Using the Bethe-ansatz density-functional theory, we study a one-dimensional Hubbard model of confined attractively interacting fermions in the presence of a uniformly distributed disorder. The strongly-correlated Luther-Emery nature of the attractive one-dimensional Hubbard model is fully taken into account as the reference system in the density-functional theory. The effects of the disorder are investigated on the atomic-density waves in the weak-to-intermediate attractive interaction and on the spin-singlet dimers of doubly occupied sites in the strongly attractive regime. It is found that atomic density waves are sensitive to the disorder and the spin-singlet dimers of doubly occupied sites are quite unstable against the disorder. We also show that very weak disorder could smear the singularities in the stiffness, thus suppress the spin-singlet pairs.
We show that in transition metal compounds containing structural metal dimers there may exist in the presence of different orbitals a special state with partial formation of singlets by electrons on one orbital, while others are effectively decoupled and may give e.g. long-range magnetic order or stay paramagnetic. Similar situation can be realized in dimers spontaneously formed at structural phase transitions, which can be called orbital-selective Peierls transition. This can occur in case of strongly nonuniform hopping integrals for different orbitals and small intra-atomic Hund's rule coupl…
We report results of nuclear magnetic resonance (NMR) experiments in SrCu2(BO3)2, a quasi two-dimensional spin system with a singlet ground state. When magnetic field is applied along the c-axis, each of the quadrupole split Cu resonance lines splits further into four lines. The spin-echo intensity for some of the split lines oscillates against the separation time between the pai/2 and the pai rf-pulses. These phenomena are due to strong nuclear spin-spin coupling mediated by the electronic spin system, which exists only within a pair of nuclei. Thus the results provides direct evidence for th…
We present single crystal inelastic neutron scattering measurements of the S=1 dimerized quasi-two-dimensional antiferromagnet Ba$_3$Mn$_2$O$_8$. The singlet-triplet dispersion reveals nearest-neighbor and next-nearest-neighbor ferromagnetic interactions between adjacent bilayers that compete against each other. Although the inter-bilayer exchange is comparable to the intra-bilayer exchange, this additional frustration reduces the effective coupling along the c-axis and leads to a quasi-two dimensional behavior. In addition, the obtained exchange values are able to reproduce the four critical …
The spinel, MgTi$_2$O$_4$, undergoes a transition into a dimerized state at low temperatures that is expected to be a spin singlet. However, no signature of a singlet-triplet transition has been discovered, in part due to the difficulty of predicting the energy of the transition from theory. We find that the dimers of MgTi$_2$O$_4$ can be described by a Heisenberg model with very small interactions between different dimers. Using high-accuracy first-principles quantum Monte Carlo combined with a novel model-fitting approach, we predict the singlet-triplet gap of these dimers to be 350(50) meV,…
We present a Raman light scattering study of the two-dimensional quantum spin system SrCu2(BO3)2 and show that the magnetic excitation spectrum has a rich structure, including several well-defined bound state modes at low temperature, and a scattering continuum and quasielastic light scattering contributions at high temperature. The key to the understanding of the unique features of SrCu2(BO3)2 is the presence of strong interactions between well-localized triplet excitations in the network of orthogonal spin dimers realized in this compound. Based on our analysis of the Heisenberg model releva…
We use quantum Monte Carlo simulations to study effects of disorder on the quantum phase transition occurring versus the ratio g=J/J' in square-lattice dimerized S=1/2 Heisenberg antiferromagnets with intra- and inter-dimer couplings J and J'. The dimers are either randomly distributed (as in the classical dimer model), or come in parallel pairs with horizontal or vertical orientation. In both cases the transition violates the Harris criterion, according to which the correlation-length exponent should satisfy nu >= 1. We do not detect any deviations from the three-dimensional O(3) universality…
Using x-ray absorption spectroscopy at the Ru-$L_{2,3}$ edge we reveal that the Ru$^{4+}$ ions remain in the $S$=1 spin state across the rare 4d-orbital ordering transition and spin-gap formation. We find using local spin density approximation + Hubbard U (LSDA+U) band structure calculations that the crystal fields in the low temperature phase are not strong enough to stabilize the $S$=0 state. Instead, we identify a distinct orbital ordering with a significant anisotropy of the antiferromagnetic exchange couplings. We conclude that La$_{4}$Ru$_{2}$O$_{10}$ appears to be a novel material in wh…
