Direct Observation of Magnon Fractionalization in the Quantum Spin Ladder
B. Thielemann, Ch. Rüegg, H. M. Rønnow, A. M. Läuchli, J. -S. Caux, B. Normand, D. Biner, K. W. Krämer, H. -U. Güdel, J. Stahn, K. Habicht, K. Kiefer, M. Boehm,… 展开作者
B. Thielemann, Ch. Rüegg, H. M. Rønnow, A. M. Läuchli, J. -S. Caux, B. Normand, D. Biner, K. W. Krämer, H. -U. Güdel, J. Stahn, K. Habicht, K. Kiefer, M. Boehm, D. F. McMorrow, J. Mesot
We measure by inelastic neutron scattering the spin excitation spectra as a function of applied magnetic field in the quantum spin-ladder material (C5H12N)2CuBr4. Discrete magnon modes at low fields in the quantum disordered phase and at high fields in the saturated phase contrast sharply with a spinon continuum at intermediate fields characteristic of the Luttinger-liquid phase. By tuning the magnetic field, we drive the fractionalization of magnons into spinons and, in this deconfined regime, observe both commensurate and incommensurate continua.
Fractionalization of an electronic quasiparticle into spin, charge and orbital parts is a fundamental and characteristic property of interacting electrons in one dimension. However, real materials are never strictly one-dimensional and the fractionalization phenomena are hard to observe. Here we studied the spin and orbital excitations of the anisotropic ladder material CaCu2O3, whose electronic structure is not one-dimensional. Combining high-resolution resonant inelastic x-ray scattering experiments with theoretical model calculations we show that: (i) spin-orbital fractionalization occurs i…
We employ a variational Monte Carlo approach to efficiently obtain the dynamical structure factor for the spin-1/2 $J_1-J_2$ Heisenberg model on the square lattice. Upon increasing the frustrating ratio $J_2/J_1$, the ground state undergoes a continuous transition from a Néel antiferromagnet to a $\mathbb{Z}_{2}$ gapless spin liquid. We identify the characteristic spectral features in both phases and highlight the existence of a broad continuum of excitations in the proximity of the spin-liquid phase. The magnon branch, which dominates the spectrum of the unfrustrated Heisenberg model, gradual…
Three magnon bound states in all total spin sectors of general nonintegrable exactly rung-dimerized spin ladder are obtained by Bethe Ansatz. Basing on this result a dispersion law for $m$-magnon ($m>3$) bound states is conjectured. It is suggested that (depending on correlations between coupling constants) a behavior of the gas parameter (density of rung-triplets) on the boundary of the rung-dimerized phase may either be smooth or have a leap.
We consider the two-chain Heisenberg ladder with antiferromagnetic interactions. Our approach is based on the description of spin excitations as triplets above a strong-coupling singlet ground state. It is shown that interaction with dynamic phonons drastically changes the dispersion of the elementary triplet excitation (magnon) and the spin structure factor. It is also shown that a new triplet excitation appears in the spectrum: the bound state of a magnon and a phonon.
An exactly solvable effective model is suggested for an asymmetric spin ladder with dimerized rungs. Magnon mode truncation originated from magnon decay (recently observed in the 1D compound ${\rm IPA-CuCl}_3$) is naturally described within this model. Using Bethe Ansatze we described a one-magnon sector and obtained relations between interaction constants of the model and experimentally observable quantities such as the gap and truncation energies, spin velocity and the truncation wave vector. It is also shown that structure factor turns to zero at the truncation point.
Inelastic neutron scattering is used to measure spin excitations in fully deuterated single crystal samples of the strong-leg antiferromagnetic S=1/2 spin ladder compound (C$_7$H$_{10}$N)$_2$CuBr$_4$. Sharp resolution-limited magnons are observed across the entire one-dimensional Brillouin zone. The results validate the previously proposed {\it symmetric} spin ladder model and provide a reliable estimate of the relevant exchange interactions.
