High-Field Quantum Disordered State in $α$-RuCl3: Spin Flips, Bound States, and a Multi-Particle Continuum
A. Sahasrabudhe, D. A. S. Kaib, S. Reschke, R. German, T. C. Koethe, J. Buhot, D. Kamenskyi, C. Hickey, P. Becker, V. Tsurkan, A. Loidl, S. H. Do, K. Y. Choi, M… 展开作者
A. Sahasrabudhe, D. A. S. Kaib, S. Reschke, R. German, T. C. Koethe, J. Buhot, D. Kamenskyi, C. Hickey, P. Becker, V. Tsurkan, A. Loidl, S. H. Do, K. Y. Choi, M. Grüninger, S. M. Winter, Zhe Wang, R. Valenti, P. H. M. van Loosdrecht
Layered $α$-RuCl3 has been discussed as a proximate Kitaev spin liquid compound. Raman and THz spectroscopy of magnetic excitations confirm that the low-temperature antiferromagnetic ordered phase features a broad Raman continuum, together with two magnon-like excitations at 2.7 and 3.6 meV, respectively. The continuum strength is maximized as long-range order is suppressed by an external magnetic field. The state above the field-induced quantum phase transition around 7.5 T is characterized by a gapped multi-particle continuum out of which a two-particle bound state emerges, together with a well-defined single-particle excitation at lower energy. Exact diagonalization calculations demonstrate that Kitaev and off-diagonal exchange terms in the Fleury-Loudon operator are crucial for the occurrence of these features in the Raman spectra. Our study firmly establishes the partially-polarized quantum disordered character of the high-field phase.
We systematically investigate the mode dispersion and spectral weight of the elementary excitation spectra in one-dimensional continuum and lattice electron systems by using the RPA, the Luttinger liquid model, and the Hubbard model. Both charge and spin excitations are studied in details and compared among the theoretical models. For the lattice Hubbard model we use both Bethe-ansatz equations and Lanczos-Gagliano method to calculate dispersion and spectral weight separately. We discuss the theoretically calculated elementary excitation spectra in terms of the experimental inelastic light (Ra…
We observe multiple step-like jumps in a Dy-Fe-Ga-based ferrimagnetic alloy in its magnetic hysteresis curve at 2 K. The observed jumps have a stochastic character with respect to their magnitude and the critical field of occurrence, and the jumps do not show any temporal effect. The jump size distribution follows a power law variation indicating the scale invariance nature of the jumps. We have invoked a simple two-dimensional random bond Ising-type spin system to model the dynamics. Our computational work can qualitatively reproduce the jumps and their scale invariant character. It also eluc…
We study a quantum phase transition of electrons on a two-dimensional square lattice. Our lattice model preserves the full $\mathrm{O}(4)$ symmetry of free spin-$\frac{1}{2}$ Dirac fermions on a bipartite lattice. In particular, it not only preserves the usual $\mathrm{SO}(4)$ (spin-charge) symmetry like in the half-filling Hubbard model, but also preserves a $\mathbb{Z}_2$ spin-charge flip symmetry. Using sign-problem-free Monte Carlo simulation, we find a second order quantum phase transition from a massless Dirac phase to a massive phase with spontaneously chosen spin order or charge order,…
We examine a novel type of disorder in quantum antiferromagnets. Our model consists of localized spins with antiferromagnetic exchanges on a bipartite quasiperiodic structure, which is geometrically disordered in such a way that no frustration is introduced. In the limit of zero disorder, the structure is the perfect Penrose rhombus tiling. This tiling is progressively disordered by augmenting the number of random "phason flips" or local tile-reshuffling operations. The ground state remains Néel ordered, and we have studied its properties as a function of increasing disorder using linear spin …
By numerical exact diagonalization techniques, we obtain the quantum phase diagram of the lattice fractional quantum Hall (FQH) systems in the presence of quenched disorder. By implementing an array of local potential traps representing the disorder, we show that the system undergoes a series of quantum phase transitions as the disorder and/or the interaction is tuned. As the strength of potential traps is increased, the FQH state turns into a compressible liquid, and then into a topologically trivial insulator. We use numerically calculated energy gap, quantum degeneracy, Chern number, entang…
Two-dimensional disordered quantum antiferromagnets are studied by means of a continuum description in which disorder is introduced by a random distribution of couplings (spin stiffnesses) in the ordered phase of the Nonlinear Sigma Model. Quenched soliton (skyrmion) correlation functions are evaluated and used, along with the quenched magnetization, to characterize the phase structure of the system. When magnetic dilution is exponentially suppressed, the introduction of disorder only modifies the subleading terms in the large distance behavior of the soliton correlation functions, yielding th…
A continuum of magnetic states has been observed by neutron scattering from the spin-1 chain compound CsNiCl3 in its disordered gapped one-dimensional phase. Results using both triple-axis and time-of-flight spectrometers show that around the antiferromagnetic point Qc=pi, the continuum lies higher in energy than the Haldane gapped excitations. At 6K the integrated intensity of the continuum is about 12(2)% of the total spectral weight. This result is considerably larger than the 1-3% weight predicted by the non-linear sigma model for the 3-particle continuum.