We analyze the stability of the spontaneously dimerized spin-liquid phase of the frustrated Heisenberg antiferromagnet - the J1-J2 model. The lowest triplet excitation, corresponding to breaking of a singlet bond, is found to be stable in the region 0.38 < J2/J1 < 0.62. In addition we find a stable low-energy collective singlet mode, which is closely related to the spontaneous violation of the discrete symmetry. Both modes are gapped in the quantum disordered phase and become gapless at the transition point to the Neel ordered phase (J2/J1=0.38). The spontaneous dimerization vanishes at the tr…
We report the dependence of magnetic properties on the crystalline disorder in Li$_{2}$RuO$_3$ with Ru honeycomb lattice. This oxide exhibits unconventional Ru-dimer transition below $T_\text{d} \sim$ 540 K. We demonstrate that the cell parameters, related to the coherence of the dimer formation, are strongly dependent on the synthesis procedure. We show that the magnetic behavior at the dimer transition is closely related to the lattice parameters. In particular, we revealed that samples with well-ordered dimers exhibit a first-order magnetic transition with the onset exceeding 550~K, higher …
Motivated by the magnetic properties of the spin-chain compounds LiCuSbO$_4$$\equiv$LiSbCuO$_4$ and Rb$_2$Cu$_2$Mo$_3$O$_{12}$, we study the ground state of the Heisenberg chain with dimerized nearest-neighbor ferromagnetic (FM)($J_1, J_1^\prime<0$) and next-nearest-neighbor antiferromagnetic ($J_2>0$) couplings. Using the density-matrix renormalization group technique and spin-wave theory we find a first-order transition between a fully-polarized FM and an incommensurate spiral state at $2α=β/(1+β)$, where $α$ is the frustration ratio $J_2/|J_1|$ and $β$ the degree of dimerization $J_1^\prime…
We investigate the influence of quenched disorder on the dynamic structure factor of Heisenberg bilayers on the square, triangular, and kagome lattice in the quantum paramagnetic phase. Perturbative continuous unitary transformations and white graphs are employed to calculate the one-triplon contribution up to high orders in perturbation about the dimer limit for bimodal and continuous disorder. For the square lattice we find that the lifetime of the gap mode is increased by stronger quantum correlations while stronger disorder effects are observed for the triangular lattice due to geometric f…
Low-temperature magnetization (M) measurements down to 0.1 K have been performed in magnetic fields up to 14.5 T for a single piece of a tiny single-crystalline sample (0.2 mg weight) of the spin-gap system YbAl3C3. At the base temperature of 0.1 K, several metamagnetic transitions were clearly observed for H // c in the range 6 T < H < 9 T whereas only two transitions were observed, one at 4.8 T and the other at 6.6 T, for H // a. At fields above 9 T, the magnetization becomes almost saturated for both H // a and H // c. The present results indicate that a singlet-triplet crossover occurs in …
We analyze the spectrum of the 2D S=1/2 frustrated Heisenberg model near the transition from the spontaneously dimerized spin-liquid phase into the Neel ordered phase. Two excitation branches: the triplet magnon, and the collective singlet mode, both become gapless at the transition point. However we find that the length scales associated with these modes are well separated at the quantum transition. While in the quantum disordered phase the singlet excitation has finite spectral weight and reflects the existence of spontaneous dimer order, near the transition point the size of the singlet bou…
We demonstrate that any SU(2) singlet wavefunction can be characterized by a set of Valence Bond occupation numbers, testing dimer presence/vacancy on pairs of sites. This genuine quantum property of singlet states (i) shows that SU(2) singlets share some of the intuitive features of hardcore quantum dimers, (ii) gives rigorous basis for interesting albeit apparently ill-defined quantities introduced recently in the context of Quantum Magnetism or Quantum Information to measure respectively spin correlations and bipartite entanglement and, (iii) suggests a scheme to define consistently a wide …
Recent results on the nature of the quantum critical point between Neel and valence bond solid(VBS) ordered phases of two dimensional quantum magnets are examined by an attack from the VBS side. This approach leads to an appealingly simple physical description, and further insight into the properties of the transition.
