We determined the crystal structure of Cs$_2$LiRuCl$_6$, which was synthesized first in this work, and investigated its magnetic properties. Cs$_2$LiRuCl$_6$ has a hexagonal structure composed of linear chains of face-sharing RuCl$_6$ and LiCl$_6$ octahedra. In two-thirds of the structural chains, Ru$^{3+}$ and Li$^+$ sites are almost ordered, while in the other chains their sites are disordered. This situation is analogous to the ground state of the antiferromagnetic Ising model on a triangular lattice. Using electron paramagnetic resonance, we evaluated the $g$ factors of Ru$^{3+}$ with effective spin-1/2 as $g_c\,{=}\,2.72$ and $g_{ab}\,{=}\,1.50$ for magnetic fields $H$ parallel and perpendicular to the $c$ axis, respectively. Magnetization curves for $H\,{\parallel}\,c$ and $H\,{\perp}\,c$ are highly anisotropic. However, these magnetization curves approximately coincide when normalized by the $g$ factors. It was found from the magnetization and specific heat results that Cs$_2$LiRuCl$_6$ can be described as a coupled one-dimensional $S\,{=}\,1/2$ Heisenberg-like antiferromagnet with $J/k_{\rm B}\,{\simeq}\,3.7$ K. Three-dimensional ordering occurs at $T_{\rm N}\,{=}\,0.48$ K. A magnetic phase diagram for $H\,{\parallel}\,c$ is also presented.
Interplay between Kondo effect and trends to antiferromagnetic and spin glass ordering in perfect and disordered bipartite Kondo lattices is considered. Ginzburg-Landau equation is derived from the microscopic effective action written in three mode representation (Kondo screening, antiferromagnetic correlations and spin liquid correlations). The problem of local constraint is resolved by means of Popov-Fedotov representation for localized spin operators. It is shown that the Kondo screening enhances the trend to a spin liquid crossover and suppresses antiferromagnetic ordering in perfect Kondo…
Bi3Mn4O12(NO3), in which the Mn4+ ions carry S=3/2, is the first honeycomb lattice system that shows no long-range magnetic order. Using neutron scattering, we have determined that short-range antiferromagnetic correlations develop at low temperatures. Applied magnetic fields induce a magnetic transition, in which the short-range order abruptly expands into a long-range order.
We report the results of dc and ac magnetization measurements as a function of temperature (1.8 - 300 K) for the spin chain compound, Ca$_3$CoRhO$_6$, which has been recently reported to exhibit a partially disordered antiferromagnetic (PDAF) structure in the range 30 - 90 K and spin-glass freezing below 30 K. We observe an unexpectedly large frequency dependence of ac susceptibility in the T range 30 - 90 K, typical of superparamagnets. In addition, we find that there is no difference in the isothermal remanent magnetization behavior for the two regimes below 90 K. These findings call for mor…
In addition to the well known ferromagnetism, double perovskites are also expected to exhibit antiferromagnetic (AF) order driven by electron delocalisation. This has been seen in model Hamiltonian studies and confirmed via ab initio calculations. The AF phases should occur, for example, on sufficient electron doping of materials like Sr_2FeMoO_6 (SFMO) via La substitution for Sr. Clear experimental indication of such AF order is limited, possibly because of increase in antisite disorder with La doping on SFMO, although intriguing signatures of non ferromagnetic behaviour are seen. We study th…
Using the determinant quantum Monte Carlo method, we investigate the antiferromagnetic phase transition that is induced by the Zeeman field in a disordered interacting two-dimensional Dirac fermion system. At a fixed interaction strength $U$, the antiferromagnetic correlation is enhanced as the magnetic filed increases, and when the magnetic field is larger than a $B_{c}(U)$, the antiferromagnetic correlation shall be suppressed by the increased magnetic field. The impact of Zeeman field $B$, Coulomb repulsion $U$ and disorder $Δ$ is not isolated. The intensity of magnetic field effect on the …
