Appearance Transitions (Cambridge Univ. Press, 1999).
M. Tinkham, Introduction to Superconductivity (McGraw-Hill, 1966).
Acknowledgments: We acknowledge S. Chernyshev, M. Hermanns, P. Lee, Y. Lee, and S. Sondhi for their comments. Funding: C.B. and R.J.C. were accurate by the U.S. Department of Energy, Office of Science, Basal Activity Sciences, through DE-SC-0019331. S.A.K. was accurate by the National Science Foundation admission DMR-1608055. M.R.N. was accurate by the U.S. Department of Energy, Office of Science, Basal Activity Sciences, Abstracts Sciences and Engineering Division. T.S. was accurate by the National Science Foundation admission DMR-1608505, and partially through a Simons Investigator Award. Aggressive interests: The authors acknowledge no aggressive interests.
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