We report on the 63/65Cu-nuclear magnetic resonance and nuclear quadrupole resonance (NQR) studies of trivalent Yb zigzag chain compound YbCuS2. Sharp NQR signals were observed in the paramagnetic (PM) state. Below TO = 0.95 K, the multi peaks induced by the internal magnetic fields arising from the antferromagnetic (AFM) ordered moments appear and coexist with the PM signal down to 0.85 K, evidencing the first-order AFM phase transition at TO. In addition, the nuclear spin-lattice relaxation rate 1/T1 abruptly decreases below TO and shows the T-linear behavior below 0.5 K. The significant large 1/T1T value strongly suggests the presence of the novel gapless spin excitation in low temperature region.
Properties of the zigzag spin chains with various nearest-neighbor and next-nearest-neighbor interactions are studied by making use of the transfer-matrix renormalization group method. Thermodynamic quantities of the systems (temperature dependence of the susceptibility and the specific heat), as well as the field dependence of the magnetization are analyzed numerically with a high accuracy in the thermodynamic limit. The results have been compared with the recent experimental data on Rb$_2$Cu$_{2}$Mo$_{3}$O$_{12}$.
A modified density matrix renormalization group (DMRG) algorithm is applied to the zigzag spin-1/2 chain with frustrated antiferromagnetic exchange $J_1$, $J_2$ between first and second neighbors. The modified algorithm yields accurate results up to $J_2/J_1 \approx 4$ for the magnetic gap $Δ$ to the lowest triplet state, the amplitude $B$ of the bond order wave (BOW) phase, the wavelength $λ$ of the spiral phase, and the spin correlation length $ξ$. The $J_2/J_1$ dependences of $Δ$, $B$, $λ$ and $ξ$ provide multiple comparisons to field theories of the zigzag chain. The twist angle of the spi…
A magnetic toroidal moment is a fundamental electronic degree of freedom in the absence of both spatial inversion and time-reversal symmetries and gives rise to novel multiferroic and transport properties. We elucidate essential model parameters of the nonlinear transport in the space-time ($\mathcal{PT}$) symmetric collinear antiferromagnetic metals accompanying a magnetic toroidal moment. By analyzing the longitudinal and transverse components of the second-order nonlinear conductivity on a two-dimensionally stacked zigzag chain based on the nonlinear Kubo formula, we show that an effective …
We investigate the low-lying excitation of the antiferromagnetic zigzag spin ladder with Möbius boundary condition. Using the Lanczos and Householder diagonalization methods, we calculate the excitation spectrum of the zigzag ladder in the momentum space. We then show that the topological defect generated by the Möbius boundary provides the clear evidence of the single spinon excitation. On the basis of the obtained single-spinon dispersion curve, we analyze the two-spinon scattering states under the usual periodic boundary condition. We further discuss the interaction effect between the spino…
In this work, we investigate the microscopic nature of the magnetism in honeycomb iridium-based systems by performing a systematic study of how the effective magnetic interactions in these compounds depend on various electronic microscopic parameters. We show that the minimal model describing the magnetism in A$_2$IrO$_3$ includes both isotropic and anisotropic Kitaev-type spin-exchange interactions between nearest and next-nearest neighbor Ir ions, and that the magnitude of the Kitaev interaction between next-nearest neighbor Ir magnetic moments is comparable with nearest neighbor interaction…
We determine the magnetic phase diagram of the antiferromagnetic(AF) zigzag spin chain in the strongly frustrated region, using the density matrix renormalization group method. We find the magnetization plateau at 1/3 of the full moment accompanying the spontaneous symmetry breaking of the translation, the cusp singularities above and/or below the plateau, and the even-odd effect in the magnetization curve. We also discuss the formation mechanisms of the plateau and cusps briefly.
We demonstrate that domain-wall type excitations carrying the fractional magnetization $S^z=\pm1/3$ and their bound state describe the low-energy physics around the 1/3 plateau of the antiferromagnetic zigzag spin chain in the strongly frustrated region. In particular we discuss the relevance of such domain-wall excitations to the characteristic properties of the magnetization curves: the 1/3 plateau accompanying the spontaneous symmetry breaking of the translation, the cusp singularities above and/or below the plateau, and the even-odd effect in the magnetization curve.
