# K. S. D. BeachUniversity of Mississippi | UM · Department of Physics and Astronomy

K. S. D. Beach

PhD

## About

66

Publications

5,673

Reads

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1,833

Citations

Introduction

Kevin Beach is an Associate Professor in the Department of Physics and Astronomy at The University of Mississippi. He is an MIT-trained physicist and a former Fellow of the Alexander von Humboldt Foundation. His main research focus is theory and numerics for condensed matter systems.

Additional affiliations

August 2014 - June 2020

January 2008 - June 2014

**University of Alberta**

Position

- Professor (Assistant)

January 2007 - December 2007

**University of Würzburg**

Position

- Fellow

Education

September 1999 - August 2004

September 1997 - August 1999

September 1993 - May 1997

## Publications

Publications (66)

Twisted bilayer graphene near the magic angle is known to have a cascade of insulating phases at integer filling factors of the low-energy bands. In this Letter we address the nature of these phases through an unrestricted, large-scale Hartree-Fock calculation on the lattice that self-consistently accounts for all electronic bands. Using numericall...

The thermal activation process by which a system passes from one local energy minimum to another is a recurring motif in physics, chemistry, and biology. For instance, biopolymer chains are typically modeled in terms of energy landscapes, with folded and unfolded conformations represented by two distinct wells separated by a barrier. The rate of tr...

Twisted bilayer graphene near the magic angle is known to have a cascade of insulating phases at integer filling factors of the low-energy bands. In this Letter we address the nature of these phases through an unrestricted, self-consistent Hartree-Fock calculation on the lattice that accounts for \emph{all} electronic bands. Using numerically unbia...

The thermal activation process by which a system passes from one local energy minimum to another is a recurring motif in physics, chemistry, and biology. For instance, biopolymer chains are typically modeled in terms of energy landscapes, with folded and unfolded conformations represented by two distinct wells separated by a barrier. The rate of tr...

The dynamical behavior of a quantum many-particle system is characterized by the lifetime of its excitations. When the system is perturbed, observables of any nonconserved quantity decay exponentially, but those of a conserved quantity relax to equilibrium with a power law. Such processes are associated with a dynamical exponent z that relates the...

The dynamical behavior of a quantum many-particle system is characterized by the lifetime of its excitations. When the system is perturbed, observables of any non-conserved quantity decay exponentially, but those of a conserved quantity relax to equilibrium with a power law. Such processes are associated with a dynamical exponent $z$ that relates t...

We introduce a generalization of the Fredkin spin chain with tunable three-body interactions expressed in terms of conventional spin-half operators. Of the model's two free parameters, one controls the preference for Ising antiferromagnetism and the other controls the strength of quantum fluctuations. In this formulation, the so-called t-deformed m...

We introduce a generalization of the Fredkin spin chain with tunable three-body interactions expressed in terms of conventional spin-half operators. Of the model's two free parameters, one controls the preference for Ising antiferromagnetism and the other controls the strength of quantum fluctuations. In this formulation, the so-called $t$-deformed...

The structural dynamics of a biopolymer is governed by a process of diffusion through its conformational energy landscape. In pulling experiments using optical tweezers, features of the energy landscape can be extracted from the probability distribution of the critical force at which the polymer unfolds. The analysis is often based on rate equation...

The structural dynamics of a biopolymer is governed by a process of diffusion through its conformational energy landscape. In pulling experiments using optical tweezers, features of the energy landscape can be extracted from the probability distribution of the critical force at which the polymer unfolds. The analysis is often based on rate equation...

We address the problem of free fermions interacting with frozen gauge fields. In particular, we consider a tight-binding model of fermions on the square lattice in which (i) flux 0 or $\pi$ is threaded through each plaquette and (ii) each nearest-neighbor link is decorated with an Ising degree of freedom that describes the local modulation of the h...