The spin-fermion model has long been used to describe the quantum-critical behavior of 2d electron systems near an antiferromagnetic (AFM) instability. Recently, the standard procedure to integrate out the fermions to obtain an effective action for spin waves has been questioned in the clean case. We show that in the presence of disorder, the single fermion loops display two crossover scales: upon lowering the energy, the singularities of the clean fermionic loops are first cut off, but below a second scale new singularities arise that lead again to marginal scaling. In addition, impurity line…
We study the temperature-dependent quantum correction to conductivity due to the interplay of spin density fluctuations and weak disorder for a two-dimensional metal near an antiferromagnetic (AFM) quantum critical point. AFM spin density fluctuations carry large momenta around the ordering vector $\mathbf{Q}$ and, at lowest order of the spin-fermion coupling, only scatter electrons between "hot spots" of the Fermi surface which are connected by $\mathbf{Q}$. Earlier, it was seen that the quantum interference between AFM spin density fluctuations and soft diffusive modes of the disordered meta…
In disordered spin systems with antiferromagnetic Heisenberg exchange, transitions into and out of a magnetic-field-induced ordered phase pass through a unique regime. Using quantum Monte Carlo simulations to study the zero-temperature behavior, these intermediate regions are determined to be a Bose-Glass phase. The localization of field-induced triplons causes a finite compressibility and hence glassiness in the disordered phase.
The disordered antiferromagnet \pfn (\pfns) is investigated in a wide temperature range by combining Mössbauer spectroscopy and neutron diffraction experiments. It is demonstrated that the magnetic ground state is a {\it microscopic} coexistence of antiferromagnetic and a spin-glass orders. This speromagnet-like phase features frozen-in short-range fluctuations of the Fe$^{3+}$ magnetic moments that are transverse to the long-range ordered antiferromagnetic spin component.
Motivated by puzzling spin-glass behaviors observed in many pyrochlore-based magnets, effects of magnetoelastic coupling to local lattice distortions were recently studied by the authors for a bond-disordered antiferromagnet on a pyrochlore lattice [Phys. Rev. Lett. 107, 047204 (2011)]. Here, we extend the analyses with focusing on the critical property of the spin-glass transition which occurs concomitantly with a nematic transition. Finite-size scaling analyses are performed up to a larger system size with 8192 spins to estimate the transition temperature and critical exponents. The exponent…
We perform the calculation of the dc resistivity as a function of temperature of the "strange-metal" state that emerges in the vicinity of a spin-density-wave phase transition in the presence of weak disorder. This scenario is relevant to the phenomenology of many important correlated materials, such as, e.g., the pnictides, the heavy-fermion compounds and the cuprates. To accomplish this task, we implement the memory-matrix approach that allows the calculation of the transport coefficients of the model beyond the quasiparticle paradigm. Our computation is also inspired by the $ε=3-d$ expansio…
We report inelastic time-of-flight and triple-axis neutron scattering measurements of the excitation spectrum of the coupled antiferromagnetic spin-1 Heisenberg chain system CsNiCl3. Measurements over a wide range of wave-vector transfers along the chain confirm that above T_N CsNiCl3 is in a quantum-disordered phase with an energy gap in the excitation spectrum. The spin correlations fall off exponentially with increasing distance with a correlation length xi=4.0(2) sites at T=6.2K. This is shorter than the correlation length for an antiferromagnetic spin-1 Heisenberg chain at this temperatur…
Using the recently introduced adaptive density-matrix renormalization-group method, we study the many spin-spin correlations of the spin-$1/2$ antiferromagnetic Heisenberg chain with random coupling constants, namely, the mean value of the bulk and of the end-to-end correlations, the typical value of the bulk correlations, and the distribution of the bulk correlations. Our results are in striking agreement with the predictions of the strong-disorder renormalization group method. We do not find any hint of logarithmic corrections neither in the bulk average correlations, which were recently rep…