We theoretically investigate an antiferromagnetic structure in a hexagonal zigzag-chain compound Ce$_3$TiBi$_5$, which shows a linear magnetoelectric effect in metals and an unfamiliar temperature dependence of magnetic susceptibility. We find a partial magnetic order as a coexistence of a nonmagnetic zigzag chain and staggered antiferromagnetic zigzag chains. The latter accompany in-plane magnetic toroidal dipole moments which are responsible for the observed behaviors. We show that magnetic frustration arising from competing exchange interactions plays an important role to realize such a par…
A family of spin-orbit coupled honeycomb Mott insulators offers a playground to search for quantum spin liquids (QSLs) via bond-dependent interactions. In candidate materials, a symmetric off-diagonal $Γ$ term, close cousin of Kitaev interaction, has emerged as another source of frustration that is essential for complete understanding of these systems. However, the ground state of honeycomb $Γ$ model remains elusive, with a suggested zigzag magnetic order. Here we attempt to resolve the puzzle by perturbing the $Γ$ region with a staggered Heisenberg interaction which favours the zigzag orderin…
We investigated the magnetic ground state and low-energy excitations of the spin chains compounds SrCuO$_{2}$ (zigzag chains) and Sr$_{2}$CuO$_{3}$ (linear chains) in the presence of quantum impurities induced by lightly doping ($\leq 1 \%$) with Zn$^{2+}$ ($S = 0$), Co$^{2+}$ ($S =1/2$) and Ni$^{2+}$ ($S = 1$) impurities at the Cu$^{2+}$ site. We show that the ground states and the nature of low-lying excitations (i.e., gapped or gapless) depend on the spin state and symmetry of the defects. For Ni doped chains a spin gap is observed but for Zn and Co doping the excitations remain gapless. Co…
We study the quantum phases and phase transitions of the Kane-Mele Hubbard (KMH) model on a zigzag ribbon of honeycomb lattice at a finite size via the weak-coupling renormalization group (RG) approach. In the non-interacting limit, the KM model is known to support topological edge states where electrons show helical property with orientations of the spin and momentum being locked. The effective inter-edge hopping terms are generated due to finite-size effect. In the presence of an on-site Coulomb repulsive interaction and the inter-edge hoppings, special focus is put on the stability of the t…
We report the magnetic structure of TaFe1+yTe3 single crystals by means of neutron diffraction measurements. TaFe1+yTe3 possesses a layered structure with a formation of two-leg zigzag ladders along the b-axis. We find that TaFe1+yTe3 undergoes an antiferromagnetic transition at 178 K with Fe1 spins of the intra-ladders ferromagnetically aligned while spins of the inter-ladders antiferromagneitcally coupled. Furthermore, spins of the neighboring interstitial Fe2 (y) ions order parallel to the Fe1 spins of each ladder. These findings are distinct from the magnetic structure of the recently-disc…
Unique features of non-magnetic insulator phase are shown, and phase diagram for t-t' Hubbard model on square lattice is presented. Using the path-integral renormalization group method, we find antiferromagnetic phase for small next-nearest neighbor transfer t' and stripe (or collinear) phase for large t' in the Mott insulating region of strong on site interaction U. For intermediate t'/t~ 0.7 at large U/t>7, we find longer-period antiferromagnetic insulator phase with 2*4 structure. In the Mott insulating region, we also find quantum spin liquid (in other words, non-magnetic insulator) phase …
The crystal structure, magnetic ground state, and the temperature dependent microscopic spin-spin correlations of the frustrated honeycomb lattice antiferromagnet Na2Co2TeO6 have been investigated by powder neutron diffraction. A three dimensional (3D) long-range antiferromagnetic ordering has been found below TN ~ 24.8 K. The magnetic ground state, determined to be zigzag antiferromagnet [k = (1/2 0 0)], occurs due to the competing exchange interactions up to third nearest neighbors within the honeycomb lattice. The exceptional existence of a limited magnetic correlation length along the c ax…
By using the variational Monte Carlo technique, we study the spin-$1/2$ XXZ antiferromagnetic model (with easy-plane anisotropy) on the kagome lattice. A class of Gutzwiller projected fermionic states with a spin Jastrow factor is considered to describe either spin liquids (with $U(1)$ or $Z_2$ symmetry) or magnetically ordered phases (with ${\bf q}=(0,0)$ or ${\bf q}=(4π/3,0)$). We find that the magnetic states are not stable in the thermodynamic limit. Moreover, there is no energy gain to break the gauge symmetry from $U(1)$ to $Z_2$ within the spin-liquid states, as previously found in the …
$^{51}$V NMR studies on CaV2O4 single crystals and $^{17}$O NMR studies on $^{17}$O-enriched powder samples are reported. The temperature dependences of the $^{17}$O NMR line width and nuclear spin-lattice relaxation rate give strong evidence for a long-range antiferromagnetic transition at Tn = 78 K in the powder. Magnetic susceptibility measurements show that Tn = 69 K in the crystals. A zero-field $^{51}$V NMR signal was observed at low temperatures (f $\approx$ 237 MHz at 4.2 K) in the crystals. The field swept spectra with the field in different directions suggest the presence of two anti…