We study the low-lying spectrum of the bilinear-biquadratic Heisenberg model in the dimerized and Haldane phases using a tensor renormalization method. At the (Takhtajan-Babujian) critical point the finite size spectrum predicted by the Wess-Zumino-Witten (WZW) model can only partly be confirmed. We find a singlet-singlet gap which does not fit into the WZW systematics. The results obtained are compared to Bethe Ansatz, exact diagonalization, and density matrix renormalization group (DMRG) calculations for specific parameters.
In the spin ladder compound BiCu$_2$PO$_6$ there exists a decisive dynamics of spin excitations that we classify and characterize using inelastic light scattering. We observe low-energy singlets and a broad triplon continuum extending from 36 cm$^{-1}$ to 700 cm$^{-1}$ in ($aa$), ($bb$), and ($cc$) light scattering polarizations. Though isolated spin ladder physics can roughly account for the observed excitations at high energies, frustration and interladder interactions need to be considered to fully describe the spectral distribution and scattering selection rules at low and intermediate ene…
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…
Short-range resonating-valence bond states in an orbitally degenerate magnet on a honeycomb lattice is studied. A quantum-dimer model is derived from the Hamiltonian which represents the superexchange interaction and the dynamical Jahn-Teller (JT) effect. We introduce two local units termed "spin-orbital singlet dimer", where two spins in a nearest-neighbor bond form a singlet state associated with an orbital polarization along the bond, and "local JT singlet", where an orbital polarization is quenched due to the dynamical JT effect. A derived quantum-dimer model consists of the hopping of the…
{Extensive calculations in the short-range RVB (Resonating valence bond) subspace on both the trimerized and the regular (non-trimerized) Heisenberg model on the kagome lattice show that short-range dimer singlets capture the specific low-energy features of both models. In the trimerized case the singlet spectrum splits into bands in which the average number of dimers lying on one type of bonds is fixed. These results are in good agreement with the mean field solution of an effective model recently introduced. For the regular model one gets a continuous, gapless spectrum, in qualitative agreem…
We use a strong-disorder renormalization group (SDRG) method and ground-state quantum Monte Carlo (QMC) simulations to study S=1/2 spin chains with random couplings, calculating disorder-averaged spin and dimer correlations. The QMC simulations demonstrate logarithmic corrections to the power-law decaying correlations obtained with the SDRG scheme. The same asymptotic forms apply both for systems with standard Heisenberg exchange and for certain multi-spin couplings leading to spontaneous dimerization in the clean system. We show that the logarithmic corrections arise in the valence-bond (sing…
Between Mott and cluster Mott: spin-orbit entangled dimer singlets in Ba$_3$CeRu$_2$O$_9$
University of Cologne（科隆大学）；Stockholm University（斯德哥尔摩大学）；Deutsches Elektronen-Synchrotron DESY（德国电子同步辐射中心）；IFW Dresden（德累斯顿联邦物理研究所）；Technische Universität Dresden（德累斯顿工业大学）
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…
Dynamical dimer structure factor of the triangular $S=1/2$ Heisenberg antiferromagnet
Technische Universität München（慕尼黑工业大学）；Université Libre de Bruxelles（布鲁塞尔自由大学）；Max-Planck-Institut für Physik komplexer Systeme（马克斯·普朗克复杂系统物理研究所）；Munich Center for Quantum Science and Technology (MCQST)（慕尼黑量子科学与技术中心）
Magnetic susceptibility and specific heat versus temperature measurements on polycrystalline samples of Sr3Cr2O8 and the isostructural, nonmagnetic compound Sr3V2O8 are reported. A Curie-Wiess fit to the high-temperature susceptibility data for Sr3Cr2O8 indicates that the Cr atoms are in the rare Cr^5+ (Spin S = 1/2) valence state as expected from the composition. The ground state was found to be a spin singlet with an excitation gap Delta/k_ B = 61.9(1) K to the magnetic triplet states, and a weak interdimer coupling of 6(2) K was inferred. The specific heat and susceptibility measurements on…