In their ideal structure, double perovskites like Sr_2FeMoO_6 have alternating Fe and Mo along each cubic axes, and a homogeneous ferromagnetic metallic ground state. Imperfect annealing leads to the formation of structural domains. The moments on mislocated Fe atoms that adjoin each other across the domain boundary have an antiferromagnetic coupling between them. This leads to a peculiar magnetic state, with ferromagnetic domains coupled antiferromagnetically. At short distance the system exhibits ferromagnetic correlation while at large lengthscales the net moment is strongly suppressed due …
In the quest for quantum spin liquids in three spatial dimensions (3D), we study the half-filled Hubbard model on the simple cubic lattice with hopping processes up to third neighbors. Employing the variational cluster approach (VCA), we determine the zero-temperature phase diagram: In addition to a paramagnetic metal at small interaction strength $U$ and various antiferromagnetic insulators at large $U$, we find an intermediate-$U$ antiferromagnetic metal. Most interestingly, we also identify a non-magnetic insulating region, extending from intermediate to strong $U$. Using VCA results in the…
Inverse Melting of 3D Antiferromagnetic Order in Multi-sublattice Magnetic Perovskites
University College London（伦敦大学学院）；STFC Rutherford Appleton Laboratory（STFC卢瑟福阿普顿实验室）；National Institute for Materials Science（材料科学国家研究所）；Hokkaido University（北海道大学）；Osaka University（大阪大学）；Durham University（杜伦大学）
We investigate the effect of hole and electron doping to half-filling in the periodic Anderson model on a triangular lattice by the Hartree-Fock approximation at zero temperature. At half-filling, the system exhibits a partially disordered insulating state, in which a collinear antiferromagnetic order on an unfrustrated honeycomb subnetwork coexists with nonmagnetic state at the remaining sites. We find that the carrier doping destabilizes the partially disordered state, resulting in a phase separation to a doped metallic state with different magnetic order. The partially disordered state is r…
We present Quantum Monte-Carlo simulations of an exchange-anisotropic spin-1/2 Heisenberg model on a square lattice with nearest and next-nearest neighbor interactions. The ground state phase diagram shows two classical magnetically ordered phases for dominating antiferromagnetic S^z-interactions and for large quantum fluctuations a ferromagnetic order in the x-y plane. In between a finite region is detected where neither classical nor quantum mechanical order, e.g. long-ranged dimer correlations, are found.
We report our theoretical results on the emergence of a partially-disordered state at zero temperature and its detailed nature in the periodic Anderson model on a triangular lattice at half filling. The partially-disordered state is characterized by coexistence of a collinear antiferromagnetic order on an unfrustrated honeycomb subnetwork and nonmagnetic state at the remaining sites. This state appears with opening a charge gap between a noncollinear antiferromagnetic metal and Kondo insulator while changing the hybridization and Coulomb repulsion. We also find a characteristic crossover in th…
Effects of disorder and external field on the competing spin-Peierls and antiferromagnetic states are studied theoretically in terms of the numerical transfer matrix method applied to a quasi one-dimensional spin 1/2 Heisenberg model coupled to the lattice degree of freedom. We show that, at temperatures above the impurity-induced antiferromagnetic phase, inhomogeneous spin-Peierls lattice distortions remain to exist showing a re-entrant behavior. This feature can be drastically altered by very weak perturbations, e.g., the staggered magnetic field or the change in interchain exchange coupling…