Two-leg spin-1/2 ladder systems consisting of a ferromagnetic leg and an antiferromagnetic leg are considered where the spins on the legs interact through antiferromagnetic rung couplings $J_1$. These ladders can have two geometrical arrangements either zigzag or normal ladder and these systems are frustrated irrespective of their geometry. This frustration gives rise to incommensurate spin density wave, dimer and spin fluid phases in the ground state. The magnetization in the systems decreases linearly with $J^2_1$, and the systems show an incommensurate phase for $0.0<J_1<1.0$. The spin-spin…
We report a single-crystal study on the magnetism of the rare-earth compound PrTiNbO$_6$ that experimentally realizes the zigzag pseudospin-$\frac{1}{2}$ quantum antiferromagnetic chain model. Random crystal electric field caused by the site mixing between non-magnetic Ti$^{4+}$ and Nb$^{5+}$, results in the non-Kramers ground state quasi-doublet of Pr$^{3+}$ with the effective pseudospin-$\frac{1}{2}$ Ising moment. Despite the antiferromagnetic intersite coupling of about 4 K, no magnetic freezing is detected down to 0.1 K, whilst the system approaches its ground state with almost zero residu…
Spin excitation in a nano-graphite ribbon with zigzag edges is investigated theoretically. Due to the strongly localized nature of the states near Fermi energy, the effective Hamiltonian for the low energy physics is given by Heisenberg Hamiltonian with the nearest neighbor exchange coupling. The action corresponding to the effective Hamiltonian is mapped to that of the O(3) nonlinear sigma model. It is shown that the spin excitation has a gap when the number of the zigzag lines is even, whereas the excitation becomes gapless in case of the odd number of the zigzag lines.
The layered honeycomb magnet alpha-RuCl3 has been proposed as a candidate to realize a Kitaev spin model with strongly frustrated, bond-dependent, anisotropic interactions between spin-orbit entangled jeff=1/2 Ru4+ magnetic moments. Here we report a detailed study of the three-dimensional crystal structure using x-ray diffraction on untwinned crystals combined with structural relaxation calculations. We consider several models for the stacking of honeycomb layers and find evidence for a crystal structure with a monoclinic unit cell corresponding to a stacking of layers with a unidirectional in…
In order to clarify a key role of orbital degree of freedom in the spin S=1 Haldane system, we investigate ground-state properties of the t2g-orbital degenerate Hubbard model on the linear chain by using numerical techniques. Increasing the Hund's rule coupling in multi-orbital systems, in general, there occurs a transition from an antiferromagnetic to a ferromagnetic phase. We find that the antiferromagnetic phase is described as the Haldane system with spin gap, while in the ferromagnetic phase, there exists the gapless excitation with respect to orbital degree of freedom. Possible relevance…
Stimulated by recent studies of quantum phases with broken local inversion symmetry, we study the magnetoelectric effect in locally noncentrosymmetric metals. We consider three-dimensional (3D) coupled zigzag chains and demonstrate that the antiferromagnetic moment is induced by the electric current through a staggered antisymmetric spin-orbit coupling. This current-induced magnetism is much larger than that in globally noncentrosymmetric metals. We provide an intuitive understanding of the current-induced antiferromagnetic moment by showing the inverse magnetoelectric effect, that is, the fer…
The spin-1/2 antiferromagnetic zig-zag ladder is studied by exact diagonalization of small systems in the regime of weak inter-chain coupling. A gapless phase with quasi long-range spiral correlations has been predicted to occur in this regime if easy-plane (XY) anisotropy is present. We find in general that the finite zig-zag ladder shows three phases: a gapless collinear phase, a dimer phase and a spiral phase. We study the level crossings of the spectrum,the dimer correlation function, the structure factor and the spin stiffness within these phases, as well as at the transition points. As t…
Recent DFT calculations for Ba2CoO4 (BCO) and neutron scattering experiments for SrRuO3 (SRO) have shown that oxygen develops a magnetic polarization. Moreover, DFT calculations for these compounds also unveiled unexpected nodes in the spin density, both along Co-O and Ru-O. For BCO, the overall antiferromagnetic state in its triangular lattice contains unusual zigzag spin patterns. Here, using simple model calculations supplemented by DFT we explain and extend these results. We predict that ligands that in principle should be spinless, such as O$^{2-}$, will develop a net polarization when th…