One-magnon problem for spin-ladders with exact singlet-rung ground state in arbitrary oriented constant and staggered magnetic fields is studied. For two special configurations when the both fields are parallel or perpendicular to each other the exact formulas for eigenstates and eigenvectors are presented. In the parallel case the one-magnon contribution to the ESR absorption line shape is obtained. The latter is non-Lorentzian, has finite range and a gap around resonance. For low fields both the range of the spectrum and the width of the gap are controlled by the width of the one-magnon zone…
We investigate the resonant two-magnon Raman scattering in the two-dimensional (2D) and ladder-type Mott insulators by using a half-filled Hubbard model in the strong coupling limit. By performing numerical diagonalization calculations for small clusters, we find that the model can reproduce the experimental features in the 2D that the Raman intensity is enhanced when the incoming photon energy is not near the absorption edge but well above it. In the ladder-type Mott insulators, the Raman intensity is found to resonate with absorption spectrum in contrast to the 2D system. The difference betw…
We have used a combination of neutron resonant spin-echo and triple-axis spectroscopies to determine the energy and linewidth of the magnon resonance in IPA-Cu(Cl$_{0.95}$Br$_{0.05}$)$_3$, a model spin-1/2 ladder antiferromagnet where Br substitution induces bond randomness. We find that the bond defects induce a blueshift, $δΔ$, and broadening, $δΓ$, of the magnon gap excitation compared to the pure compound. At temperatures exceeding the energy scale of the inter-ladder exchange interactions, $δΔ$ and $δΓ$ are temperature independent within the experimental error, in agreement with Matthiess…
We have used a combination of neutron resonant spin-echo and triple-axis spectroscopies to determine the energy, fine structure, and linewidth of the magnon resonance in the model spin-1/2 ladder antiferromagnet IPA-CuCl_3 at temperatures T << Delta_0 /k_B, where Delta_0 is the spin gap at T=0. In this low-temperature regime we find that the results deviate substantially from the predictions of the non-linear sigma model proposed as a description of magnon excitations in one-dimensional quantum magnets and attribute these deviations to real-space and spin-space anisotropies in the spin Hamilto…
We suggest a family of spin-ladder models with exact singlet-rung vacuum. In addition to the terms describing interactions along legs and rungs the Hamiltonian has frustration and cyclic terms. Using coordinate Bethe ansatze all one- and two-dimensional states are constructed. The explicit formula for zero-temperature Raman scattering cross section is derived. The corresponding line-shapes are strongly asymmetric and have no more than one van-Hove singularity. The results are in good agreement with experimental data.
Phonon-assisted 2-magnon absorption is studied at T=4 K in the spin-1/2 two-leg ladders of Ca_14-x La_x Cu_24 O_41 (x=5 and 4) for polarization of the electrical field parallel to the legs and the rungs, respectively. Two peaks at about 2140 and 2800 1/cm reflect van-Hove singularities in the density of states of the strongly dispersing 2-magnon singlet bound state, and a broad peak at about 4000 1/cm is identified with the 2-magnon continuum. Two different theoretical approaches (Jordan-Wigner fermions and perturbation theory) describe the data very well for J_parallel = 1050 - 1100 1/cm and …
We study the excitation spectrum of the two-leg antiferromagnetic S=1/2 Heisenberg ladder. Our approach is based on the description of the excitations as triplets above a strong-coupling singlet ground state. The quasiparticle spectrum is calculated by treating the excitations as a dilute Bose gas with infinite on-site repulsion. We find singlet (S=0) and triplet (S=1) two-particle bound states of the elementary triplets. We argue that bound states generally exist in any dimerized quantum spin model.
