arXiv subjects

Kedar Damle

Publications and source records attributed to Kedar Damle.

At least 19 recordsLinked to original sources

arXiv2026

Vacancy-induced local moments in quantum paramagnetic phases: An SU($N$) designer Hamiltonian study

We explore the effects of non-magnetic impurities (vacancy disorder) on the quantum paramagnetic phases stabilized by SU($N$) designer Hamiltonians on bipartite lattices. Using the results of our quantum Monte Carlo simulations, we demonstrate that isolated vacancies seed emergent spin $S=1/2$ moments in their vicinity when the low-temperature state has valence bond solid order. Indeed, our quantum Monte Carlo results for the low-temperature susceptibility in such regimes shows clear evidence of the vacancy-induced Curie tails associated with these emergent moments, and our zero-temperature projector Monte Carlo results on the ground-state wavefunction in the valence bond basis provide additional evidence in support of this picture. Further, for such designer Hamiltonians on the Lieb lattice with two additional sites on each bond of a square lattice, we identify a low-temperature spin liquid-like regime with no sign of spin or valence bond order. This liquid-like regime serves as a test bed for validating a recently-developed argument concerning the effects of vacancy disorder in such low temperature regimes. Consistent with this argument, we find that isolated vacancies do not seed emergent local moments in such spin liquids. Instead, in the presence of vacancy disorder, emergent local moments are associated with the presence of monomers in maximum-density dimer packings of the corresponding diluted lattice.

cond-mat.str-el↗

arXiv2026

$Z_3$ confined and deconfined Coulomb liquids in $S_{\rm eff} = 3/2$ pyrochlore magnets

We identify an interesting regime in the physics of pyrochlore magnets in which spin-orbit and crystal field effects lead to {\em two} low-lying magnetic doublets that can be modeled as an effective spin $S=3/2$ degree of freedom that sees a dominant easy-axis antiferromagnetic exchange $J>0$ favoring the local $[111]$ axes, which competes with a comparably strong single-ion anisotropy $\Delta = J+\mu/2$ (with $|\mu| \ll J$) favoring the perpendicular planes. For a precise analysis, we study the $T/J \rightarrow 0$ limit in which $w \equiv \exp(-\mu/T)$ is the control variable. In this limit, we find {\em two topologically distinct} zero-field Coulomb phases separated by a first-order $Z_3$ confinement transition at $w_c \approx 2.02$. Both Coulomb phases admit a description in terms of the fluctuations of a coarse-grained divergence-free polarization field. However, the flux of this polarization field is restricted to integer multiples of $3$, and only charges that are multiples of 3 are deconfined in one of these phases, while all integer fluxes are allowed and all integer charges are deconfined in the other phase. Experimental systems with small negative $\mu$ ({\em i.e.}, $-J \ll \mu < 0$) are therefore predicted to exhibit signatures of this topological transition when cooled below $T_c \approx 1.42|\mu|$.

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arXiv2025

Random geometry of maximum-density dimer packings of the site-diluted kagome lattice

Recent work that analyzed the effect of vacancy disorder on a short-range resonating valence bond spin liquid state of kagome-lattice antiferromagnets argued that such spin liquids are stable to vacancy disorder. The argument relied crucially on a numerical study that identified the following property of the site-diluted kagome lattice: maximum-density dimer packings (maximum matchings) of any connected component of such site-diluted kagome lattices have at most one unmatched vertex that hosts a monomer. Here, we provide an inductive proof of a stronger result that implies this property: If a connected cluster of such a lattice has an odd number of vertices, its Gallai-Edmonds decomposition~\cite{Lovas_Plummer_1986} has exactly one ${\mathcal R}$-type region that spans the entire connected cluster and hosts a single monomer of any maximum-density dimer packing. If on the other hand it has an even number of sites, it admits perfect matchings (fully-packed dimer coverings with no monomers) and its Gallai-Edmonds decomposition consists of a single ${\mathcal P}$-type region that spans the entire cluster. Our proof also applies to the site-diluted Archimedean star lattice, the site-diluted pyrochlore lattice (corner-sharing tetrahedra), the site-diluted hyperkagome lattice, and, more generally, to any lattice satisfying a certain local connectivity property. It does not apply to bond-diluted versions of such lattices.