We provide a route to generate nematic order in a spin-1/2 system. Unlike the well-known magnon-binding mechanism, our spin nematics requires neigher the frustration effect nor a spin polarization in a high field or in the vicinity of a ferromagnet, but instead appears next to the spin singlet phase. We start from a state consisting of a quantum spin-1/2 singlet dimer placed on each site of a triangular lattice, and show that inter-dimer ring exchange interactions efficiently dope the SU(2) triplets that itinerate and interact, easily driving a stable singlet state to either Bose Einstein cond…
Covalently linked acene dimers are of interest as candidates for intramolecular singlet fission. We report many-electron calculations of the energies and wavefunctions of the optical singlets, the lowest triplet exciton and the triplet-triplet biexciton, as well as the final states of excited state absorptions from these states in a family of phenyl-linked pentacene dimers. While it is difficult to distinguish between the triplet and the triplet-triplet from their transient absorptions in the 500-600 nm region, by comparing theoretical transient absorption spectra against published and unpubli…
Spin excitations are explored in the electron-doped spin-orbit Mott insulator (Sr$_{1-x}$La$_{x}$)$_3$Ir$_2$O$_7$. As this bilayer square lattice system is doped into the metallic regime, long-range antiferromagnetism vanishes, yet a spectrum of gapped spin excitation remains. Excitation lifetimes are strongly damped with increasing carrier concentration, and the energy integrated spectral weight becomes nearly momentum independent as static spin order is suppressed. Local magnetic moments, absent in the parent system, grow in metallic samples and approach values consistent with one $J=\frac{1…
We report quadruple configuration interaction calculations within the extended Pariser$-$Parr$-$Pople Hamiltonian on the excited states of aggregates of polyenes, crystalline acenes, and covalently linked dimers of acene molecules. We determine the precise energy orderings and analyze the cluster wave functions in order to arrive at a comprehensive physical understanding of singlet fission in these diverse families of materials. Our computational approach allows us to retain a very large number of basis states and thereby obtain the correct relative energy orderings of one electron$-$one hole …
We investigate the ground-state property and the excitation gap of a one-dimensional spin-orbital model with isotropic spin and anisotropic orbital exchange interactions, which represents the strong-coupling limit of a two-orbital Hubbard model including the Hund's rule coupling ($J$) at quarter filling, by using a density-matrix renormalization group method. At J=0, spin and orbital correlations coincide with each other with a peak at $q=π/2$, corresponding to the SU(4) singlet state. On the other hand, spin and orbital states change in a different way due to the Hund's rule coupling. With in…
This article explore the behavior of singlet pairs in Inorganic crystals and take the Ti-Ti and Bi-Bi dimer for example, including the transition in critical temperture by directly or indirectly. Through the analysis, It is proposed that with the decrease of temperature, the strength of spin-orbit coupling (SOC) increases and the phase difference also increases. In the one-dimensional spin 1/2 chain system of TiOCl and BiOCl crystals, the possibility of reversible parameter modulation is proposed by the calculation of the first principle analyzing the process and analysis of the structural pha…
Heat capacity and susceptibility measurements have been performed on the diluted spin dimer compound Ba3(Mn1-xVx)2O8. The parent compound Ba3Mn2O8 is a spin dimer system based on pairs of antiferromagnetically coupled S = 1, 3d2 Mn5+ ions such that the zero field groundstate is a product of singlets. Substitution of non-magnetic S = 0, 3d0 V5+ ions leads to an interacting network of unpaired Mn moments, the low temperature properties of which are explored in the limit of small concentrations, 0<x<0.05. The zero-field heat capacity of this diluted system reveals a progressive removal of magneti…
An effective spin-orbit Hamiltonian is derived for a spin-1/2 trimerized kagome antiferromagnet in the second-order of perturbation theory in the ratio of two coupling constants. Low-energy singlet states of the obtained model are mapped to a quantum dimer model on a triangular lattice. The quantum dimer model is dominated by dimer resonances on a few shortest loops of the triangular lattice. Characteristic energy scale for the dimer model constitutes only a small fraction of the weaker exchange coupling constant.