Three-magnon problem for exactly rung-dimerized spin ladder is brought up separately at all total spin sectors. At first a special duality transformation of the $\rm Schr\ddot odinger$ equation is found within general outlook. Then the problem is treated within Coordinate Bethe Ansatze. A straightforward approach is developed to obtain pure scattering states. At values S=0 and S=3 of total spin the $\rm Schr\ddot odinger$ equation has the form inherent in the $XXZ$ chain. For $S=1,2$ solvability holds only in five previously found {\it completely integrable} cases. Nevertheless a partial S=1 B…
The magnon thermal conductivity $κ_{\mathrm{mag}}$ of the hole doped spin ladders in $\rm Sr_{14-x}Ca_xCu_{24}O_{41}$ has been investigated at low doping levels $x$. The analysis of $κ_{\mathrm{mag}}$ reveals a strong doping and temperature dependence of the magnon mean free path $l_{\mathrm{mag}}$ which is a local probe for the interaction of magnons with the doped holes in the ladders. In particular, this novel approach to studying charge degrees of freedom via spin excitations shows that charge ordering of the holes in the ladders leads to a freezing out of magnon-hole scattering processes.
We present a theory of phonon-assisted optical two-magnon absorption in two-leg spin-ladders. Based on the strong intra-rung-coupling limit we show that collective excitations of total spin S=0, 1 and 2 exist outside of the two-magnon continuum. It is demonstrated that the singlet collective state has a clear signature in the optical spectrum.
The magnetic excitations of the quasi-2D spin-1/2 Heisenberg antiferromagnet on an anisotropic triangular lattice Cs2CuCl4 are explored throughout the 2D Brillouin zone using inelastic neutron scattering. In the spin liquid phase above the transition to magnetic order extended excitation continua are observed, characteristic of fractionalization of S=1 spin waves into pairs of deconfined S=1/2 spinons and the hallmark of a resonating-valence-bond (RVB) state. The weak inter-layer couplings stabilize incommensurate spiral order at low temperatures and in this phase sharp magnons carrying a smal…
Motivated by the ever-increasing experimental effort devoted to the properties of frustrated quantum magnets in a magnetic field, we present a careful and detailed theoretical analysis of a one-dimensional version of this problem, a frustrated ladder with a magnetization plateau at m=1/2. We show that even for purely isotropic Heisenberg interactions, the magnetization curve exhibits a rather complex behavior that can be fully accounted for in terms of simple elementary excitations. The introduction of anisotropic interactions (e.g., Dzyaloshinskii-Moriya interactions) modifies significantly t…
The effect of disorder is studied on the field-induced quantum phase transition in the frustrated spin-ladder compound H8C4SO2Cu2(Cl[1-x]Brx)4 using bulk magnetic and thermodynamic measurements. The parent material (x=0) is a quantum spin liquid, which in applied fields is known to form a magnon condensate with long-range helimagnetic order. We show that bond randomness introduced by a chemical substitution on the non-magnetic halogene site destroys this phase transition at very low concentrations, already for x=0.01. The extreme fragility of the magnon condensate is attributed to random frust…
We study the exchange physics in high spin Mott insulating systems with $S=3/2$ which is realizable in ultracold atomic systems. The high symmetry of SO(5) or SU(4) therein renders stronger quantum fluctuations than the usual spin-1/2 systems. A spontaneous plaquette ground state without any site and bond spin orders is rigorously proved in a ladder spin-3/2 model, whose topological excitations exhibit fractionalization behavior. The generalization to the SU(N) plaquette state is also investigated.
We argue that the Raman intensity in a spin S two-leg spin-ladder has a pseudo-resonance peak, whose width is very small at large S. The pseudo-resonance originates from the existence of a local minimum in the magnon excitation spectrum, and is located slightly below twice the magnon energy at the minimum. The physics behind the peak is similar to the excitonic scenario for the neutron and Raman resonances in a d-wave superconductor.