The Fredkin model describes a spin-half chain segment subject to three-body, correlated-exchange interactions and twisted boundary conditions. The model is frustration free, and its ground-state wave function is known exactly. Its low-energy physics is that of a strong xy ferromagnet with gapless excitations and an unusually large dynamical exponen...

The Fredkin model describes a spin-half chain segment subject to three-body, correlated-exchange interactions and twisted boundary conditions. The model is frustration-free, and its ground state wave function is known exactly. Its low-energy physics is that of a strong xy ferromagnet with gapless excitations and an unusually large dynamical exponen...

Interaction-driven topological phase transitions in Dirac semimetals are investigated by means of large-scale quantum Monte Carlo simulations. The interaction among Dirac fermions is introduced by coupling them to Ising spins that realize the quantum dynamics of the two-dimensional transverse field Ising model. The ground-state phase diagram, in wh...

The strange correlator [Phys. Rev. Lett. 112, 247202 (2014)] has been proposed as a measure of symmetry protected topological order in one- and two-dimensional systems. It takes the form of a spin-spin correlation function, computed as a mixed overlap between the state of interest and a trivial local product state. We demonstrate that it can be com...

We investigate the problem of Dirac fermions living in a background of nematic fluctuations. In our scheme, the local nematic order is carried by Ising spins, and these gauge-like variables are given their own dynamics in the form of a two-dimensional transverse field Ising model. On the basis of quantum Monte Carlo simulations, we present a comple...

We show that the particular distribution of mass deposited on the surface of a nanomechanical resonator can be estimated by tracking the evolution of the device’s resonance frequencies during the process of desorption. The technique, which relies on analytical models we have developed for the multimodal response of the system, enables mass sensing...

We show that the particular distribution of mass deposited on the surface of
a nanomechanical resonator can be estimated by tracking the evolution of the
device's resonance frequencies during the process of desorption. The technique,
which relies on analytical models we have developed for the multimodal response
of the system, enables mass sensing...

The energy landscapes that drive structure formation in biopolymers are difficult to measure. Here we validate experimentally a novel method to reconstruct landscape profiles from single-molecule pulling curves using an inverse Weierstrass transform (IWT) of the Jarzysnki free-energy integral. The method was applied to unfolding measurements of a D...

The conformational diffusion coefficient for intrachain motions in biopolymers, D, sets the timescale for structural dynamics. Recently, force spectroscopy has been applied to determine D both for unfolded proteins and for the folding transitions in proteins and nucleic acids. However, interpretation of the results remains unsettled. We investigate...

We study high-Q nanostrings that are joined end-to-end to form coupled linear arrays. Whereas isolated individual resonators exhibit sinusoidal vibrational modes with an almost perfectly harmonic spectrum, the modes of the interacting strings are substantially hybridized. Even far-separated strings can show significantly correlated displacement. Th...

We construct a family of short-range resonating-valence-bond wave functions
on a layered cubic lattice, allowing for a tunable anisotropy in the amplitudes
assigned to nearest-neighbour valence bonds along one axis. Monte Carlo
simulations reveal that four phases are stabilized over the full range of the
anisotropy parameter. They are separated fro...

We present variational results for the ground state of the antiferromagnetic
quantum Heisenberg model with frustrating next-nearest-neighbour interactions.
The trial wave functions employed are of resonating-valence-bond type,
elaborated to account for various geometric motifs of adjacent bond pairs. The
calculation is specialized to a square-latti...

We describe a general procedure to calibrate the detection of a nano/micro-mechanical resonator's displacement as it undergoes thermal Brownian motion. A brief introduction to the equations of motion for such a resonator is presented, followed by a detailed derivation of the corresponding power spectral density (PSD) function, which is identical in...

We construct energy-optimized resonating valence bond wavefunctions as a
means to sketch out the zero-temperature phase diagram of the square-lattice
quantum Heisenberg model with competing nearest- (J1) and
next-nearest-neighbour (J2) interactions. Our emphasis is not on achieving an
accurate representation of the magnetically disordered intermedi...