We determined the crystal structure of Ba$_2$CoSi$_2$O$_6$Cl$_2$, which was synthesized in this work, and investigated its quantum magnetic properties using single crystals. This compound should be described as a two-dimensionally coupled spin-1/2 XY-like spin dimer system. Ba$_2$CoSi$_2$O$_6$Cl$_2$ exhibits a stepwise magnetization process with a plateau at half of the saturation magnetization, irrespective of the field direction, although all the Co$^{2+}$ sites are equivalent. This indicates that spin triplets are localized owing to the almost perfect frustration of interdimer exchange inte…
The singlet dynamics which plays a major role in the physics of the spin-1/2 Quantum Heisenberg Antiferromagnet (QHAF) on the Kagome lattice can be approximately described by projecting onto the nearest-neighbor valence bond (NNVB) singlet subspace. We re-visit here the effective Quantum Dimer Model which originates from the latter NNVB-projected Heisenberg model via a non-perturbative Rokhsar-Kivelson-like scheme. By using Lanczos exact diagonalisation on a 108-site cluster supplemented by a careful symmetry analysis, it is shown that a previously-found 36-site Valence Bond Crystal (VBC) in f…
Magnetic excitations in the isostructural spin-dimer systems Sr3Cr2O8 and Ba3Cr2O8 are probed by means of high-field electron spin resonance at sub-terahertz frequencies. Three types of magnetic modes were observed. One mode is gapless and corresponds to transitions within excited states, while two other sets of modes are gapped and correspond to transitions from the ground to the first excited states. The selection rules of the gapped modes are analyzed in terms of a dynamical Dzyaloshinskii-Moriya interaction, suggesting the presence of phonon-assisted effects in the low-temperature spin dyn…
When spin-orbit-entangled $d$-electrons reside on a honeycomb lattice, rich quantum states are anticipated to emerge, as exemplified by the $d^5$ Kitaev materials. Distinct yet equally intriguing physics may be realized with a $d$-electron count other than $d^5$. We found that the layered ruthenate Ag$_3$LiRu$_2$O$_6$ with $d^4$ Ru$^{4+}$ ions at ambient pressure forms a honeycomb lattice of spin-orbit-entangled singlets, which is a playground for frustrated excitonic magnetism. Under pressure, the singlet state does not develop the expected excitonic magnetism but experiences two successive t…
We study ground state properties in the bilayer Kitaev model by means of the dimer expansion. The existence of parity symmetries in the system reduces the computational cost significantly. This allows us to expand the ground state energy and interlayer spin-spin correlation up to 30th order in the interdimer Kitaev coupling. The numerical calculations clarify that the dimer singlet state is indeed realized in the wide parameter region.