We report inelastic time-of-flight and triple-axis neutron scattering measurements of the excitation spectrum of the coupled antiferromagnetic spin-1 Heisenberg chain system CsNiCl3. Measurements over a wide range of wave-vector transfers along the chain confirm that above T_N CsNiCl3 is in a quantum-disordered phase with an energy gap in the excitation spectrum. The spin correlations fall off exponentially with increasing distance with a correlation length xi=4.0(2) sites at T=6.2K. This is shorter than the correlation length for an antiferromagnetic spin-1 Heisenberg chain at this temperatur…
Inelastic neutron scattering reveals a broad continuum of excitations in Pr$_2$Zr$_2$O$_7$, the temperature and magnetic field dependence of which indicate a continuous distribution of quenched transverse fields ($Δ$) acting on the non-Kramers Pr$^{3+}$ crystal field ground state doublets. Spin-ice correlations are apparent within 0.2 meV of the Zeeman energy. A random phase approximation provides an excellent account of the data with a transverse field distribution $ρ(Δ)\propto (Δ^2+Γ^2)^{-1}$ where $ Γ=0.27(1)$ meV. Established during high temperature synthesis due to an underlying structura…
We present a large-$S$ study of a quantum spin ice Hamiltonian, introduced by Huang, et al., [PRL 112, 167203 (2014)], on the kagome lattice. This model involves a competition between the frustrating Ising term of classical kagome ice, a Zeeman magnetic field $h$,and a nearest-neighbor transverse spin-flip term $S^x_iS^x_j - S^y_iS^y_j$. Recent Quantum Monte Carlo (QMC) simulations by Carrasquilla, et al., [Nature Communications, 6, 7421, (2015)], uncovered lobes of a disordered phase for large Ising interaction and $h\neq 0$ -- a putative quantum spin liquid phase. Here, we examine the nature…
Existing theoretical works differ on whether three-dimensional Dirac and Weyl semimetals are stable to a short-range-correlated random potential. Numerical evidence suggests the semimetal to be unstable, while some field-theoretic instanton calculations have found it to be stable. The differences go beyond method: the continuum field-theoretic works use a single, perfectly linear Weyl cone, while numerical works use tight-binding lattice models which inherently have band curvature and multiple Weyl cones. In this work, we bridge this gap by performing exact numerics on the same model used in a…
Motivated by the lack of direct evidence with inelastic neutron scattering of the well documented bound state of Heisenberg ferromagnets, we use the time-dependent Thermal Density Matrix Renormalization Group algorithm to study the temperature dependence of the dynamical spin structure factor of Heisenberg ferromagnetic spin chains. For spin-1/2, we show that the bound state appears as a well defined excitation with significant spectral weight in the temperature range $J/12 \lesssim T \lesssim J/3$, pointing to the possibility of detecting it with inelastic neutron scattering near $k=π$ provid…
Observation of spin-conserving two-spinon continuum in the $S$=1/2 antiferromagnetic chain system Sr$_2$CuO$_3$ using Cu $K$-edge resonant inelastic x-ray scattering
We report measurements of optical absorption in the zig-zag antiferromagnet $α$-RuCl$_3$ as a function of temperature, $T$, magnetic field, $B$, and photon energy, $\hbarω$ in the range $\sim$ 0.3 to 8.3 meV, using time-domain terahertz spectroscopy. Polarized measurements show that 3-fold rotational symmetry is broken in the honeycomb plane from 2 K to 300 K. We find a sharp absorption peak at 2.56 meV upon cooling below the Néel temperature of 7 K at $B=0$ that we identify as magnetic-dipole excitation of a zero-wavevector magnon, or antiferromagnetic resonance (AFMR). With application of $B…
Protected by the interplay of on-site Coulomb interactions and spin-orbit coupling, Sr$_2$IrO$_4$ at high pressure is a rare example of a Mott insulator with a paramagnetic ground state. Here, using optical Raman scattering, we measure both the phonon and magnon evolution in Sr$_2$IrO$_4$ under pressure, and identify three different magnetically-ordered phases, culminating in a spin-disordered state beyond 18 GPa. A strong first-order structural phase transition drives the magnetic evolution at $\sim$10 GPa with reduced structural anisotropy in the IrO$_6$ cages, leading to increasingly isotro…
Disorder in Weyl semimetals and superconductors is surprisingly subtle, attracting attention and competing theories in recent years. In this brief review, we discuss the current theoretical understanding of the effects of short-ranged, quenched disorder on the low energy-properties of three-dimensional, topological Weyl semimetals and superconductors. We focus on the role of non-perturbative rare region effects on destabilizing the semimetal phase and rounding the expected semimetal-to-diffusive metal transition into a cross over. Furthermore, the consequences of disorder on the resulting natu…
Quantum spin models with spatially dependent interactions, known as compass models, play an important role in the study of frustrated quantum magnetism. One example is the Kitaev model on the honeycomb lattice with spin-liquid ground states and anyonic excitations. Another example is the geometrically frustrated quantum $120^\circ$ model on the same lattice whose ground state has not been unambiguously established. To generalize the Kitaev model beyond the exactly solvable limit and connect it with other compass models, we propose a new model, dubbed "the tripod model", which contains a contin…