We present the Raman-scattering spectra of CaV2O5 and MgV2O5. The magnetic contribution in the Raman spectra of CaV2O5 is found in the form of a strong asymmetric line, centered at 2Delta, with a tail on the high-energy side. Our analysis of its spectral shape shows that the magnetic ordering in CaV2O5 can be described using a S=1/2 two-leg ladder Heisenberg antiferromagnetic model with Jparallel/Jperp = 0.1, and a small interladder exchange. The spin gap and exchange constant are estimated to be Delta = 400 cm-1 (570 K) and Jperp = 640 K. No magnon bound states are found. In contrast to CaV2O…
The $S=1/2$ quantum spin ladder system with the anisotropic ferromagnetic exchange interaction on the rung under magnetic field is investigated using the numerical diagonalization and the density matrix renormalization group (DMRG) analyses. It is found that the nematic-spin-correlation-dominant Tomonaga-Luttinger liquid (TLL) appears in some high magnetic field. It is included in the TLL phase where the two-magnon bound state is realized. For some suitable parameters, after the field-induced phase transition from this two-magnon-bound TLL phase to the single-magnon TLL one, the re-entrant tra…
The weakly coupled quasi-one-dimensional spin ladder compound (CH$_3$)$_2$CHNH$_3$CuCl$_3$ is studied by neutron scattering in magnetic fields exceeding the critical field of Bose-Einstein condensation of magnons. Commensurate long-range order and the associated Goldstone mode are detected and found to be similar to those in a reference 3D quantum magnet. However, for the upper two massive magnon branches the observed behavior is totally different, culminating in a drastic collapse of excitation bandwidth beyond the transition point.
Using a combined analytical and numerical approach, we study the collective spin and orbital excitations in a spin-orbital chain under a crystal field. Irrespective of the crystal field strength, these excitations can be universally described by fractionalized fermions. The fractionalization phenomenon persists and contrasts strikingly with the case of a spin chain, where fractionalized spinons cannot be individually observed but confined to form magnons in a strong magnetic field. In the spin-orbital chain, each of the fractional quasiparticles carries both spin and orbital quantum numbers, a…
We study the spin-1/2 two-leg Heisenberg ladder with four-spin ring exchanges under a magnetic field. We introduce an exact duality transformation which is an extension of the spin-chirality duality developed previously and yields a new self-dual surface in the parameter space. We then determine the magnetic phase diagram using the numerical approaches of the density-matrix renormalization-group and exact diagonalization methods. We demonstrate the appearance of a magnetization plateau and the Tomonaga-Luttinger liquid with dominant vector-chirality quasi-long-range order for a wide parameter …
Transient localization from fractionalization: vanishingly small heat conductivity in gapless quantum magnets
Technical University of Munich（慕尼黑工业大学）；Munich Center for Quantum Science and Technology (MCQST)（慕尼黑量子科学与工程中心）；The Ohio State University（俄亥俄州立大学）；Imperial College London（帝国理工学院）
The ground state of the two-leg Heisenberg ladder is identified as an RVB type spin liquid, which is generated by Gutzwiller projection of tight-binding bands with flux pi per plaquet. Explicit trial wave functions for the magnon and hole excitations are formulated in terms of spinons and holons.