cond-mat.dis-nn↗

arXiv2025

$S = 1$ pyrochlore magnets with competing anisotropies: A tale of two Coulomb phases, $Z_2$ flux confinement and $XY$-like transitions

We argue that the low-temperature physics of $S=1$ pyrochlore magnets with a predominantly Ising-like easy-axis exchange coupling $J$ that favors the local tetrahedral body diagonals, and a comparably large easy-plane single-ion anisotropy $\Delta =J + \mu$ ($|\mu| \ll J$) that favors the plane perpendicular to these local axes will exhibit interesting new phenomena due to the competition between $J$ and $\Delta$. In the $T/J \rightarrow 0$ limit, we find three low temperature phases as a function of $\mu/T$: a short-range correlated paramagnetic phase, and two topologically-distinct Coulomb liquids separated by a $Z_2$ flux confinement transition. Both Coulomb liquids are described at long-wavelengths by a fluctuating divergence-free polarization field and have characteristic pinch-point singularities in their structure factor. In one Coulomb phase, the flux of this polarization field is confined to {\em even} integers, while it takes on all integer values in the other Coulomb phase. Experimental realizations with $|\mu| \ll J$ and negative are predicted to exhibit signatures of a transition from a flux-deconfined Coulomb phase to the flux-confined Coulomb phase as they are cooled below $T_{c_2} \approx 1.57|\mu|$, while realizations with positive $\mu \ll J$ will show signatures of a transition from a flux-deconfined Coulomb liquid to a short-range correlated paramagnet via a continuous $XY$-like transition at $T_{c_1} \approx 0.98 \mu$.

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arXiv2025

Flux confinement-deconfinement transition of dimer-loop models on three-dimensional bipartite lattices

Motivated by recent work that mapped the low-temperature properties of a class of frustrated spin $S=1$ kagome antiferromagnets with competing exchange and single-ion anisotropies to the fully-packed limit (with each vertex touched by exactly one dimer or nontrivial loop) of a system of dimers and nontrivial (length $s > 2$) loops on the honeycomb lattice, we study this fully-packed dimer-loop model on the three-dimensional bipartite cubic and diamond lattices as a function of $w$, the relative fugacity of dimers. We find that the $w \rightarrow 0$ O($1$) loop-model limit is separated from the $w \rightarrow \infty$ dimer limit by a geometric phase transition at a nonzero finite critical fugacity $w_c$: The $w>w_c$ phase has short loops with an exponentially decaying loop-size distribution, while the $w w_c$, but become deconfined in the small-$w$ phase. Although both phases are unstable to a nonzero fugacity for the charge $\pm 1/2$ excitations, the destruction of the $w >w_c$ Coulomb liquid is characterized by an interesting slow crossover, since test charges with $q=\pm 1/2$ are confined in this phase.

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arXiv2024

Magnetic effects of non-magnetic impurities in gapped short-range resonating valence bond spin liquids

We study the effect of a small density $n_v$ of quenched non-magnetic impurities, i.e. vacancy disorder, in gapped short-range resonating valence bond (RVB) spin liquid states and valence bond solid (VBS) states of quantum magnets. We argue that a large class of short-range RVB liquids are stable at small $n_v$ on the kagome lattice, while the corresponding states on triangular, square, and honeycomb lattices are unstable at any nonzero $n_v$ due to the presence of emergent vacancy-induced local moments. In contrast, VBS states are argued to be generically unstable (independent of lattice geometry) at nonzero $n_v$ due to such a local-moment instability. Our arguments rely in part on an analysis of the statistical mechanics of maximally-packed dimer covers of the diluted lattice, and are fully supported by our computational results on $O(N)$ symmetric designer Hamiltonians.

