arXiv subjects

Theja N. De Silva

Publications and source records attributed to Theja N. De Silva.

At least 19 recordsLinked to original sources

arXiv2026

Superconducting and correlated phases of an effective Hubbard model on the BCC lattice

We investigate the electronic phases of an effective Hubbard model on the body-centered-cubic lattice, motivated by alkali-doped fulleride molecular solids. The model incorporates renormalized on-site interactions and an effective inverted Hund's coupling originating from electron-phonon interactions. To access complementary interaction regimes, we employ two theoretical approaches. In the intermediate-coupling regime, the on-site repulsive interaction is approximated by a long-range interaction in momentum space, yielding an exactly solvable Hatsugai-Kohmoto model supplemented by a BCS-type pairing term. Within this framework, we analyze the superconducting instability and demonstrate a first-order normal-superconducting phase transition, characterized by a discontinuous jump of the order parameter. In the strong-coupling regime, where pairing fluctuations are suppressed, we apply the spin rotationally invariant slave-boson formalism to map out the temperature-interaction phase diagram. This analysis reveals first-order transitions between a Fermi-liquid phase, an antiferromagnetic phase, and a Mott insulating phase, with a narrow intermediate region where all three phases compete. The resulting phase diagram captures the interplay of itinerancy, magnetic order, and Mott localization in three dimensions and provides a unified perspective on superconducting and correlation-driven phenomena in fulleride-inspired lattice systems.

cond-mat.str-el↗

arXiv2025

Quantum Filtering of Hydrogen Isotopes through Graphene

Driven by the growing demand in the energy, medical, and industrial sectors, we investigate a hydrogen isotope separation technique that offers both a high separation factor and economic feasibility. Our findings reveal that filtering isotopes through two-dimensional graphene layers provides an exceptionally efficient quantum-mechanical method for isotope separation. Using a recently developed analytical pairwise potential between hydrogen isotopes and carbon atoms in graphene, we examine the classical trajectories of isotopes near the graphene layer, as well as the quantum-mechanical tunneling properties of isotopes through the graphene layer. Using various quantum-mechanical methods, we calculate both the isotope tunneling probabilities and the quantum-mechanical isotope sticking probabilities. Our study shows that quantum filtering through graphene layers can be an effective technique for enriching deuterium by separating it from protium.

cond-mat.mes-hall↗

arXiv2024

Two-band atomic superfluidity in the presence of an orbital Feshbach resonance

We study static superfluid properties of alkali-earth-like Fermi atomic systems in the presence of orbital Feshbach resonance. Using a two-band description of the ground state and excited state and a mean-field approximation of the intra-band atomic pairing, we investigate the phase transitions and crossover between BCS and Bose-Einstein-condensate (BEC) superfluidity. At zero temperature, we find exact BCS analytical solutions for the mean-field gap equations and number equations. Using these exact solutions, we calculate various static properties, such as superfluid order parameters, chemical potentials, density variations, density profiles, correlation and coherence lengths, ground-state energy, and Tan's contact density across the entire BCS-BEC crossover region. We anticipate that our closed-form analytical results can be used as a benchmark for future experimental and theoretical investigations and will have an impact on the current understanding of two-band superconductors such as MgB$_2$.

cond-mat.quant-gas↗

arXiv2021

Massive electrons and unconventional room-temperature superconductivity in superhydrides

The search for room-temperature superconducting materials has been at the center of modern research for decades. The recent discovery of high-temperature superconductivity, under extreme pressure in hydrogen-rich materials, is a tremendous achievement in this research front. This discovery offers a route in the search for room temperature superconductivity at ambient pressure. The superconductivity of these hydrogen-rich materials was confirmed by the observation of zero-resistance, isotope effects, effect of magnetic field, and other standard properties. However, some of the experimental features were puzzling as they were not consistent with the known superconductivity theories. These debatable features have lead to a series of recent publications downplaying the existence of superconductivity in these superhydrides. Here we propose a concept of massive electrons under pressure and successfully explain all non-standard experimental observations. Our massive electron concept explains the large effective mass of the quasiparticles, the reason for the high critical temperatures for moderate electron-phonon couplings, and a 3-5 orders of magnitude larger conductivity causing a narrow resistivity broadening at the transition in the presence of magnetic field. We anticipate our findings will lead to a new directions and tweaks in current research in the search for ambient-pressure, room-temperature superconductors.

cond-mat.supr-con↗

arXiv2020

Asymptotic behavior of correlation functions of one-dimensional polar-molecules on optical lattices

We combine a slave-spin approach with a mean-field theory to develop an approximate theoretical scheme to study the density, spin, and, pairing correlation functions of fermionic polar molecules. We model the polar molecules subjected to a one-dimensional periodic optical lattice potential using a generalized $t-J$ model, where the long-range part of the interaction is included through the exchange interaction parameter. For this model, we derive a set of self-consistent equations for the correlation functions, and evaluate them numerically for the long-distance behaviour. We find that the pairing correlations are related to spin correlations through the density and the slave-spin correlations. Further, our calculations indicates that the long-range character of the interaction can be probed through these correlation functions. In the absence of exact solutions for the one-dimensional $t-J$ model, our approximate theoretical treatment can be treated as a useful tool to study one dimensional long-range correlated fermions.

