arXiv subjects

Yen Lee Loh

Publications and source records attributed to Yen Lee Loh.

At least 19 recordsLinked to original sources

arXiv2022

Superfluid response of an atomically thin, gate-tuned van der Waals superconductor

A growing number of two-dimensional superconductors are being discovered in the family of layered van der Waals (vdW) materials. Due to small sample volume, their characterization has been largely limited to electrical transport measurements. As a consequence, characterization of the diamagnetic response of the superfluid to an applied magnetic field, a defining property of any superconductor, has been lacking. Here, we use a local magnetic probe to directly measure the superfluid response of the tunable, gate-induced superconducting state in MoS$_2$. We find that the backgate changes the superconducting transition temperature non-monotonically whereas the superfluid stiffness at low temperature and the normal state conductivity monotonically increase with backgate voltage. In some devices, we find direct signatures in agreement with a Berezinskii-Kosterlitz-Thouless transition, whereas in others we find a broadened, shallow onset of the superfluid response. We show that the observed behavior is consistent with disorder playing an important role in determining the superconducting properties in superconducting MoS$_2$. Our work demonstrates that magnetic property measurements are within reach for vdW superconductors and reveals that the superfluid response significantly deviates from simple BCS-like behavior.

cond-mat.supr-con↗

arXiv2021

Path-Integral Treatment of Quantum Bouncers

The one-sided bouncer and the symmetric bouncer involve a one-dimensional particle in a piecewise linear potential. For such problems, the time-dependent quantum mechanical propagator cannot be found in closed form. The semiclassical Feynman path integral is a very appealing approach, as it approximates the propagator by a closed-form expression (a sum over a finite number of classical paths). In this paper we solve the classical path enumeration problem. We obtain closed-form expressions for the initial velocity, bounce times, focal times, action, van Vleck determinant, and Morse index for each classical path. We calculate the propagator within the semiclassical approximation. The numerical results agree with eigenfunction expansion results away from caustics. We derive mappings between the one-sided bouncer and symmetric bouncer, which explains why each bounce of the one-sided bouncer increases the Morse index by 2 and results in a phase change of $π$. We interpret the semiclassical Feynman path integral to obtain visualizations of matter wave propagation based on interference between classical paths, in analogy with the traditional visualization of light wave propagation as interference between classical ray paths.

quant-ph↗

arXiv2019

Superconductor-Insulator Transition and Fermi-Bose Crossovers

The direct transition from an insulator to a superconductor (SC) in Fermi systems is a problem of long-standing interest, which necessarily goes beyond the standard BCS paradigm of superconductivity as a Fermi surface instability. We introduce here a simple, translationally-invariant lattice fermion model that undergoes a SC-insulator transition (SIT) and elucidate its properties using analytical methods and quantum Monte Carlo simulations. We show that there is a fermionic band insulator to bosonic insulator crossover in the insulating phase and a BCS-to-BEC crossover in the SC. The SIT is always found to be from a bosonic insulator to a BEC-like SC, with an energy gap for fermions that remains finite across the SIT. The energy scales that go critical at the SIT are the gap to pair excitations in the insulator and the superfluid stiffness in the SC. In addition to giving insights into important questions about the SIT in solid state systems, our model should be experimentally realizable using ultracold fermions in optical lattices.

cond-mat.supr-con↗

arXiv2017

A general method for calculating lattice Green functions on the branch cut

We present a method for calculating the complex Green function $G_{ij} (ω)$ at any real frequency $ω$ between any two sites $i$ and $j$ on a lattice. Starting from numbers of walks on square, cubic, honeycomb, triangular, bcc, fcc, and diamond lattices, we derive Chebyshev expansion coefficients for $G_{ij} (ω)$. The convergence of the Chebyshev series can be accelerated by constructing functions $f(ω)$ that mimic the van Hove singularities in $G_{ij} (ω)$ and subtracting their Chebyshev coefficients from the original coefficients. We demonstrate this explicitly for the square lattice and bcc lattice. Our algorithm achieves typical accuracies of 6--9 significant figures using 1000 series terms.