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arXiv2023

Chaotic percolation in the random geometry of maximum-density dimer packings

Maximum-density dimer packings (maximum matchings) of non-bipartite site-diluted lattices, such as the triangular and Shastry-Sutherland lattices in $d=2$ dimensions and the stacked-triangular and corner-sharing octahedral lattices in $d=3$, generically exhibit a nonzero density of monomers (unmatched vertices). Following a construction in the recent literature, we use the structure theory of Gallai and Edmonds to decompose the disordered lattice into ``${\mathcal R}$-type'' regions which host the monomers of any maximum matching, and perfectly matched ``${\mathcal P}$-type'' regions from which such monomers are excluded. When the density $n_v$ of quenched vacancies lies well within the low-$n_v$ geometrically percolated phase of the disordered lattice, we find that the random geometry of these regions exhibits unusual {\em Gallai-Edmonds percolation} phenomena. In $d=2$, we find two phases separated by a critical point, namely a phase in which all ${\mathcal R}$-type and ${\mathcal P}$-type regions are small, and a percolated phase that displays a striking lack of self-averaging in the thermodynamic limit: Each sample has a single percolating region which is of type ${\mathcal R}$ with probability $f_{\mathcal R}$ and type ${\mathcal P}$ with probability $1-f_{\mathcal R}$, where $f_{\mathcal R} \approx 0.50(2)$ is independent of $n_v$ (away from the critical region). In this regime, microscopic changes in the vacancy configuration lead to chaotic changes in the large-scale structure of ${\mathcal R}$-type and ${\mathcal P}$-type regions. In $d=3$, apart from a phase with small ${\mathcal R}$-type and ${\mathcal P}$-type regions, the thermodynamic limit exhibits {\em four} distinct percolated phases separated by critical points at successively lower $n_v$, two of which again display unusual violations of self-averaging. Physical consequences are also discussed.

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arXiv2023

Ashkin-Teller phase transition and multicritical behavior in a classical monomer-dimer model

We use Monte Carlo simulations and tensor network methods to study a classical monomer-dimer model on the square lattice with a hole (monomer) fugacity $z$, an aligning dimer-dimer interaction $u$ that favors columnar order, and an attractive dimer-dimer interaction $v$ between two adjacent dimers that lie on the same principal axis of the lattice. The Monte Carlo simulations of finite size systems rely on our grand-canonical generalization of the dimer worm algorithm, while the tensor network computations are based on a uniform matrix product ansatz for the eigenvector of the row-to-row transfer matrix that work directly in the thermodynamic limit. The phase diagram has nematic, columnar order and fluid phases, and a nonzero temperature multicritical point at which all three meet. For any fixed $v/u < \infty$, we argue that this multicritical point continues to be located at a nonzero hole fugacity $z_{\rm mc}(v/u) > 0$; our numerical results confirm this theoretical expectation but find that $z_{\rm mc}(v/u) \to 0$ very rapidly as $v/u \to \infty$. Our numerical results also confirm the theoretical expectation that the corresponding multicritical behavior is in the universality class of the four-state Potts multicritical point on critical line of the two-dimensional Ashkin-Teller model.

cond-mat.stat-mech↗

arXiv2023

Competition between Neel, Haldane nematic, plaquette valence bond solid, and $(\pi,\pi)$ valence bond solid phases in SU(N) analogs of $S=1$ square-lattice antiferromagnets

We use stochastic series expansion (SSE) quantum Monte Carlo (QMC) methods to study the phases and transitions displayed by a class of sign-free designer Hamiltonians for SU($N$) analogs of spin $S=1$ quantum antiferromagnets on the square lattice. The SU($N$) spins are generators of the single-row two-column representation (complex conjugate of single-row two-column representation) on $A$ ($B$) sublattices of the square lattice, and the Hamiltonian is designed to explore the competition between the nearest neighbour antiferromagnetic exchange couplings $J$ and four-spin interactions $Q$ that favor a plaquette-ordered valence bond solid (p-VBS) ground state. We find that this state is indeed established at large $Q/J$ for all $N > 3$. For $3< N \leq 9$, the ground state exhibits a direct first order quantum phase transition from a small-$Q/J$ N\'eel ordered antiferromagnetic state to this large-$Q/J$ p-VBS state. The ground state at $Q/J=0$ for $N \geq 10$ has been previously reported to be a valence bond nematic state, dubbed the Haldane nematic in recent literature. For small nonzero $Q/J$ and $N \geq 10$, we additionally find an unusual intermediate state in which the bond energy has a Bragg peak at wavevector $(\pi,\pi)$ with no accompanying Bragg peaks at wavevectors $(\pi,0)$ and $(0, \pi)$. This $(\pi, \pi)$ state also appears to be metastable at $Q/J =0$, as evidenced by low temperature histograms of the Haldane nematic and $(\pi, \pi)$ order parameters. Deep in the p-VBS phase of the ground state phase diagram, we find the temperature-driven melting of the p-VBS order is in the Ashkin-Teller universality class. In this regime, we identify an interesting signature of Ashkin-Teller criticality in bond correlations at wavevector $(\pi, \pi)$; this is in addition to the expected critical fluctuations of the conventional p-VBS order parameter at wavevectors $(\pi,0)$ and $(0,\pi)$.