Using the recently developed transfer-matrix renormalization group method, we have studied the thermodynamic properties of two-leg antiferromagnetic ladders in the magnetic field. Based on different behavior of magnetization, we found disordered spin liquid, Luttinger liquid, spin-polarized phases and a classical regime depending on magnetic field and temperature. Our calculations in Luttinger liquid regime suggest that both the divergence of the NMR relaxation rate and the anomalous specific heat behavior observed on Cu$_2$(C$_5$H$_{12}$N$_2$)$_2$Cl$_4$}are due to quasi-one-dimensional effect…
The $1/4$-filled organic compound, $δ$-(EDT-TTF-CONMe$_{2}$)$_{2}$AsF$_6$ is a frustrated two-dimensional triangular magnetic system as shown by high-frequency (111.2 and 222.4 GHz) electron spin resonance (ESR) and structural data in the literature. The material gradually orders antiferromagnetically below 40~K but some magnetically disordered domains persist down to 4 K. We propose that in defect free regions frustration prevents true magnetic order down to at least 4 K in spite of the large first- and second-neighbor exchange interactions along chains and between chains, respectively. The a…
We review recent progress in our theoretical understanding of strongly correlated fermion systems in the presence of disorder. Results were obtained by the application of a powerful nonperturbative approach, the Dynamical Mean-Field Theory (DMFT), to interacting disordered lattice fermions. In particular, we demonstrate that DMFT combined with geometric averaging over disorder can capture Anderson localization and Mott insulating phases on the level of one-particle correlation functions. Results are presented for the ground-state phase diagram of the Anderson-Hubbard model at half filling, bot…
The title compound Ba3RuTi2O9 crystallizes with a hexagonal unit cell. It contains layers of edge shared triangular network of Ru4+ (S=1) ions. Magnetic susceptibility chi(T) and heat capacity data show no long range magnetic ordering down to 1.8K. A Curie-Weiss (CW) fitting of chi(T) yields a large antiferromagnetic CW temperature theta_CW=-166K. However, in low field, a splitting of zero field cooled (ZFC) and field cooled (FC) chi(T) is observed below ~30K. Our measurements suggest that Ba3RuTi2O9 is a highly frustrated system but only a small fraction of the spins in this system undergo a …
In the present paper we study the phase diagram of the Heisenberg model on the honeycomb lattice with antiferromagnetic interactions up to third neighbors along the line $J_2=J_3$ that include the point $J_2=J_3=J_1/2$, corresponding to the highly frustrated point where the classical ground state has macroscopic degeneracy. Using the Linear Spin-Wave, Schwinger boson technique followed by a mean field decoupling and exact diagonalization for small systems we find an intermediate phase with a spin gap and short range Néel correlations in the strong quantum limit (S=1/2). All techniques provide …
A model of disordered spin-Peierls system is considered, where domain walls are randomly distributed as a telegraph noise. For this realization of the disorder in an XX spin chain, we calculate exactly the density of states as well as several thermodynamic quantities. The resulting physical behavior should be qualitatively unchanged even for an XXZ chain, up to the isotropic XXX point. For weak disorder, besides a high energy regime where the behavior of a pure spin-Peierls system is recovered, there is a cross-over to a low energy regime with singular thermodynamic properties and enhanced ant…
For coupled-dimer magnets with quenched disorder, we introduce a generalization of the bond-operator method, appropriate to describe both singlet and magnetically ordered phases. This allows for a numerical calculation of the magnetic excitations at all energies across the phase diagram, including the strongly inhomogeneous Griffiths regime near quantum criticality. We apply the method to the bilayer Heisenberg model with bond randomness and characterize both the broadening of excitations and the transfer of spectral weight induced by disorder. Inside the antiferromagnetic phase this model fea…