The spin half Heisenberg antiferromagnet on the Kagome lattice, is mapped by Contractor Renormalization to a Spin-Pseudospin Hamiltonian on the triangular superlattice. Variationally, we find a ground state with columnar dimer order. Dimer orientation fluctuations are described by an effective O(2) model at energies above an exponentially suppressed clock mass scale. Our results explain the large density of low energy singlets observed numerically, and the non magnetic T-squared specific heat observed experimentally.
We report a structural/magnetic investigation by X-ray absorption spectroscopy (XAS), neutron diffraction, dc-susceptibility ($χ_{\mbox{dc}}$) and electron spin resonance (ESR) of the 12R-type perovskite BaTi$_{1/2}$Mn$_{1/2}$O$_{3}$. Our structural analysis by neutron diffraction supports the existence of structural trimers with chemically disordered occupancy of Mn$^{4+}$ and Ti$^{4+}$ ions, with the valence of the Mn ions confirmed by the XAS measurements. The magnetic properties are explored by combining dc-susceptibility and $X$-band ($9.4$ GHz) electron spin resonance, both in the temper…
We present a condition in which the dimer-monomer state is exactly the ground state of an extended diamond chain with spin magnitude 1/2. The Hamiltonian of the extended diamond chain includes next-nearest-neighbor exchange interactions and distortions, where the spin magnitude of the spin pair on a singlet dimer is not generally conserved. The method of deriving the condition is based on representing the Hamiltonian in a complete square form. The dimer-monomer ground state is found even if the Hamiltonian has no space-reflection symmetries.
Ground-state properties of the spin-1 two-leg antiferromagnetic ladder are investigated precisely by means of the quantum Monte Carlo method. It is found that the correlation length along the chains and the spin gap both remain finite regardless of the strength of interchain coupling, i.e., the Haldane state and the spin-1 dimer state are connected smoothly without any quantum phase transitions between them. We propose a plaquette-singlet solid state, which qualitatively describes the ground state of the spin-1 ladder quite well, and also a corresponding topological hidden order parameter. It …
We consider insulating phases of cold spin-1 bosonic particles with antiferromagnetic interactions, such as Na-23, in optical lattices. We show that spin-exchange interactions give rise to several distinct phases, which differ in their spin correlations. In two- and three- dimensional lattices insulating phases with odd number of particles per site are always nematic. For insulating states with even number of particles per site there is always a spin-singlet phase and there may also be a first-order transition into the nematic phase. The nematic phase breaks spin rotational symmetry, but prese…
The nature and extent of the spin-entanglement in the triplet-triplet biexciton with total spin zero in correlated-electron $π$-conjugated systems continues to be an enigma. Differences in the ultrafast transient absorption spectra of free triplets versus the triplet-triplet can give a measure of the entanglement. This, however, requires theoretical understandings of transient absorptions from the optical spin-singlet, the lowest spin-triplet exciton as well as from the triplet-triplet state, whose spectra are often overlapping and hence difficult to distinguish. We present a many-electron the…
The celebrated Shastry Sutherland model has a gapped dimer singlet ground state. The material SrCu$_2$(BO$_3$)$_2$ serves as a good realization of this model, upto small anisotropies arising from Dzyaloshinskii-Moriya (DM) interactions. The DM interactions admix a triplet component into the singlet ground state and give rise to weakly dispersing triplon bands. We show that an applied magnetic field splits the triplon modes and opens band gaps. Surprisingly, we are left with topological bands with Chern numbers $\pm 2$. SrCu$_2$(BO$_3$)$_2$ thus supports topologically protected triplonic edge m…
We construct and analyze a two-dimensional frustrated quantum spin model with plaquette order, in which the low-energy dynamics is controlled by spin singlets. At a critical value of frustration the singlet spectrum becomes gapless, indicating a quantum transition to a phase with dimer order. This T=0 transition belongs to the 3D Ising universality class, while at finite temperature a 2D Ising critical line separates the plaquette and dimerized phases. The magnetic susceptibility has an activated form throughout the phase diagram, whereas the specific heat exhibits a rich structure and a power…
Low-lying excited states for indeno[1,2-b]fluorene homo dimers with or without benzene spacers are calculated using the Density Matrix Renormalization group (DMRG) approach within Pariser-Parr-Pople (PPP) model Hamiltonian. DMRG calculations suggest that all the dimers studied here satisfy the essential energy conditions for SF. SF is a multiexciton generation process. As it is spin allowed, the process is very fast. By generating multiple exciton at a time SF underestimate SQ limit to enhance photo-conversion efficiency of single junction solar cells. Frontier orbital calculation through Dens…
Magnetic bound states are a general phenomenon in low dimensional antiferromagnets with gapped singlet states. Using Raman scattering on three compounds as dedicated examples we show how exchange topology, dimensionality, defects and thermal fluctuations influence the properties and the spectral weight of these states.