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arXiv2023

Flux fractionalization transition in anisotropic $S=1$ antiferromagnets and dimer-loop models

We demonstrate that the low temperature ($T$) properties of a class of anisotropic spin $S=1$ kagome (planar pyrochlore) antiferromagnets on a field-induced $\frac{1}{3}$-magnetization ($\frac{1}{2}$-magnetization) plateau are described by a model of fully-packed dimers and loops on the honeycomb (square) lattice, with a temperature-dependent relative fugacity $w(T)$ for the dimers. The fully-packed O(1) loop model ($w=0$) and the fully-packed dimer model ($w=\infty$) limits of this dimer-loop model are found to be separated by a phase transition at a finite and nonzero critical fugacity $w_c$, with interesting consequences for the spin correlations of the frustrated magnet. The $w>w_c$ phase has short loops and spin correlations dominated by power-law columnar order (with subdominant dipolar correlations), while the $w w_c$, proliferate in the $w<w_c$ phase and destroy power-law columnar order.

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arXiv2023

Band-center metal-insulator transition in bond-disordered graphene

We study the transport properties of a tight-binding model of non-interacting fermions with random hopping on the honeycomb lattice. At the particle-hole symmetric chemical potential, the absence of diagonal disorder (random onsite potentials) places the system in the well-studied chiral orthogonal universality class of disordered fermion problems, which are known to exhibit both a critical metallic phase and a dimerization-induced localized phase. Here, our focus is the behavior of the two-terminal conductance and the Lyapunov spectrum in quasi-1D geometry near the dimerization-driven transition from the metallic to the localized phase. For a staggered dimerization pattern on the square and honeycomb lattices, we find that the renormalized localization length $\xi/M$ ($M$ denotes the width of the sample) and the typical conductance display scaling behavior controlled by a crossover length-scale that diverges with exponent $\nu \approx 1.05(5)$ as the critical point is approached. However, for the plaquette dimerization pattern, we observe a relatively large exponent $\nu \approx 1.55(5)$ revealing an apparent non-universality of the delocalization-localization transition in the BDI symmetry class.

cond-mat.dis-nn↗

arXiv2022

Theory of collective topologically-protected Majorana fermion excitations of networks of localized Majorana modes

Predictions of localized Majorana modes, and ideas for manipulating these degrees of freedom, are the two key ingredients in proposals for physical platforms for Majorana quantum computation. Several proposals envisage a scalable network of such Majorana modes coupled bilinearly to each other by quantum-mechanical mixing amplitudes. Here, we develop a theoretical framework for characterizing collective topologically protected zero-energy Majorana fermion excitations of such networks of localized Majorana modes. A key ingredient in our work is the Gallai-Edmonds decomposition of a general graph, which we use to obtain an alternate ``local'' proof of a ``global'' result of Lov{á}sz and Anderson on the dimension of the topologically protected null space of {\em real skew-symmetric} (or pure-imaginary hermitean) adjacency matrices of general graphs. Our approach to Lov{á}sz and Anderson's result constructs a maximally-localized basis for the said null-space from the Gallai-Edmonds decomposition of the graph. Applied to the graph of the Majorana network in question, this gives a method for characterizing basis-independent properties of these collective topologically protected Majorana fermion excitations, and relating these properties to the correlation function of monomers in the ensemble of maximum matchings (maximally-packed dimer covers) of the corresponding network graph. Our approach can also be used to identify signatures of zero-energy excitations in systems modeled by a free-fermion Hamiltonian with a hopping matrix of this type; an interesting example is provided by vacancy-induced Curie tails in generalizations (on non-bipartite lattices) of Kitaev's honeycomb model.