Kitaev quantum spin liquids host Majorana fermions via the fractionalization of spins. In a magnetic field, the Majorana fermions were predicted to comprise a topological state, which has attracted great attention by the discovery of the half-quantized thermal Hall conductivity. Nevertheless, a reliable theory remains elusive for the field effect, especially at finite temperature. Here we present unbiased large-scale numerical results for the Kitaev model in a wide range of magnetic field and temperature. We find that the unconventional paramagnetic region showing fractional spin dynamics exte…
We provide an exact study of dynamical correlations for the quantum spin-orbital liquid phases of an SU(2)-symmetric Kitaev honeycomb lattice model. We show that the spin dynamics in this Kugel-Khomskii type model is exactly the density-density correlation function of S=1 fermionic magnons, which could be probed in resonant inelastic x-ray scattering experiments. We predict the characteristic signatures of spin-orbital fractionalization in inelastic scattering experiments and compare them to the ones of the spin-anisotropic Kitaev honeycomb spin liquid. In particular, the resonant inelastic x-…
We study the Kitaev spin-$1/2$ ladder, a model which exhibits self-localization due to fractionalization caused by exchange frustration. When a weak magnetic field is applied, the model is described by an effective fermionic Hamiltonian, with an additional time reversal symmetry breaking term. We show that this term alone is not capable of delocalizing the system but flux mobility is a prerequisite. For magnetic fields larger but comparable to the flux gap, fluxes become mobile and drive the system into a delocalized regime, featuring finite dc transport coefficients. Our findings are based on…
The magnon thermal conductivity $κ_{\mathrm{mag}}$ of the spin ladders in $\rm Sr_{14}Cu_{24-x}Zn_xO_{41}$ has been investigated at low doping levels $x=0$, 0.125, 0.25, 0.5 and 0.75. The Zn-impurities generate nonmagnetic defects which define an upper limit for $l_{\mathrm{mag}}$ and therefore allow a clear-cut relation between $l_{\mathrm{mag}}$ and $κ_{\mathrm{mag}}$ to be established independently of any model. Over a large temperature range we observe a progressive suppression of $κ_{\mathrm{mag}}$ with increasing Zn-content and find in particular that with respect to pure $\rm Sr_{14}Cu_…
Conventionally ordered magnets possess bosonic elementary excitations, called magnons. By contrast, no magnetic insulators in more than one dimension are known whose excitations are not bosons but fermions. Theoretically, some quantum spin liquids (QSLs) -- new topological phases which can occur when quantum fluctuations preclude an ordered state -- are known to exhibit Majorana fermions as quasiparticles arising from fractionalization of spins. Alas, despite much searching, their experimental observation remains elusive. Here, we show that fermionic excitations are remarkably directly evident…
We characterize phases of the compass ladder model by using degenerate perturbation theory, symmetry fractionalization, and numerical techniques. Through degenerate perturbation theory we obtain an effective Hamiltonian for each phase of the model, and show that a cluster model and the Ising model encapsulate the nature of all phases. In particular, the cluster phase has a symmetry-protected topological order, protected by a specific $\mathbb{Z}_2\times \mathbb{Z}_2$ symmetry, and the Ising phase has a $\mathbb{Z}_2$-symmetry-breaking order characterized by a local order parameter expressed by…
We analyze the formation of multi-particle bound states in ladders with frustrated kinetic energy in two component bosonic and two component fermionic systems. We focus on the regime of light doping relative to insulating states at half-filling, spin polarization close to 100 percent, and strong repulsive interactions. A special feature of these systems is that the binding energy scales with single particle tunneling $t$ rather than exchange interactions, since effective attraction arises from alleviating kinetic frustration. For two component Fermi systems on a zigzag ladder we find a bound s…
The discovery of the almost ideal, two-chain spin-ladder material (C_5H_12N)_2CuBr_4 has once again focused attention on this most fundamental problem in low-dimensional quantum magnetism. Within the bond-operator framework, three qualitative advances are introduced which extend the theory to all finite temperatures and magnetic fields in the gapped regime. This systematic description permits quantitative and parameter-free experimental comparisons, which are presented for the specific heat, and predictions for thermal renormalization of the triplet magnon excitations.
A lithium dopant in a cuprate spin ladder acts as a vacant (non-magnetic) site accompanied by an extra hole bound by a Coulomb potential. We find that, although the undoped ladder spin gap is not essentially altered by Li doping, a dopant-magnon bound state appears within the gap. Contrary to the case of Zn-doped ladders, we predict that, in the Li-doped ladder, the spin liquid character is very robust against antiferromagnetism. We also predict the spatial dependence of the density of states in the vicinity of the dopant which could be measured by local spectroscopic probes.