Band structure calculations within the local spin-density approximation are presented in order to investigate the electronic and magnetic properties of the zigzag spin-chain compound In$_2$VO$_5$. The essential structural feature of the system is a double chain of VO$_6$-octahedra, which leads to competing intrachain and interchain magnetic couplings. Frustration of the spin-chains is expected for the proposed antiferromagnetic ordering at low temperatures. However, the band calculations show that the experimental room temperature crystal structure is incompatible with antiferromagnetism. Both…
The frustrated spin-1/2 model with weakly anisotropic ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor exchanges is studied with use of variational mean-field approach, scaling estimates of the infrared divergencies in the perturbation theory and finite-size calculations. The ground state phase diagram of this model contains three phases: the ferromagnetic phase, the commensurate spin-liquid phase and the incommensurate phase. The non-trivial behavior of the boundaries between these phases and the character of the phase transitions in case of weak anisotropy are deter…
The interplay between Kondo effect, indirect magnetic interaction and geometrical frustration is studied in the Kondo lattice on the one-dimensional zigzag ladder. Using the density-matrix renormalization group (DMRG), the ground state and various short- and long-range spin- and density-correlation functions are calculated for the model at half-filling as a function of the antiferromagnetic Kondo interaction down to $J=0.3t$ where $t$ is the nearest-neighbor hopping on the zigzag ladder. Geometrical frustration is shown to lead to at least two critical points: Starting from the strong-$J$ limi…
The ground state properties of a frustrated spin-1/2 system is studied on a trellis ladder which is composed of two zigzag ladders interacting through rung interactions. The presence of rung interaction between the zigzag ladders induces a non-magnetic ground state, although, each of zigzag ladders has ferromagnetic order in weak antiferromagnetic leg interaction limit. The rung interaction also generates rung dimers and opens spin gap which increases rapidly with rung interaction strength. The correlation between spins decreases exponentially with the distance between them.
We use exact diagonalization and the modified Lanczos method to study the finite energy and finite momentum spectral weight of the longitudinal and transverse spin excitations of the anisotropic zig-zag ladder. We find that the spin excitations form continua of gapless or gapped spinons in the different regions of the phase diagram. The results obtained are consistent with a picture previously proposed that in the anisotropic case there is a transition from a gapped regime to a gapless regime, for small interchain coupling. In this regime we find a sharp low-energy peak in the structure functi…
We use high-resolution, tunable angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations to study the electronic properties of single crystals of MnBi2Te4, a material that was predicted to be the first intrinsic antiferromagnetic (AFM) topological insulator. We observe both bulk and surface bands in the electronic spectra, in reasonable agreement with the DFT calculations results. In striking contrast to the earlier literatures showing a full gap opening between two surface band manifolds along (0001) direction, we observed a gapless Dirac cone remain …
The random antiferromagnetic two-leg and zigzag spin-1/2 ladders are investigated using the real space renormalization group scheme and their complete phase diagrams are determined. We demonstrate that the first system belongs to the same universality class of the dimerized random spin-1/2 chain. The zigzag ladder, on the other hand, is in a random singlet phase at weak frustration and disorder. Otherwise, we give additional evidence that it belongs to the universality class of the random antiferromagnetic and ferromagnetic quantum spin chains, although the universal fixed point found in the l…
The effect of next-nearest-neighbor hopping $t_{2}$ on the ground-state phase diagram of the one-dimensional Kondo lattice is studied with density-matrix renormalization-group techniques and by comparing with the phase diagram of the classical-spin variant of the same model. For a finite $t_{2}$, i.e., for a zigzag-ladder geometry, indirect antiferromagnetic interactions between the localized spins are geometrically frustrated. We demonstrate that $t_{2}$ at the same time triggers several magnetic phases which are absent in the model with nearest-neighbor hopping only. For strong $J$, we find …
We numerically study the Heisenberg models on triangular lattices by extending it from the simplest equilateral lattice with only the nearest-neighbor exchange interaction. We show that, by including an additional weak next-nearest-neighbor interaction, a quantum spin-liquid phase is stabilized against the antiferromagnetic order. The spin gap (triplet excitation gap) and spin correlation at long distances decay algebraically with increasing system size at the critical point between the antiferromagnetic phase and the spin-liquid phase. This algebraic behavior continues in the spin-liquid phas…