cond-mat.str-el↗

arXiv2021

Quantum half-orphans in kagome antiferromagnets

We numerically study the effects of non-magnetic impurities (vacancies) in the spin-$S$ Heisenberg antiferromagnet on the kagomé lattice. For a range of low but nonzero temperatures, and spin values that extend down to $S=2$, we find that the magnetization response to an external magnetic field is consistent with the response of emergent "half-orphan" degrees of freedom that are expected to dominate the response of the corresponding classical magnet in a similar temperature range whenever there are two vacancies on the same triangle. Specifically, for all spin values we have considered (from $S=1/2$ to $S=4$), there is a large enhancement of the local susceptibility of the lone spin on such a triangle with two vacancies; in the presence of a uniform magnetic field $h$, this lone-spin behaves effectively as an almost free spin $S$ in an effective field $h/2$. Quite remarkably, in the zero temperature limit, the ground state in the presence of a half-orphan has a non-zero total spin value $S_{GS}$ that shows a trend similar to $S/2$ when $S\ge 2$. These qualitative aspects of the response differ strikingly from the more conventional response of diluted samples without such half-orphan degrees of freedom. We discuss how these findings could be checked experimentally.

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arXiv2020

Nonlinear Fluctuating Hydrodynamics for the Classical XXZ Spin Chain

Using the framework of nonlinear fluctuating hydrodynamics (NFH), we examine equilibrium spatio-temporal correlations in classical ferromagnetic spin chains with nearest neighbor interactions. In particular, we consider the classical XXZ-Heisenberg spin chain (also known as Lattice Landau Lifshitz or LLL model) evolving deterministically and chaotically via Hamiltonian dynamics, for which energy and $z$-magnetization are the only locally conserved fields. For the easy-plane case, this system has a low-temperature regime in which the difference between neighboring spin's angular orientations in the XY plane is an \textit{almost conserved} field. According to the predictions of NFH, the dynamic correlations in this regime exhibit a heat peak and propagating sound peaks, all with anomalous broadening. We present a detailed molecular dynamics test of these predictions and find a reasonably accurate verification. We find that, in a suitable intermediate temperature regime, the system shows two sound peaks with Kardar-Parisi-Zhang (KPZ) scaling and a heat peak where the expected anomalous broadening is less clear. In high temperature regimes of both easy plane and easy axis case of LLL, our numerics show clear diffusive spin and energy peaks and absence of any sound modes, as one would expect. We also simulate an integrable version of the XXZ-model, for which the ballistic component instead moves with a broad range of speeds rather than being concentrated in narrower peaks around the sound speed.

cond-mat.stat-mech↗

arXiv2020

Extremal statistics of entanglement eigenvalues can track the many-body localized to ergodic transition

Some interacting disordered many-body systems are unable to thermalize when the quenched disorder becomes larger than a threshold value. Although several properties of nonzero energy density eigenstates (in the middle of the many-body spectrum) exhibit a qualitative change across this many-body localization (MBL) transition, many of the commonly-used diagnostics only do so over a broad transition regime. Here, we provide evidence that the transition can be located precisely even at modest system sizes by sharply-defined changes in the distribution of extremal eigenvalues of the reduced density matrix of subsystems. In particular, our results suggest that $p* = \lim_{λ_2 \rightarrow \ln(2)^{+}}P_2(λ_2)$, where $P_2(λ_2)$ is the probability distribution of the second lowest entanglement eigenvalue $λ_2$, behaves as an ''order-parameter'' for the MBL phase: $p*> 0$ in the MBL phase, while $p* = 0$ in the ergodic phase with thermalization. Thus, in the MBL phase, there is a nonzero probability that a subsystem is entangled with the rest of the system only via the entanglement of one subsystem qubit with degrees of freedom outside the region. In contrast, this probability vanishes in the thermal phase.

cond-mat.dis-nn↗

arXiv2019

Efficient quantum cluster algorithms for frustrated transverse field Ising antiferromagnets and Ising gauge theories