We study the evolution of magnetic excitations in the disordered two-dimensional antiferromagnet Sr2Ir1-xRuxO4. A gigantic magnetic gap greater than 40 meV opens at x = 0.27 and increases with Ru concentration, rendering the dispersive magnetic excitations in Sr2IrO4 almost momentum independent. Up to a Ru concentration of x = 0.77, both experiments and first-principles calculations show the Ir Jeff = 1/2 state remains intact. The magnetic gap arises from the local interaction anisotropy in the proximity of the Ru disorder. Under the coherent potential approximation, we reproduce the experimen…
The dynamical spin susceptibility is studied in the magnetically-disordered phase of heavy-Fermion systems near the antiferromagnetic quantum phase transition. In the framework of the $S=1/2$ Kondo lattice model, we introduce a perturbative expansion treating the spin and Kondo-like degrees of freedom on an equal footing. The dynamical spin susceptibility displays a two-component behaviour in agreement with the inelastic neutron scattering (INS) experiments performed in $CeCu_{6}$, $Ce_{1-x}La_{x}Ru_{2}Si_{2}$ and $UPt_{3}$: a quasielastic q-independent peak as in a Fermi liquid theory, and a …
The half-doped manganite, (Eu0.4La0.1)(Sr0.4Ca0.1)MnO3, has been found to exhibit sharp step-like metamagnetic transitions below 5 K. The number of magnetic steps increases with decreasing temperature and this number suddenly rises from 3 at 2 K to ~50 at 1.7 K. The self-similar character of the multiple magnetic steps at reduced temperature identifies these steps as avalanche-like transitions. The occurrence of a multitude of magnetic steps at low temperatures, the decreasing effect of magnetic field sweep rate on the step-like metamagnetism, and the decreasing irreversibility of transition i…
We study the heat transport of S = 1/2 chain compound CuCl_2$\cdot$2((CH_3)_2SO) along the b axis (vertical to the chain direction) at very low temperatures. The zero-field thermal conductivity (κ) shows a distinct kink at about 0.9 K, which is related to the long-range antiferromagnetic (AF) transition. With applying magnetic field along the c axis, κ(H) curves also show distinct changes at the phase boundaries between the AF and the high-field disordered states. These results indicate a strong spin-phonon interaction and the magnetic excitations play a role in the b-axis heat transport as ph…
We study the conductivity of a 3D disordered metal close to the antiferromagnetic instability within the framework of the spin-fermion model using the diagrammatic technique. We calculate the interaction correction $δσ(ω,T)$ to the conductivity, assuming that the latter is dominated by the disorder scattering, and the interaction is weak. Although the fermionic scattering rate shows critical behaviour on the entire Fermi surface, the interaction correction is dominated by the processes near the hot spots, narrow regions of the Fermi-surface corresponding to the strongest spin-fermion scatterin…
A molecular Mott insulator $κ$-(ET)$_2$B(CN)$_4$ [ET = bis(ethylenedithio)tetrathiafulvalene] with a distorted triangular lattice exhibits a quantum disordered state with gapped spin excitation in the ground state. $^{13}$C nuclear magnetic resonance, magnetization, and magnetic torque measurements reveal that magnetic field suppresses valence bond order and induces long-range magnetic order above a critical field $\sim 8$ T. The nuclear spin-lattice relaxation rate $1/T_1$ shows persistent evolution of antiferromagnetic correlation above the transition temperature, highlighting a quantum spin…
Neutron scattering is used to study NiCl$_{2-2x}$Br$_{2x}\cdot$4SC(NH$_2$)$_2$, $x=0.06$, a bond-disordered modification of the well-known gapped $S=1$ antiferromagnetic quantum spin system NiCl$_{2}\cdot$4SC(NH$_2$)$_2$. The magnetic excitation spectrum throughout Brillouin zone is mapped out at $T=60$ mK using high-resolution time-of-flight spectroscopy. It is found that the dispersion of spin excitation is renormalized, as compared to that in the parent compound. The lifetime of excitations near the bottom of the band is substantially decreased. No localized states are found below the gap e…