We investigate the excitation spectrum of the triangular-lattice antiferromagnetic $XXZ$ model using series expansions and mean field Schwinger bosons approaches. The single-magnon spectrum computed with series expansions exhibits rotonic minima at the middle points of the edges of the Brillouin zone, for all values of the anisotropy parameter in the range $0\leq J^z/J\leq1$. Based on the good agreement with series expansions for the single-magnon spectrum, we compute the full dynamical magnetic structure factor within the mean field Schwinger boson approach to investigate the relevance of the…
The zero-field excitation spectrum of the strong-leg spin ladder (C$_7$H$_10$N)$_2$CuBr$_4$ (DIMPY) is studied with a neutron time-of-flight technique. The spectrum is decomposed into its symmetric and asymmetric parts with respect to the rung momentum and compared with theoretical results obtained by the density matrix renormalization group method. Additionally, the calculated dynamical correlations are shown for a wide range of rung and leg coupling ratios in order to point out the evolution of arising excitations, as e.g. of the two-magnon bound state from the strong to the weak coupling li…
We investigate the low-energy properties of N-leg integer-spin ladders and tubes with an antiferromagnetic (AF) intrachain coupling. In the odd-leg tubes, the AF rung coupling causes the frustration. To treat all ladders and tubes systematically, we apply Sénéchal's method [Phys. Rev. B 52, 15319 (1995)], based on the nonlinear sigma model. This strategy is valid in the weak interchain (rung) coupling regime. We show that all frustrated tubes possess six-fold degenerate spin-1 magnon bands, as the lowest excitations, while other ladders and tubes have a standard triply degenerate bands. We als…
We report on zero-field muon spin rotation, electron spin resonance and polarized Raman scattering measurements of the coupled quantum spin ladder Ba2CuTeO6. Zero-field muon spin rotation and electron spin resonance probes disclose a successive crossover from a paramagnetic through a spin-liquid-like into a magnetically ordered state with decreasing temperature. More significantly, the two-magnon Raman response obeys a T-linear scaling relation in its peak energy, linewidth and intensity. This critical scaling behavior presents an experimental signature of proximity to a quantum critical point…
A theoretical description is presented for low-temperature magnetic-field induced three-dimensional (3D) ordering transitions in strongly anisotropic quantum antiferromagnets, consisting of weakly coupled antiferromagnetic spin-1/2 chains and ladders. First, effective continuum field theories are derived for the one-dimensional subsystems. Then the Luttinger parameters, which determine the low-temperature susceptibilities of the chains and ladders, are calculated from the Bethe ansatz solution for these effective models. The 3D ordering transition line is obtained using a random phase approxim…
We identify fingerprints of a proximate quantum spin-liquid (QSL), observable by finite-temperature dynamical thermal transport within a minimal version of the idealized Kitaev model on a two-leg ladder, if subjected to inevitably present Heisenberg couplings. Using exact diagonalization and quantum typicality, we uncover (i) an insulator-conductor crossover induced by fracton recombination at infinitesimal Heisenberg coupling, (ii) low- and high-energy signatures of fractons, which survive far off the pure QSL point, and (iii) a non-monotonous current life-time versus Heisenberg couplings. Gu…
The temperature dependence of the specific heat and spin susceptibility of the spin ladders with cyclic four-spin interactions in the rung-singlet phase is explored by making use of the transfer-matrix renormalization group method. The values of spin gap are extracted from the specific heat and susceptibility, respectively. It is found that for different relative strength between interchain and intrachain interactions, the spin gap is approximately linear with the cyclic four-spin interaction in the region far away from the critical point. Furthermore, we show that the dispersion for the one-t…
The challenge of one-dimensional systems is to understand their physics beyond the level of known elementary excitations. By high-resolution neutron spectroscopy in a quantum spin ladder material, we probe the leading multiparticle excitation by characterizing the two-magnon bound state at zero field. By applying high magnetic fields, we create and select the singlet (longitudinal) and triplet (transverse) excitations of the fully spin-polarized ladder, which have not been observed previously and are close analogs of the modes anticipated in a polarized Haldane chain. Theoretical modelling of …