Working within the Stochastic Series Expansion (SSE) framework, we construct efficient quantum cluster algorithms for transverse field Ising antiferromagnets on the pyrochlore lattice and the planar pyrochlore lattice, for the fully frustrated square lattice Ising model in a transverse field (dual to the 2+1 dimensional odd Ising gauge theory), and for a transverse field Ising model with multi-spin interactions on the square lattice, which is dual to a 2+1 dimensional even Ising gauge theory (and reduces to the two dimensional quantum loop model in a certain limit). Our cluster algorithms use a microcanonical update procedure that generalizes and exploits the notion of "pre-marked motifs" introduced earlier in the context of a quantum cluster algorithm for triangular lattice transverse field Ising antiferromagnets. We demonstrate that the resulting algorithms are significantly more efficient than the standard link percolation based quantum cluster approach. We also introduce a new canonical update scheme that leads to a further improvement in measurement of some observables arising from its ability to make one-dimensional clusters in the "imaginary time" direction. Finally, we demonstrate that refinements in the choice of premarking strategies can lead to additional improvements in the efficiency of the microcanonical updates. As a first example of the physics that can be studied using these algorithmic developments, we obtain evidence for a power-law ordered intermediate-temperature phase associated with the two-step melting of long-range order in the fully frustrated square lattice transverse field Ising model.

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arXiv2019

Phase diagram of a system of hard cubes on the cubic lattice

We study the phase diagram of a system of $2\times2\times2$ hard cubes on a three dimensional cubic lattice. Using Monte Carlo simulations, we show that the system exhibits four different phases as the density of cubes is increased: disordered, layered, sublattice ordered, and columnar ordered. In the layered phase, the system spontaneously breaks up into parallel slabs of size $2\times L \times L$ where only a very small fraction cubes do not lie wholly within a slab. Within each slab, the cubes are disordered; translation symmetry is thus broken along exactly one principal axis. In the solid-like sublattice ordered phase, the hard cubes preferentially occupy one of eight sublattices of the cubic lattice, breaking translational symmetry along all three principal directions. In the columnar phase, the system spontaneously breaks up into weakly interacting parallel columns of size $2\times 2\times L$ where only a very small fraction cubes do not lie wholly within a column. Within each column, the system is disordered, and thus translational symmetry is broken only along two principal directions. Using finite size scaling, we show that the disordered-layered phase transition is continuous, while the layered-sublattice and sublattice-columnar transitions are discontinuous. We construct a Landau theory written in terms of the layering and columnar order parameters, which is able to describe the different phases that are observed in the simulations and the order of the transitions. Additionally, our results near the disordered-layered transition are consistent with the $O(3)$ universality class perturbed by cubic anisotropy as predicted by the Landau theory.

cond-mat.stat-mech↗

arXiv2018

Statistical properties of worm algorithms for two dimensional frustrated Ising models

We study the distribution of lengths and other statistical properties of worms constructed by Monte Carlo worm algorithms in the power-law three-sublattice ordered phase of frustrated triangular and kagome lattice Ising antiferromagnets. Viewing each step of the worm construction as a position increment (step) of a random walker, we demonstrate that the persistence exponent $\theta$ and the dynamical exponent $z$ of this random walk depend only on the universal power-law exponents of the underlying critical phase, and not on the details of the worm algorithm or the microscopic Hamiltonian. Further, we argue that the detailed balance criterion obeyed by such worm algorithms and the power-law correlations of the underlying equilibrium system together give rise to two related properties of this random walk: First, the steps of the walk are expected to be power-law correlated in time. Second, the position distribution of the walker relative to its starting point is given by the equilibrium position distribution of a particle in an attractive logarithmic central potential of strength $\eta_m$, where $\eta_m$ is the universal power-law exponent of the equilibrium defect-antidefect correlation function of the underlying spin system. We derive a scaling relation, $z = (2-\eta_m)/(1-\theta)$, that allows us to express the dynamical exponent $z(\eta_m)$ of this process in terms of its persistence exponent $\theta(\eta_m)$. Our measurements of $z(\eta_m)$ and $\theta(\eta_m)$ are consistent with this relation over a range of values of the universal equilibrium exponent $\eta_m$, and yield subdiffusive ($z>2$) values of $z$ in the entire range. Thus we demonstrate that the worms represent a discrete-time realization of a fractional Brownian motion characterized by these properties.

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