math-ph↗

arXiv2014

Bias-free simulation of diffusion-limited aggregation on a square lattice

We identify sources of systematic error in traditional simulations of the Witten-Sander model of diffusion-limited aggregation (DLA) on a square lattice. We present an algorithm that reduces these biases to below $10^{-12}$. We grow clusters of $10^8$ particles on $65536\times 65536$ lattices. We verify that lattice DLA clusters inevitably grow into anisotropic shapes, dictated by the anisotropy of the aggregation process. We verify that the fractal dimension evolves from the continuum DLA value, $D=1.71$, for small disk-shaped clusters, towards Kesten's bound of $D=3/2$ for highly anisotropic clusters with long protruding arms.

cond-mat.dis-nn↗

arXiv2014

Dynamical Conductivity Across The Disorder-Tuned Superconductor-Insulator Transition

A quantum phase transition is a dramatic event marked by large spatial and temporal fluctuations, where one phase of matter with its ground state and tower of excitations reorganizes into a completely different phase. We provide new insight into the disorder-driven superconductor-insulator transition (SIT) in two dimensions, a problem of great theoretical and experimental interest, with the dynamical conductivity σ(ω) and the bosonic (pair) spectral function P(ω) calculated from quantum Monte Carlo simulations. We identify characteristic energy scales in the superconducting and insulating phases that vanish at the transition due to enhanced quantum fluctuations, despite the persistence of a robust fermionic gap across the SIT. Disorder leads to enhanced absorption in σ(ω) at low frequencies compared to the SIT in a clean system. Disorder also expands the quantum critical region, due to a change in the universality class, with an underlying T=0 critical point with a finite low-frequency conductivity.

cond-mat.supr-con↗

arXiv2013

Optical Lattice Emulators: Bose and Fermi Hubbard Models

This chapter is a pedagogical review of the Hubbard model for bosons with repulsion and for fermions with attraction and repulsion primarily using two methods, one chosen for its simplicity and insights (mean field theory) and the other chosen for its accuracy and reliability (quantum Monte Marlo methods). From a comparison of the two methods we glean valuable information into the effects of fluctuations that dominate quantum phase transitions. The chapter includes an in-depth comparison with experiments. We conclude with a discussion of future developments where the technical methods expounded on here, mean field theory and quantum Monte Carlo, could be useful.

cond-mat.quant-gas↗

arXiv2013

Visualizing Angular Momentum Eigenstates using the Spin Coherent State Representation

Orbital angular momentum eigenfunctions are readily understood in terms of spherical harmonic wavefunctions. However, the quantum mechanical phenomenon of spin is often said to be mysterious and hard to visualize, with no classical analogue. Many textbooks give a heuristic and somewhat unsatisfying picture of a precessing spin vector. Here we advocate for the "spin wavefunction" in the spin coherent state representation as a striking, elegant, and mathematically meaningful visual tool. We also demonstrate that cartographic projections such as the Hammer projection are useful for visualizing wavefunctions defined on spherical surfaces.

physics.ed-ph↗

arXiv2013

Single and two-particle energy gaps across the disorder-driven superconductor-insulator transition

The competition between superconductivity and localization raises profound questions in condensed matter physics. In spite of decades of research, the mechanism of the superconductor-insulator transition (SIT) and the nature of the insulator are not understood. We use quantum Monte Carlo simulations that treat, on an equal footing, inhomogeneous amplitude variations and phase fluctuations, a major advance over previous theories. We gain new microscopic insights and make testable predictions for local spectroscopic probes. The energy gap in the density of states survives across the transition, but coherence peaks exist only in the superconductor. A characteristic pseudogap persists above the critical disorder and critical temperature, in contrast to conventional theories. Surprisingly, the insulator has a two-particle gap scale that vanishes at the SIT, despite a robust single-particle gap.