The superconducting pairing instability — as determined by a divergence of the two-particle susceptibility — is obtained in the mean field (BCS) approximation in the thermodynamic limit. The usual practice is to examine this property for a finite lattice. We illustrate that, while the conclusions remain unchanged, the technical features are very di...

Low-mass, high-Q, silicon nitride nanostrings are at the cutting edge of
nanomechanical devices for sensing applications. Here we show that the addition
of a chemically functionalizable gold overlayer does not adversely affect the Q
of the fundamental out-of-plane mode. Instead the device retains its mechanical
responsiveness while gaining sensitiv...

Monte Carlo sampling of quantum spin models is only practical when it is
possible to gauge away simultaneously all negative signs in the
coefficients of the ground state wavefunction. The existence of such a
transformation is related to the possibility of establishing a bipartite
pattern of magnetic order on the lattice and to the choice of a
so-ca...

High-stress silicon nitride nanostrings are a promising system for sensing
applications because of their ultra-high mechanical quality factors (Qs). By
performing thermomechanical calibration across multiple vibrational modes, we
are able to assess the roles of the various dissipation mechanisms in these
devices. Specifically, we possess a set of n...

Inspired by recent experiments on 3He films between one and two atoms thick,
we consider a bilayer Hubbard model on a triangular lattice. Our results are
obtained in the framework of a cluster dynamical mean-field calculation with a
quantum Monte Carlo impurity solver. For appropriate model parameters, we
observe an enhancement of the effective mas...

Using the tight binding Hamiltonian for a honeycomb lattice, we develop a computational technique for the construction and time evolution of Gaussian wave packets in graphene. Employing this approach, we study the scattering across barriers and compute reflection and transmission coefficients. Given the nature of energy dispersion in graphene, it i...

When the linear Heisenberg spin chain is given non-uniform exhange
couplings, its ground state becomes frozen in a quasi-static singlet
bond pattern. This so-called random-singlet phase has long been
understood via a renormalization-group (RG) procedure that decimates the
bonds from strongest to weakest; the flow equations indicate that even
infini...

Inspired by recent experiments on bilayer 3He, we consider a bilayer Hubbard model on a triangular lattice. For appropriate model parameters, we observe a band-selective Mott transition at a critical chemical potential, mu_c, corresponding to the solidification of the fermions in the first layer. The growth of the effective mass on the metallic sid...

A quantum phase transition is typically induced by tuning an external parameter that appears as a coupling constant in the Hamiltonian. Another route is to vary the global symmetry of the system, generalizing, e.g., SU(2) to SU(N). In that case, however, the discrete nature of the control parameter prevents one from identifying and characterizing t...

We report the results of dynamical mean field calculations for the metallic Kondo lattice model subject to an applied magnetic field. High-quality spectral functions reveal that the picture of rigid, hybridized bands, Zeeman-shifted in proportion to the field strength, is qualitatively correct. We find evidence of a zero-temperature magnetization p...

Frustrated quantum magnets are not amenable to simulation using conventional quantum Monte Carlo because of the infamous sign problem. In the overcomplete basis of singlet product states, updates have a many-to-one property that allows for grouping of updates around plaquettes in such a way that the negative sampling weights are almost entirely eli...

Quantum Monte Carlo methods are sophisticated numerical techniques for simulating interacting quantum systems. In some cases, however, they suffer from the notorious "sign problem" and become too inefficient to be useful. A recent publication [J. Wojtkiewicz, Phys. Rev. B 75, 174421 (2007)] claims to have solved the sign problem for a certain class...

We investigate the evolution of the heavy fermion ground state under application of a strong external magnetic field. We present a richer version of the usual hybridization mean field theory that allows for hybridization in both the singlet and triplet channels and incorporates a self-consistent Weiss field. We show that for a magnetic field streng...

The Heisenberg chain with antiferromagnetic, powerlaw exchange has a quantum phase transition separating spin liquid and Neel ordered phases at a critical value of the powerlaw exponent alpha. The behaviour of the system can be explained rather simply in terms of a resonating valence bond state in which the amplitude for a bond of length r goes as...