The frustrated spin-1/2 chain with weakly anisotropic ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor exchanges is studied. We focus on the excitation spectrum and the low-temperature thermodynamics in the ferromagnetic region of the ground state phase diagram. It is shown that the excitation spectrum of the model is characterized by the existence of the multi-magnon bound states. These excitations determine the low-temperature magnetic susceptibility. The energy of the bound magnon complexes is found using the scaling estimates of the perturbation theory and numeric…
We have investigated polycrystalline samples of the zigzag chain system BaTb$_2$O$_4$ with a combination of magnetic susceptibility, heat capacity, neutron powder diffraction, and muon spin relaxation measurements. Despite the onset of Tb$^{3+}$ short-range antiferromagnetic correlations at $|θ_{CW}|$ $=$ 18.5 K and a very large effective moment, our combined measurements indicate that BaTb$_2$O$_4$ remains paramagnetic down to 0.095 K. The magnetic properties of this material show striking similarities to the pyrochlore antiferromagnet Tb$_2$Ti$_2$O$_7$, and therefore we propose that BaTb$_2$…
We report the revised crystal structure, static and dynamic magnetic properties of quasi-two dimensional honeycomb-lattice silver delafossite Ag3Co2SbO6. The magnetic susceptibility and specific heat data are consistent with the onset of antiferromagnetic long range order at low temperatures with Néel temperature TN ~ 21.2 K. In addition, the magnetization curves revealed a field-induced (spin-flop type) transition below TN in moderate magnetic fields. The GGA+U calculations show the importance of the orbital degrees of freedom, which maintain a hierarchy of exchange interaction in the system.…
We show strong numerical evidence in favor of an unexpected virtually gapless spectrum, with edge states localized at the boundaries, in frustrated spin-1/2 antiferromagnetic ladders with an odd number of legs. These features can be accurately reproduced by using a projected BCS wave function with a non-trivial pairing, that mixes even and odd reflection symmetries. This approach gives the correct classification of the excitations and provides a simple and very appealing picture of an unconventional spin-liquid phase stabilized by frustration.
NiPSe3 is regarded as a bandwidth-controlled Mott insulator, distinct from the widely studied Mott insulating magnetic graphene MPSe3 (M = Mn and Fe) family. By employing high-pressure synchrotron X-ray diffraction, we observe two structural transitions as a function of pressure. With the help of first-principles calculations, we discover the antiferromagnetic (AFM) moment directions of NiPSe3 switch from out-of-plane to in-plane and the honeycomb layers slide relative to each other at the first structural transition. The in-plane AFM order persists until the second structural transition, wher…
Exploring Low-Dimensional Magnetism in Cobalt Vanadates, ${A}$CoV$_{2}$O$_{7}$~(${A}$~=~Ca, Sr) : Crystal Growth and Magnetic Properties of Effective Spin-1/2 Zigzag Chains
Three-dimensional zigzag correlations in the van der Waals Kitaev magnet RuBr$_3$
Deutsches Elektronen-Synchrotron DESY（德国电子同步加速器）；Tohoku University（东北大学）；University of Tokyo（东京大学）；Masaryk University（马萨里克大学）；Max-Planck-Institut für Festkörperforschung（马克斯·普朗克固体研究所）
By means of quantum Monte Carlo (QMC) calculations from first principles, we study the ground-state properties of the narrowest zigzag graphene nanoribbon, with an infinite linear acene structure. We show that this quasi-one-dimensional system is correlated and its ground state is made of localized $π$ electrons whose spins are antiferromagnetically (AFM) ordered. The AFM stablization energy (36(3) meV per carbon atom) and the absolute magnetization (1.13(1) $μ_\textrm{B}$ per unit cell) predicted by QMC are sizable, and they suggest the survival of antiferromagnetic correlations above room te…
Na$_2$IrO$_3$ was one of the first materials proposed to feature the Kane-Mele type topological insulator phase. Contemporaneously it was claimed that the very same material is in a Mott insulating phase which is described by the Kitaev-Heisenberg (KH) model. First experiments indeed revealed Mott insulating behavior in conjunction with antiferromagnetic long-range order. Further refined experiments established antiferromagnetic order of zigzag type which is not captured by the KH model. Since then several extensions and modifications of the KH model were proposed in order to describe the expe…
As one of the most promising Kitaev quantum-spin-liquid (QSL) candidates, $α$-RuCl$_3$ has received a great amount of attention. However, its ground state exhibits a long-range zigzag magnetic order, which defies the QSL phase. Nevertheless, the magnetic order is fragile and can be completely suppressed by applying an external magnetic field. Here, we explore the evolution of magnetic excitations of $α$-RuCl$_3$ under an in-plane magnetic field, by carrying out inelastic neutron scattering measurements on high-quality single crystals. Under zero field, there exist spin-wave excitations near th…
We derive upper bounds for spin velocity in half-integer-spin Heisenberg antiferromagnetic chains. We relate these upper bounds to the instability of the gapless state, which is observed in frustrated systems.
