arXiv subjects

Bruce Normand

Publications and source records attributed to Bruce Normand.

27 records · Page 2Linked to original sources

arXiv2017

The amplitude mode in three-dimensional dimerized antiferromagnets

The amplitude (Higgs) mode is a ubiquitous collective excitation related to spontaneous breaking of a continuous symmetry. We combine quantum Monte Carlo (QMC) simulations with stochastic analytic continuation to investigate the dynamics of the amplitude mode in a three-dimensional dimerized quantum spin system. We characterize this mode by calculating the spin and dimer spectral functions near the quantum critical point, finding that both the energies and the intrinsic widths satisfy field-theoretical scaling predictions. While the line width of the spin response is close to that observed in neutron scattering experiments on TlCuCl$_3$, the dimer response is significantly broader. Our results demonstrate that highly non-trivial dynamical properties are accessible by modern QMC and analytic continuation methods.

cond-mat.str-el↗

arXiv2016

Self-consistent spin-wave analysis of the 1/3 magnetization plateau in the kagome antiferromagnet

We propose a modified spin-wave theory to study the 1/3 magnetization plateau of the antiferromagnetic Heisenberg model on the kagome lattice. By the self-consistent inclusion of quantum corrections, the 1/3 plateau is stabilized over a broad range of magnetic fields for all spin quantum numbers, S. The values of the critical magnetic fields and the widths of the magnetization plateaus are fully consistent with recent numerical results from exact diagonalization and infinite projected entangled paired states.

cond-mat.str-el↗

arXiv2015

Multiplicative logarithmic corrections to quantum criticality in three-dimensional dimerized antiferromagnets

We investigate the quantum phase transition in an $S = 1/2$ dimerized Heisenberg antiferromagnet in three spatial dimensions. By performing large-scale quantum Monte Carlo simulations and detailed finite-size scaling analyses, we obtain high-precision results for the quantum critical properties at the transition from the magnetically disordered dimer-singlet phase to the antiferromagnetically ordered Néel phase. This transition breaks O($N$) symmetry with $N = 3$ in $D = 3 + 1$ dimensions. This is the upper critical dimension, where multiplicative logarithmic corrections to the leading mean-field critical properties are expected; we extract these corrections, establishing their precise forms for both the zero-temperature staggered magnetization, $m_s$, and the Néel temperature, $T_N$. We present a scaling Ansatz for $T_N$, including logarithmic corrections, which agrees with our data and indicates exact linearity with $m_s$, implying a complete decoupling of quantum and thermal fluctuation effects even arbitrarily close to the quantum critical point. We also demonstrate the predicted $N$-independent leading and subleading logarithmic corrections in the size-dependence of the staggered magnetic susceptibility. These logarithmic scaling forms have not previously been identified or verified by unbiased numerical methods and we discuss their relevance to experimental studies of dimerized quantum antiferromagnets such as TlCuCl$_3$.

cond-mat.str-el↗

arXiv2015

Interlayer electronic hybridization leads to exceptional thickness-dependent vibrational properties in few-layer black phosphorus

Stacking two-dimensional (2D) materials into multi-layers or heterostructures, known as van der Waals (vdW) epitaxy, is an essential degree of freedom for tuning their properties on demand. Few-layer black phosphorus (FLBP), a material with high potential for nano- and optoelectronics applications, appears to have interlayer couplings much stronger than graphene and other 2D systems. Indeed, these couplings call into question whether the stacking of FLBP can be governed only by vdW interactions, which is of crucial importance for epitaxy and property refinement. Here, we perform a theoretical investigation of the vibrational properties of FLBP, which reflect directly its interlayer coupling, by discussing six Raman-observable phonons, including three optical, one breathing, and two shear modes. With increasing sample thickness, we find anomalous redshifts of the frequencies for each optical mode but a blueshift for the armchair shear mode. Our calculations also show splitting of the phonon branches, due to anomalous surface phenomena, and strong phonon-phonon coupling. By computing uniaxial stress effects, inter-atomic force constants, and electron densities, we provide a compelling demonstration that these properties are the consequence of strong and highly directional interlayer interactions arising from electronic hybridization of the lone electron-pairs of FLBP, rather than from vdW interactions. This exceptional interlayer coupling mechanism controls the stacking stability of BP layers and thus opens a new avenue beyond vdW epitaxy for understanding the design of 2D heterostructures.