We propose a simple modification of the density matrix renormalization group (DMRG) method in order to tackle strongly disordered quantum spin chains. Our proposal, akin to the idea of the adaptive time-dependent DMRG, enables us to reach larger system sizes in the strong disorder limit by avoiding most of the metastable configurations which hinder the performance of the standard DMRG method. We benchmark our adaptive method by revisiting the random antiferromagnetic XXZ spin-1/2 chain for which we compute the random-singlet ground-state average spin-spin correlation functions and von Neumann …
Two-dimensional disordered quantum antiferromagnets are studied by means of a continuum description in which disorder is introduced by a random distribution of couplings (spin stiffnesses) in the ordered phase of the Nonlinear Sigma Model. Quenched soliton (skyrmion) correlation functions are evaluated and used, along with the quenched magnetization, to characterize the phase structure of the system. When magnetic dilution is exponentially suppressed, the introduction of disorder only modifies the subleading terms in the large distance behavior of the soliton correlation functions, yielding th…
The magnetic correlations in the charge- and orbital-ordered manganite La(0.5)Sr(1.5)MnO(4) have been studied by elastic and inelastic neutron scattering techniques. Out of the well-defined CE-type magnetic structure with the corresponding magnons a competition between CE-type and ferromagnetic fluctuations develops. Whereas ferromagnetic correlations are fully suppressed by the static CE-type order at low temperature, elastic and inelastic CE-type correlations disappear with the melting of the charge-orbital order at high temperature. In its charge-orbital disordered phase, La(0.5)Sr(1.5)MnO(…
Site dilution of spin-gapped antiferromagnets leads to localized free moments, which can order antiferromagnetically in two and higher dimensions. Here we show how a weak magnetic field drives this order-by-disorder state into a novel disordered-free-moment phase, characterized by the formation of local singlets between neighboring moments and by localized moments aligned antiparallel to the field. This disordered phase is characterized by the absence of a gap, as it is the case in a Bose glass. The associated field-driven quantum phase transition is consistent with the universality of a super…
We investigated the crystal and magnetic structures of the spin-1/2 frustrated antiferromagnet Cu3Mo2O9 in which the spin system consists of antiferromagnetic chains and dimers. The space group at room temperature has been reported to be orthorhombic Pnma (No. 62). We infer that the space group above TN = 7.9 K is monoclinic P2_1/m (No. 11) from the observation of reflections forbidden in Pnma in x-ray powder diffraction experiments at room temperature. We determined the magnetic structure of Cu3Mo2O9 in neutron powder diffraction experiments. Magnetic moments on dimer sites lie in the ac plan…
We observe multiple step-like jumps in a Dy-Fe-Ga-based ferrimagnetic alloy in its magnetic hysteresis curve at 2 K. The observed jumps have a stochastic character with respect to their magnitude and the critical field of occurrence, and the jumps do not show any temporal effect. The jump size distribution follows a power law variation indicating the scale invariance nature of the jumps. We have invoked a simple two-dimensional random bond Ising-type spin system to model the dynamics. Our computational work can qualitatively reproduce the jumps and their scale invariant character. It also eluc…
We study antiferromagnetic two-leg spin-1/2 ladders with strong bond randomness, using the real space renormalization group method. We find the low-temperature spin susceptibility of the system follows non-universal power laws, and the ground state spin-spin correlation is short-ranged. Our results suggest that there is no phase transition when the bond randomness increases from zero; for strong enough randomness the system is in a Griffith region with divergent spin susceptibility and short-range spin-spin correlation.