Recent studies have revealed that the randomness-induced quantum spin liquid (QSL)-like state is stabilized in certain frustrated quantum magnets in two and three dimensions. In order to clarify the nature of this gapless QSL-like state, we investigate both zero- and finite-temperature properties of the random-bond one-dimensional (1D) $s=12$ Heisenberg model with the competing nearest-neighbor and next-nearest-neighbor antiferromagnetic interactions, $J_1$ and $J_2$, by means of the exact diagonalization, density-matrix renormalization-group and Hams--de Raedt methods. We find that, on increa…
We study the spectrum and the nature of the excitations of an antiferromagnetic (AFM) Heisenberg chain with staggered long range interactions, both numerically using the time-dependent density matrix renormalization group (tDMRG) method and by means of a multi-spinon approximation that qualitatively explains its main features. The unfrustrated long-range nature of the exchange effectively increases the dimensionality of the system and the chain is able to undergo true symmetry breaking and develop long range order, transitioning from a gapless spin liquid to a gapless ordered AFM phase. We cal…
We report an investigation on structure and magnetic properties of the $S=3/2$ zigzag spin chain compound BaCoTe$_2$O$_7$. Neutron diffraction measurements reveal BaCoTe$_2$O$_7$ crystallizes in the noncentrosymmetric space group $Ama2$ with a canted $\uparrow\uparrow\downarrow\downarrow$ spin structure along the quasi-one-dimensional zigzag chain and a moment size of $1.89(2)μ_B$ at 2 K. Magnetic susceptibility and specific heat measurements yield an antiferromagnetic phase transition at $T_N=6.2$ K. A negative Curie-Weiss temperature $Θ_{CW}=-74.7(2)$ K and an empirical frustration parameter…
We study a generalized quantum spin ladder with staggered long range interactions that decay as a power-law with exponent $α$. Using large scale quantum Monte Carlo (QMC) and the density matrix renormalization group (DMRG) simulations, we show that this model undergoes a transition from a rung-dimer phase characterized by a non-local string order parameter, to a symmetry broken Néel phase. We find evidence that the transition is second order.In the magnetically ordered phase, the spectrum exhibits gapless modes, while excitations in the gapped phase are well described in terms of triplons -- b…
A fundamental problem posed from the study of correlated electron compounds, of which heavy-fermion systems are prototypes, is the need to understand the physics of states near a quantum critical point (QCP). At a QCP, magnetic order is suppressed continuously to zero temperature and unconventional superconductivity often appears. Here, we report pressure ($P$) -dependent $^{115}$In nuclear quadrupole resonance (NQR) measurements on heavy-fermion antiferromagnet CeRh$_{0.5}$Ir$_{0.5}$In$_5$. These experiments reveal an antiferromagnetic (AF) QCP at $P_{\rm c}^{\rm AF}$ = 1.2 GPa where a dome o…
We study a generalized quantum spin ladder with staggered long range interactions that decay as a power-law with exponent $α$. Using large scale quantum Monte Carlo (QMC) and density matrix renormalization group (DMRG) simulations, we show that this model undergoes a transition from a rung-dimer phase characterized by a non-local string order parameter, to a symmetry broken Néel phase. We find evidence that the transition is second order. In the magnetically ordered phase, the spectrum exhibits gapless modes, while excitations in the gapped phase are well described in terms of triplons -- boun…
We study the nearest neighboring spin-1/2 Kitaev-Heisenberg-Gamma ($KJΓ$) model on the honeycomb lattice in the parameter region of ferromagnetic (FM) Kitaev and antiferromagnetic (AFM) Heisenberg couplings relevant for honeycomb iridates, using a coupled-chain analysis. Starting from the gapless Luttinger liquid phase of a decoupled $KJΓ$ chain, the inter-chain interactions in the two-dimensional model is treated within a self-consistent mean field approach based on the Luttinger liquid theory. In the FM Gamma region, our analysis recovers the reported 120$^\circ$ magnetic order, previously o…