Triangular lattice quasi-two-dimensional Mott insulators based on BEDT-TTF molecule and its analogies present a possibility to produce exotic phases by coupling charge and spin degrees of freedom. In this work we discuss magnetic properties of one of such materials, $κ$-(BEDT-TTF)$_2$Hg(SCN)$_2$Cl, which is found at the border of the phase transition between a Mott insulator into a charge ordered state. Our magnetic susceptibility and cantilever magnetisation measurements demonstrate how the charge degree of freedom defines magnetic properties for few different charge phases observed in this m…
We study a model of strongly correlated S=1/2 fermions on the planar pyrochlore, or checkerboard, lattice, at fractional (1/8) filling. Starting with the extended Hubbard model in the limit of strong Coulomb repulsion, low-energy configurations can be mapped onto hard-core dimer configurations whose dimers carry a spin degree of freedom. An effective Hamiltonian similar to the kinetic term of a quantum dimer model on the square lattice which rotates two parallel dimers (in a spin-singlet configuration) by 90 degrees naturally emerges. We also introduce an additional term in the Hamiltonian, a …
The nature of the randomness-induced quantum spin liquid state, the random-singlet state, is investigated in two dimensions (2D) by means of the exact-diagonalization and the Hams-de Raedt methods for several frustrated lattices, e.g., the triangular, the kagome and the J_1-J_2 square lattices. Properties of the ground state, the low-energy excitations and the finite-temperature thermodynamic quantities are investigated. The ground state and the low-lying excited states consist of nearly isolated singlet-dimers, clusters of resonating singlet-dimers, and orphan spins. Low-energy excitations ar…
We study the ground state of three-leg $S=1/2$ Heisenberg tube using the density-matrix renormalization group method. The dimerization order-parameter and spin-excitation gap are calculated in a wide range of leg exchange interactions. We confirm that a gapped state with the dimerization order is realized even when the leg exchange interactions are ferromagnetic. Furthermore, the topological configuration of spin-singlet pairs in the ordered state is determined from the results of the Berry phase of each spin coupling as well as the structure factor of singlet-singlet correlation functions. We…
The dynamics of a doped hole in the orthogonal-dimer spin system is investigated systematically in one, two and three dimensions. By combining the bond-operator method with the self-consistent Born approximation, we argue that a dispersive quasi-particle state in the dimer phase is well defined even for quasi-two-dimensional systems. On the other hand, a doped hole in the plaquette-singlet phase hardly itinerates, forming an almost localized mode. We further clarify that although the quasi-particle weight in the dimer phase is decreased in the presence of the interchain coupling, it is not sup…
Recently it was shown that by means of an STM it is experimentally possible to stimulate clock transitions between the singlet and the non-magnetic triplet state of a Heisenberg-coupled spin dimer (Bae et al., Science Advances 4, eaau4159). This leads to more strongly protected clock transitions while ordinary ones only provide first-order protection against magnetic noise. However, large decoherence times of clock like states normally refer to ensembles of spins which do not dephase. In the cited experiment only one single dimer is manipulated and not an ensemble. For this reason we simulate …