We have studied the antiferromagnetic Ising chain in a transverse magnetic field $h_{x}$ and uniform longitudinal field $h_{z}$. Using the density matrix renormalization group calculation combined with a finite-size scaling the ground state phase diagram in ($h_{x},h_{z}$) plane is determined. It is shown that there is an order-disordered transition line in this plane and the critical properties belong to the universality class of the two-dimensional Ising model. Based on the perturbation theory in $h_{z}$ the scaling behavior of the mass gap in the vicinity of the critical point ($h_{x}=1/2,h…
Only 3% hole doping by Li is sufficient to suppress the long-range antiferromagnetic order in La2CuO4. Spin dynamics in such a disordered state was investigated with measurements of the dynamic magnetic structure factor S(omega,q), using cold neutron spectroscopy, for La2(Cu0.94Li0.06)O4. The S(omega,q) is found to sharply peak at (pi,pi), and its dynamics to be relaxational. Confirming theoretical expectation for the quantum disordered 2D S=1/2 Heisenberg antiferromagnet, the energy scale saturates at a finite value at low temperatures. Possible connection to the ``pseudo spin gap'' phenomeno…
The Hubbard model on the honeycomb lattice undergoes a quantum phase transition from a semimetallic to a Mott insulating phase and from a disordered to an anti-ferromagnetically phase. We show that these transitions occur simultaneously and we calculate the critical coupling $U_c=3.835(14)$ as well as the critical exponents $ν=1.181(43)$ and $β=0.898(37)$ which are expected to fall into the $SU(2)$ Gross-Neveu universality class. For this we employ Hybrid Monte Carlo simulations, extrapolate the single particle gap and the spin structure factors to the thermodynamic and continuous time limits,…
We investigate low-energy properties of two-dimensional quantum spin systems with the ladder and plaquette structures, which are described by a generalized antiferromagnetic Heisenberg model with both of the bond and spin alternations. By exploiting a non-linear $σ$ model technique and a modified spin wave approach, we evaluate the spin gap and the spontaneous magnetization to discuss the quantum phase transition between the ordered and disordered states. We argue how the spin-gapped phase is driven to the antiferromagnetic phase in the phase diagram.
We present variational results for the ground state of the antiferromagnetic quantum Heisenberg model with frustrating next-nearest-neighbour interactions. The trial wave functions employed are of resonating-valence-bond type, elaborated to account for various geometric motifs of adjacent bond pairs. The calculation is specialized to a square-lattice cluster consisting of just sixteen sites, large enough that the system can accommodate nontrivial singlet dimer correlations but small enough that exhaustive enumeration of states in the total spin zero sector is still feasible. A symbolic computa…
We investigate ground state properties of the half-filled staggered-flux Hubbard model on a square lattice. Energy gaps to charge and spin excitations and magnetic as well as dimer orders are calculated as a function of interaction strength $U/t$ by means of constrained-path quantum Monte Carlo method. It is found that the system is a semi-metal at $U/t\lesssim 5.6$ and a Mott insulator with long-range antiferromagnetic order at $U/t \gtrsim 6.6$. In the range $5.6\lesssim U/t\lesssim 6.6$, the ground state is an correlated insulator where both magnetic and dimer orders are absent. Furthermore…
We uncover a novel mechanism for inducing a gapful phase in interacting many-body quantum chains. The mechanism is nonperturbative, being triggered only in the presence of both strong interactions and strong aperiodic (disordered) modulation. In the context of the critical antiferromagnetic spin-1/2 XXZ chain, we identify an emerging dimerization which removes the system from criticality and stabilizes the novel phase. This mechanism is shown to be quite general in strongly interacting quantum chains in the presence of strongly modulated quasiperiodic disorder which is, surprisingly, perturbat…
We study the effects of disorder on long-range antiferromagnetic correlations in the half-filled, two dimensional, repulsive Hubbard model at T=0. A mean field approach is first employed to gain a qualitative picture of the physics and to guide our choice for a trial wave function in a constrained path quantum Monte Carlo (CPQMC) method that allows for a more accurate treatment of correlations. Within the mean field calculation, we observe both Anderson and Mott insulating antiferromagnetic phases. There are transitions to a paramagnet only for relatively weak coupling, U < 2t in the case of b…