cond-mat.supr-con↗

arXiv2013

Emergent granularity and pseudogap near the superconductor-insulator transition

In two dimensions there is a direct superconductor-to-insulator quantum phase transition driven by increasing disorder. We elucidate, using a combination of inhomogeneous mean field theory and quantum Monte Carlo techniques, the nature of the phases and the mechanism of the transition. We make several testable predictions specifically for local spectroscopic probes. With increasing disorder, the system forms superconducting blobs on the scale of the coherence length embedded in an insulating matrix. In the superconducting state, the phases on the different blobs are coherent across the system whereas in the insulator long range phase coherence is disrupted by quantum fluctuations. As a consequence of this emergent granularity, we show that the single-particle energy gap in the density of states survives across the transition, but coherence peaks exist only in the superconductor. A characteristic pseudogap persists above the critical disorder and critical temperature, in contrast to conventional theories. Surprisingly, the insulator has a two-particle gap scale that vanishes at the SIT, despite a robust single-particle gap.

cond-mat.supr-con↗

arXiv2013

Theoretical Studies of Superconductor-Insulator Transitions

In this article we study superconductor-insulator transitions within the general framework of an attractive Hubbard model. This is a well-defined model of s-wave superconductivity which permits different tuning parameters (disorder and field). Furthermore, it allows a comparison of various analytical and computational approaches in order to gain a complete understanding of the various effects of amplitude and phase fluctuations. We present a systematic pedagogical approach, aiming to equip the lay reader with enough apparatus to be able to understand the numerical calculations, reproduce some of the simpler results, and be able to tackle future problems related to inhomogeneous phases. We go into considerable detail on mean-field theory (MFT) and the Bogoliubov-de Gennes (BdG) approach, as these are a first line of attack which can capture much of the physics, but we also outline cases where this fails to capture phase fluctuations and more sophisticated Quantum Monte Carlo (QMC) calculations are necessary. We discuss the behavior of many observables, including densities of states, superfluid stiffness, and dynamical conductivity, for the disorder-tuned superconductor-insulator transition. We also discuss SITs tuned by parallel magnetic field, which are quite different due to pairbreaking.

cond-mat.supr-con↗

arXiv2013

Aspects of Localization across the 2D Superconductor-Insulator Transition

It is well known that the metal-insulator transition in two dimensions for non-interacting fermions takes place at infinitesimal disorder. In contrast, the superconductor-to-insulator transition takes place at a finite critical disorder (on the order of V_c ~ 2t), where V is the typical width of the distribution of random site energies and t is the hopping scale. In this article we compare the localization/delocalization properties of one and two particles. Whereas the metal-insulator transition is a consequence of single-particle Anderson localization, the superconductor-insulator transition (SIT) is due to pair localization - or, alternatively, fluctuations of the phase conjugate to pair density. The central question we address is how superconductivity emerges from localized single-particle states. We address this question using inhomogeneous mean field theory and quantum Monte Carlo techniques and make several testable predictions for local spectroscopic probes across the SIT. We show that with increasing disorder, the system forms superconducting blobs on the scale of the coherence length embedded in an insulating matrix. In the superconducting state, the phases on the different blobs are coherent across the system whereas in the insulator long-range phase coherence is disrupted by quantum fluctuations. As a consequence of this emergent granularity, we show that the single-particle energy gap in the density of states survives across the transition, but coherence peaks exist only in the superconductor. A characteristic pseudogap persists above the critical disorder and critical temperature, in contrast to conventional theories. Surprisingly, the insulator has a two-particle gap scale that vanishes at the SIT despite a robust single-particle gap.

cond-mat.supr-con↗

arXiv2013

Divergence of Dynamical Conductivity at Certain Percolative Superconductor-Insulator Transitions

Random inductor-capacitor (LC) networks can exhibit percolative superconductor-insulator transitions (SITs). We use a simple and efficient algorithm to compute the dynamical conductivity σ(ω,p) of one type of LC network on large (4000 x 4000) square lattices, where δ=p-p_c is the tuning parameter for the SIT. We confirm that the conductivity obeys a scaling form, so that the characteristic frequency scales as Ω~ |δ|^{νz} with νz \approx 1.91, the superfluid stiffness scales as Υ~ |δ|^t with t \approx 1.3, and the electric susceptibility scales as χ_E ~ |δ|^{-s} with s = 2νz - t \approx 2.52. In the insulating state, the low-frequency dissipative conductivity is exponentially small, whereas in the superconductor, it is linear in frequency. The sign of Im σ(ω) at small ωchanges across the SIT. Most importantly, we find that right at the SIT Re σ(ω) ~ ω^{t/νz-1} ~ ω^{-0.32}, so that the conductivity diverges in the DC limit, in contrast with most other classical and quantum models of SITs.