A variational approach for constructing an effective particle description of the low-energy physics of one-dimensional quantum spin chains is presented. Based on the matrix product state formalism, we compute the one- and two-particle excitations as eigenstates of the full microscopic Hamiltonian. We interpret the excitations as particles on a strongly-correlated background with non-trivial dispersion relations, spectral weights and two-particle S matrices. Based on this information, we show how to describe a finite density of excitations as an interacting gas of bosons, using its approximate …
Polarized inelastic neutron scattering experiments recently identified the amplitude (Higgs) mode in C$_9$H$_{18}$N$_2$CuBr$_4$, a two-dimensional near-quantum-critical spin-1/2 two-leg ladder compound, which exhibits a weak easy-axis exchange anisotropy. Here, we theoretically examine the dynamic spin structure factor of such planar coupled spin-ladder systems using large-scale quantum Monte Carlo simulations. This allows us to provide a quantitative account of the experimental neutron scattering data within a consistent quantum spin model. Moreover, we trance the details of the continuous ev…
We introduce a new Kondo ladder model by coupling the well-known Takhtajan-Babujan S=1 chain to a half-filled one-dimensional electron gas, and we solve it using quantum field theory techniques. Through the so-called underscreened Kondo effect, this gives novel insights into low-dimensional spin liquid systems. For instance, the optical conductivity reveals only a pseudogap at $k=π$ despite of the insulating state, a coherent magnon peak in the dynamical spin susceptibility coexists with an incoherent spinon background, the magnetization yields a fractional-like quantization. More generally, t…
Using micromechanical force magnetometry, we have measured the magnetization of the strong-leg spin-1/2 ladder compound (C$_7$H$_{10}$N)$_2$CuBr$_2$ at temperatures down to 45 mK. Low-temperature magnetic susceptibility as a function of field exhibits a maximum near the critical field H_c at which the magnon gap vanishes, as expected for a gapped one-dimensional antiferromagnet. Above H_c a clear minimum appears in the magnetization as a function of temperature as predicted by theory. In this field region, the susceptibility in conjunction with our specific heat data yields the Wilson ratio R_…
The spin dynamics of a doped 2-leg spin ladder is investigated by numerical techniques. We show that a hole pair-magnon boundstate evolves at finite hole doping into a sharp magnetic excitation below the two-particle continuum. This is supported by a field theory argument based on a SO(6)-symmetric ladder. Similarities and differences with the resonant mode of the high-T$_c$ cuprates are discussed.
Raman spectroscopy is used to study magnetic excitations in the quasi one dimensional $S=1/2$ quantum spin systems Cu(Qnx)(Cl$_{1-x}$Br$_x$)$_2$. The low energy spectrum is found to be dominated by a two-magnon continuum as expected from the numerical calculations for the Heisenberg spin ladder model. The continuum shifts to higher energies as more Br is introduced. The cutoff of the scattering increases faster than the onset indicating that the increase of exchange constant along the leg is the main effect on the magnetic properties. The upper and lower continuum thresholds are measured as a …
We study the roton-like dip in the magnon dispersion at the boundary of the Brillouin zone in the isotropic S=1/2 Heisenberg quantum antiferromagnet. This high-energy feature is sometimes seen as indication of a fractionalization of the magnons to spinons. In this article, we provide evidence that the description of the dip in terms of magnons can be improved significantly by applying more advanced evaluation schemes. In particular, we illustrate the usefulness of the application of the principle of minimal sensitivity in varied perturbation theory. Thereby, we provide an example for the appli…
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…
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 introduce an entanglement entropy analysis to quantitatively identify the confinement and deconfinement of the spinons in the spin excitations of quantum magnets. Our proposal is implemented by the parton construction of a honeycomb-lattice antiferromagnet exhibiting high-energy anomalous spectra. To obtain the quasiparticles of spin excitations for entanglement entropy calculations, we develop an effective Hamiltonian using the random phase approximation. We elaborate quantitatively the deconfinement-to-confinement transition of spinons in the anomalous spectra with the increase of the Hub…