We study a strongly correlated spin-1 Bose gas in 2D space by using the projective construction. A spin-disordered superfluid state is constructed and proposed as a candidate competing with the conventional polar condensate when interaction is antiferromagnetic. This novel state has a non-trivial topological order whose low energy excitations carry fractional spin, charge, and statistics. The spin excitations become gapless only at the edge and are described by level-1 SU(2) Kac-Moody algebra. The edge state is identical to the edge state of the chiral spin liquid or the right moving sector of…
Layered $α$-RuCl3 has been discussed as a proximate Kitaev spin liquid compound. Raman and THz spectroscopy of magnetic excitations confirm that the low-temperature antiferromagnetic ordered phase features a broad Raman continuum, together with two magnon-like excitations at 2.7 and 3.6 meV, respectively. The continuum strength is maximized as long-range order is suppressed by an external magnetic field. The state above the field-induced quantum phase transition around 7.5 T is characterized by a gapped multi-particle continuum out of which a two-particle bound state emerges, together with a w…
We show that the effect of disorder on a Mott-Hubbard insulator of interacting electrons produces a quantum phase transition from an antiferromagnetic to a paramagnetic phase governed by a percolation of the singly occupied weakly disordered sites. Near the transition, we propose that a new type of defect, formed by the resonant exchange between a spin singlet and a doubly occupied site with an attractive disorder potential, plays an important role. These resonant pair exchange defects reduce the staggered magnetization but enhance the coupling of the two spins and produce characteristic signa…
$S$ = 1/2 Ising kagome antiferromagnets (KAFs) are rare in reality. In this work, we report the magnetic ground state and field-induced transitions of an $S$ = 1/2 Ising antiferromagnet with a stacked kagome geometry, Na$_5$Co$_{15.5}$Te$_6$O$_{36}$. Upon cooling, magnetic susceptibility measurement reveals three successive magnetic transitions at $T_1$ = 45.2 K, $T_2$ = 35.4 K, and $T_3 \sim$ 20 K. When the magnetic field is applied along the Ising $c$ axis, a strikingly anomalous initial magnetization lying outside the hysteresis loop is observed below $T_3$. Through the detailed characteriz…
The question of structural disorder and its effects on magnetism is relevant to a number of spin liquid candidate materials. Although commonly thought of as a route to spin glass behavior, here we describe a system in which the structural disorder results in long-range antiferromagnetic order due to local symmetry breaking. Nd$_2$ScNbO$_7$ is shown to have a dispersionless gapped excitation observed in other neodymium pyrochlores below T$_N$ = 0.37 K through polarized and inelastic neutron scattering. However the dispersing spin waves are not observed. This excited mode is shown to occur in on…
We report on an investigation into the dynamics of the stripe phase of La5/3Sr1/3CoO4, a material recently shown to have an hour-glass magnetic excitation spectrum. A combination of magnetic susceptibility, muon-spin relaxation and nuclear magnetic resonance measurements strongly suggest that the physics is determined by a disordered configuration of charge and spin stripes whose frustrated magnetic degrees of freedom are strongly dynamic at high temperature and which freeze out in a glassy manner as the temperature is lowered. Our results broadly confirm a recent theoretical prediction, but s…
We present time-resolved magneto-optical spectroscopy on the magnetic Mott-Hubbard-insulating Kitaev spin liquid candidate $α$-RuCl$_3$ to investigate the nonequilibrium dynamics of its antiferromagnetically ordered zigzag groundstate after photoexcitation. A systematic study of the transient magnetic linear dichroism under different experimental conditions (temperature, external magnetic field, photoexcitation density) gives direct access to the dynamical interplay of charge excitations with the zigzag ordered state on ultrashort time scales. We observe a rather slow initial demagnetization (…
The temperature dependence of the static magnetic susceptibility of exchange-disordered antiferromagnetic Heisenberg spin-1/2 finite chains with an odd number of spins is investigated as a function of size and type of disorder in the exchange coupling. Two models for the exchange disorder distribution are considered. At sufficiently low temperatures each chain behaves like an isolated spin-1/2 particle. As the size of the chains increases, this analogy is lost and the chains evolve into the thermodynamic limit behavior. The present study provides a simple criterion, based on susceptibility mea…