cond-mat.supr-con↗

arXiv2012

Accurate Calculation of Off-Diagonal Green Functions on Anisotropic Hypercubic Lattices

We present a method for accurate evaluation of the Green function $G(ω,r_1,...,r_d)$ at any real frequency $ω$ and any lattice vector $(r_1,...,r_d)$ for a $d$-dimensional hypercubic lattice that may have anisotropic couplings $(Ω_1,...,Ω_d)$. In this method, we start with an integral representation of $G$, split the oscillatory integrand into combinations of Hankel functions, and deform the integration paths into the complex plane to obtain rapidly convergent integrals. We also discuss an alternative approach using the Levin collocation method. We report values of the Green function at selected frequencies on the branch cut and selected lattice vectors.

math-ph↗

arXiv2012

Cooling by corralling: a route to ultra-low entropies in optical lattices

A major motivation for cold atom experiments is the search for quantum ground states such as antiferromagnets and d-wave superfluids. The primary obstacle to this task is the difficulty of cooling to sufficiently low temperatures. We propose a way to achieve very low temperatures and entropies ($\sim 0.03k_B$ per particle) by trapping fermions in a corral formed from another species of atoms. The Fermi system can then be used as a heat sink, or it can be adiabatically evolved into other desired states. In particular, we suggest methods for generating antiferromagnetism using this technique.

cond-mat.quant-gas↗

arXiv2012

Accurate calculation of Green functions on the d-dimensional hypercubic lattice

We write the Green function of the $d$-dimensional hypercubic lattice in a piecewise form covering the entire real frequency axis. Each piece is a single integral involving modified Bessel functions of the first and second kinds. The smoothness of the integrand allows both real and imaginary parts of the Green function to be computed quickly and accurately for any dimension $d$ and any real frequency, and the computational time scales only linearly with $d$.

math-ph↗

arXiv2011

Proposal for interferometric detection of topological defects in modulated superfluids

Attractive interactions between fermions can produce a superfluid ground state, in which pairs of up and down spins swirl together in a coordinated, coherent dance. How is this dance affected by an imbalance in the population of up and down fermions? Do the extra fermions stand on the sides, or do they disrupt the dance? The most intriguing possibility is the formation of a modulated superfluid state, known as an LO phase, in which the excess fermions self-organize into domain walls where the pairing amplitude changes sign. Despite fifty years of theoretical and experimental work, there has so far been no direct observation of an LO phase. Here we propose an experiment in which two fermion clouds, prepared with unequal population imbalances, are allowed to expand and interfere. A zipper pattern in the interference fringes is unequivocal evidence of LO physics. Furthermore, because the experiment is resolved in time and in two spatial directions, we expect an observable signature even at finite temperatures (when thermal fluctuations destroy long-range LO order averaged over time).

cond-mat.quant-gas↗

arXiv2011

Fermions in 3D Optical Lattices: Cooling Protocol to Obtain Antiferromagnetism

A major challenge in realizing antiferromagnetic (AF) and superfluid phases in optical lattices is the ability to cool fermions. We determine the equation of state for the 3D repulsive Fermi-Hubbard model as a function of the chemical potential, temperature and repulsion using unbiased determinantal quantum Monte Carlo methods, and we then use the local density approximation to model a harmonic trap. We show that increasing repulsion leads to cooling, but only in a trap, due to the redistribution of entropy from the center to the metallic wings. Thus, even when the average entropy per particle is larger than that required for antiferromagnetism in the homogeneous system, the trap enables the formation of an AF Mott phase.

cond-mat.quant-gas↗