We describe a "master equation" analysis for the bond amplitudes h(r) of an RVB wavefunction. Starting from any initial guess, h(r) evolves (in a manner dictated by the spin hamiltonian under consideration) toward a steady-state distribution representing an approximation to the true ground state. Unknown transition coefficients in the master equati...

We present a valence bond theory of the spin-S quantum Heisenberg model. For nonfrustracting, local exchange and dimension d > 1, it predicts a resonating ground state with bond amplitudes h(r) ~ (a^2+r^2)^(-p/2) and decay exponent p=d+1. Different values of p can be achieved by introducing frustrating (p > d+1) or nonfrustrating (p < d+1) long-ran...

We report on a valence bond projector Monte Carlo simulation of the cubic lattice quantum Heisenberg model with additional higher-order exchange interactions in each unit cell. The model supports two different valence bond solid (VBS) ground states. In one of these states, the dimer pattern is a three-dimensional analogue of the columnar pattern fa...

We discuss a projector Monte Carlo method for quantum spin models formulated in the valence bond basis, using the S=1/2 Heisenberg antiferromagnet as an example. Its singlet ground state can be projected out of an arbitrary basis state as the trial state, but a more rapid convergence can be obtained using a good variational state. As an alternative...

We will report infrared and magneto-optical results on two heavy fermion skutterudites YbFe4Sb12 and CeRu4Sb12. Detailed temperature dependence of infrared spectra will be presented for both compounds. In addition, magneto-transmission measurements on YbFe4Sb12 in magnetic field as high as 33 Tesla, and magneto-reflection measurements on CeRu4Sb12...

Quantum spin systems such as the Heisenberg model can be simulated numerically in the valence bond basis, as an alternative to the standard basis of eigenstates of the S^zi operators [1]. One advantage of this approach is that also the triplet sector can be studied based on the configurations generated in the singlet sector [1,2]. This way an impro...

The spin singlet ground state of a quantum antiferromagnet can be expanded in the overcomplete basis of valence bond states. [1] To first approximation, the weight associated with each configuration is factorizable into a product of individual bond amplitudes. For nonfrustrated antiferromagnets with local interactions, mean field calculations indic...

In a system with an even number of SU(2) spins, there is an overcomplete set of states--consisting of all possible pairings of the spins into valence bonds--that spans the S=0 Hilbert subspace. Operator expectation values in this basis are related to the properties of the closed loops that are formed by the overlap of valence bond states. We constr...

It has long been believed that systems of interacting spins can support, in addition to the usual collinear N'eel state, a variety of paramagnetic ground states with resonating or static valence bond order. Confirmation of their existence in candidate models has been complicated by the fact that the frustrating interactions that might support these...

We report a comprehensive infrared magnetospectroscopy study of a CeRu4Sb12 compound revealing quasiparticles with a heavy effective mass m*, with a detailed analysis of optical constants in fields up to 17 T. We find that the applied magnetic field strongly affects the low-energy excitations in the system. In particular, the magnitude of m* approx...

A Zeeman field affects the metallic heavy fermion ground state in two ways: (i) it splits the spin-degerenate conduction sea, leaving spin up and spin down Fermi surfaces with different band curvature; (ii) it competes with the Kondo effect and thus suppresses the mass enhancement. Taking these two effects into account, we compute the quasiparticle...

We use quantum Monte Carlo (stochastic series expansion) and finite-size
scaling to study the quantum critical points of two S=1/2 Heisenberg
antiferromagnets in two dimensions: a bilayer and a Kondo-lattice-like system
(incomplete bilayer), each with intra- and inter-plane couplings J and J_perp.
We discuss the ground-state finite-size scaling pro...