cond-mat.mtrl-sci↗

arXiv2011

Accurate determination of the Gaussian transition in spin-1 chains with single-ion anisotropy

The Gaussian transition in the spin-one Heisenberg chain with single-ion anisotropy is extremely difficult to treat, both analytically and numerically. We introduce an improved DMRG procedure with strict error control, which we use to access very large systems. By considering the bulk entropy, we determine the Gaussian transition point to 4-digit accuracy, $D_{c}/J = 0.96845(8)$, resolving a long-standing debate in quantum magnetism. With this value, we obtain high-precision data for the critical behavior of quantities including the ground-state energy, gap, and transverse string-order parameter, and for the critical exponent, $ν= 1.472(2)$. Applying our improved technique at $J_{z} = 0.5$ highlights essential differences in critical behavior along the Gaussian transition line.

cond-mat.str-el↗

arXiv2009

Quantized squeezing and even-odd asymmetry of trapped bosons

We investigate the exact nature of the superfluid-to-Mott-insulator crossover for interacting bosons on an optical lattice in a one-dimensional, harmonic trap by high-precision density-matrix renormalization-group calculations. The results reveal an intermediate regime characterized by a cascade of microscopic steps. These arise as a consequence of individual boson "squeezing" events and display an even-odd alternation dependent on the trap symmetry. We discuss the experimental observation of this behavior, which is generic in an external trapping potential.

cond-mat.quant-gas↗

arXiv2009

Dzyaloshinskii-Moriya anisotropy and non-magnetic impurities in the $s = 1/2$ kagome system ZnCu_3(OH)_6Cl_2

Motivated by recent nuclear magnetic resonance experiments on ZnCu$_3$(OH)$_6$Cl$_2$, we present an exact-diagonalization study of the combined effects of non-magnetic impurities and Dzyaloshinskii-Moriya (DM) interactions in the $s = 1/2$ kagome antiferromagnet. The local response to an applied field and correlation-matrix data reveal that the dimer freezing which occurs around each impurity for $D = 0$ persists at least up to $D/J\simeq 0.06$, where $J$ and $D$ denote respectively the exchange and DM interaction energies. The phase transition to the ($Q = 0$) semiclassical, 120$^\circ$ state favored at large $D$ takes place at $D/J\simeq 0.1$. However, the dimers next to the impurity sites remain strong up to values $D \sim J$, far above this critical point, and thus do not participate fully in the ordered state. We discuss the implications of our results for experiments on ZnCu$_3$(OH)$_6$Cl$_2$.

cond-mat.str-el↗

arXiv2008

Frustration and entanglement in the $t_{2g}$ spin--orbital model on a triangular lattice: valence--bond and generalized liquid states

We consider the spin--orbital model for a magnetic system with singly occupied but triply degenerate $t_{2g}$ orbitals coupled into a planar, triangular lattice, as would be exemplified by NaTiO$_2$. We investigate the ground states of the model for interactions which interpolate between the limits of pure superexchange and purely direct exchange interactions. By considering ordered and dimerized states at the mean--field level, and by interpreting the results from exact diagonalization calculations on selected finite systems, we demonstrate that orbital interactions are always frustrated, and that orbital correlations are dictated by the spin state, manifesting an intrinsic entanglement of these degrees of freedom. In the absence of Hund coupling, the ground state changes from a highly resonating, dimer--based, symmetry--restored spin and orbital liquid phase, to one based on completely static, spin--singlet valence bonds. The generic properties of frustration and entanglement survive even when spins and orbitals are nominally decoupled in the ferromagnetic phases stabilized by a strong Hund coupling. By considering the same model on other lattices, we discuss the extent to which frustration is attributable separately to geometry and to interaction effects.

cond-mat.str-el↗

arXiv2003

Universal scaling at field-induced magnetic phase transitions

We study field-induced magnetic order in cubic lattices of dimers with antiferromagnetic Heisenberg interactions. The thermal critical exponents at the quantum phase transition from a spin liquid to a magnetically ordered phase are determined from Stochastic Series Expansion Quantum Monte Carlo simulations. These exponents are independent of the interdimer coupling ratios, and converge to the value obtained by considering the transition as a Bose-Einstein condensation of magnons, alpha_(BEC) = 1.5. The scaling results are of direct relevance to the spin-dimer systems TlCuCl_3 and KCuCl_3, and explain the broad range of exponents reported for field-induced ordering transitions.

cond-mat.str-el↗