A recent paper [Burovski et al., cond-mat/0507352] reports on a new, high-accuracy simulation of the classical phi^4 model (in the three-dimensional XY universality class). The authors claim that a careful scaling analysis of their data gives nu = 0.6711(1) for the thermal critical exponent. If correct, this would neatly resolve the discrepancy bet...

We discuss the effect of site dilution on both the magnetization and the density of states of quantum spins in the honeycomb lattice, described by the antiferromagnetic Heisenberg spin-S model. For this purpose a real-space Bogoliubov-Valatin transformation is used. In this work we show that for the S>1/2 the system can be analyzed in terms of line...

We propose a treatment of the subleading corrections to finite-size scaling that preserves the notion of data collapse. This approach is used to extend and improve the usual Binder cumulant analysis. As a demonstration, we present results for the two- and three-dimensional classical Ising models and the two-dimensional, double-layer quantum antifer...

The maximum entropy method is shown to be a special limit of the stochastic analytic continuation method introduced by Sandvik [Phys. Rev. B 57, 10287 (1998)]. We employ a mapping between the analytic continuation problem and a system of interacting classical fields. The Hamiltonian of this system is chosen such that the determination of its ground...

The half-filled Kondo lattice model describes the physics of the Kondo
insulators (Ce_3Bi_4Pt_3, CeRhAs, CeRhSb, YbB_12, and SmB_6) provided
that the exchange coupling J exceeds some critical value. In this
regime, local singlet formation dominates the RKKY antiferromagnetism.
An applied magnetic field, however, will interfere with the
singlet--RKK...

The Kondo lattice model, augmented by a Zeeman term, serves as a useful model of a Kondo insulator in an applied magnetic field. A variational mean field analysis of this system on a square lattice, backed up by quantum Monte Carlo calculations, reveals an interesting separation of magnetic field scales. For Zeeman energy comparable to the Kondo en...

We outline a partial-fractions decomposition method for determining the one-particle spectral function and single-particle density of states of a correlated electronic system on a finite lattice in the non self-consistent T-matrix approximation to arbitrary numerical accuracy, and demonstrate the application of these ideas to the attractive Hubbard...

We have analyzed the 3D attractive Hubbard model (AHM) in the non self-consistent T-matrix approximation. We present a technique that allows for us to determine, exactly, the single-particle density of states (DOS) at all temperatures above the superconducting temperature, in the T-matrix approximation. Since this approximation scheme should be acc...

Superconducting fluctuations become more pronounced in low-dimensional systems. We present a formulation of the Thouless criterion for the attractive Hubbard model, which includes pairing fluctuations that suppress Tc to zero in two dimensions. A self-consistent pair propagator is advocated, and the appropriate self energy for self-consistent theor...

We propose a fully microscopic theory of the anomalous normal state of the attractive Hubbard model in the low-density limit that accounts for propagator renormalization. Our analytical conclusions regarding the thermodynamic instabilities contained in the self-consistent equations associated with this formulation have been verified by our comprehe...

We examine the role of spin twists in the formation of domain walls, often called stripes, by focusing on the spin textures
found in the cluster spin glass phases of and . To this end, we derive improved analytic expressions for the spin distortions produced by a frustrating bond, both near
the core region of the bond and in the far field, and then...

We critique a Padé analytic continuation method whereby a rational polynomial function is fit to a set of input points by means of a single matrix inversion. This procedure is accomplished to an extremely high accuracy using a symbolic computation algorithm. As an example of this method in action, it is applied to the problem of determining the spe...

This short paper summarizes a calculational method for obtaining the dynamical properties of many-body theories formulated in terms of (unrenormalized) bare propagators (and more generally, in terms of meromorphic functions, or convolutions over meromorphic functions) to a very high accuracy. We demonstrate the method by applying it to a T-matrix t...

We examine the low--temperature magnetic properties of moderately doped LaSrCuO paying particular attention to the spin--glass (SG) phase and the C-IC transition as they are affected by Sr impurity disorder. New measurements of the low--temperature susceptibility in the SG phase show an increase of an anomalously small Curie constant with doping. T...