cond-mat.quant-gas↗

arXiv2020

Molecular Self-Organization of Three-Component Lipid Membranes

By constructing a Landau-like energy functional, we investigate the molecular organization of a three-component mixture in cell membranes. In the strongly interacting limit, we model the interaction between molecules using pseudospin variables and convert them into non-interacting variables using a mean-field theory. Next, we construct the two-order parameter Landau-type energy functional through the Helmholtz free energy. By analyzing the Landau free energy, we map out the phase diagram focusing on homogeneous and various phase separated states on the cell membrane.

physics.bio-ph↗

arXiv2019

Theoretical Phase diagram of unconventional alkali-doped fullerides

By constructing an effective model based on recently calculated \emph{ab }initio bare interaction parameters, we study the phase diagram of alkali doped fullerides as a function of temperature and internal pressure. We use a slave-rotor mean-field approach at the weak and intermediate coupling limits and a variational mean-field approach at the strong coupling limit, and find a good agreement with experimental phase diagram. We explain the unified description of the phase diagram including the proximity of s-wave superconducting state and the Mott-insulating state, and the existence of Jahn-Teller distorted metallic state using orbital selective physics. We ague that the double electronic occupation of two degenerate orbitals triggers both s-wave superconductivity and Jahn-Teller distortion. While the orbital ordering of two electrons causes the distortion, the remaining single electron in the third orbital causes the metal-insulator transition.

cond-mat.str-el↗

arXiv2019

Bio-energy Transport as a Phonon Dressed Vibrational Exciton in Protein Molecules

Following the ideas of Davydov's soliton theory, we study the bio-energy transport in protein molecules. By using a quantum Brownian motion model for a phonon dressed vibrational exciton, we calculate the time-dependence on the mean square distance, diffusion coefficient, and energy of the vibrational exciton. We find the time-dependence by solving the quantum Langevin equation and find oscillatory behaviors due to the super-diffusive non-ohmic dissipation. We find that the vibrational exciton gains an overall energy due to the coupling to the phonon bath; it also dissipates its energy to the environment as it propagates. The amount of energy gain and the oscillatory features depend on both temperature and the phonon-vibron coupling.

physics.bio-ph↗

arXiv2018

A statistical mechanics perspective for protein folding from $q$-state Potts model

The folding of a peptide chain into a three dimensional structure is a thermodynamically driven process such that the chain naturally evolves to form domains of similar amino acids. The formation of this domain occurs by curling the one dimensional amino acid sequence by moving similar amino acids proximity to each other. We model this formation of domains or ordering of amino acids using q-state Potts model and study the thermodynamic properties using a statistical mechanics approach. Converting the interacting amino acids into an effectively non-interacting model using a mean-field theory, we calculate the Helmholtz free energy (HFE). Then by investigating the HFE, we study the properties of protein folding transition qualitatively. We find that the protein folding phase transition is a strongly first order and the specific heat shows the experimental signatures of this phase transition. Further, we compare these mean-field results with exact transfer matrix results in one dimension and then large $q$ expansion results in two dimensions.

cond-mat.stat-mech↗

arXiv2018

Metal-Insulator-Superconductor transition of spin-3/2 atoms on optical lattices

We use a slave-rotor approach within a mean-field theory to study the competition of metallic, Mott-insulating, and superconducting phases of spin-3/2 fermions subjected to a periodic optical lattice potential. In addition to the metallic, the Mott-insulating, and the superconducting phases that are associated with the gauge symmetry breaking of the spinon field, we identify a novel emerging superconducting phase that breaks both roton and spinon field gauge symmetries. This novel superconducting phase emerges as a result of the competition between spin-0 singlet and spin-2 quintet interaction channels naturally available for spin-3/2 systems. The two superconducting phases can be distinguished from each other by quasiparticle weight. We further discuss the properties of these phases for both two-dimensional square and three-dimensional cubic lattices at zero and finite temperatures.

cond-mat.quant-gas↗

arXiv2016

An Effective Series Expansion to the Equation of State of Unitary Fermi Gases

Using universal properties and a basic statistical mechanical approach, we propose a general equation of state for unitary Fermi gases. The universal equation of state is written as a series solution to a self consistent integral equation where the general solution is a linear combination of Fermi functions. First, by truncating our series solution to four terms with already known exact theoretical inputs at limiting cases, namely the first \emph{three} virial coefficients and using the Bertsch parameter as a free parameter, we find a good agreement with experimental measurements in the entire temperature region in the normal state. This analytical equation of state agrees with experimental data up to the fugacity $z = 18$, which is a vast improvement over the other analytical equations of state available where the agreements is \emph{only} up to $z \approx 7$. Second, by truncating our series solution to four terms again using first \emph{four} virial coefficients, we find the Bertsch parameter $ξ=0.35$, which is in good agreement with the direct experimental measurement of $ξ=0.37$. This second form of equation of state shows a good agreement with self-consistent T-matrix calculations in the normal phase.

cond-mat.quant-gas↗

arXiv2016

An effective mean field theory for the coexistence of anti-ferromagnetism and superconductivity: Applications to iron-based superconductors and cold Bose-Fermi atomic mixtures

We study an effective fermion model on a square lattice to investigate the cooperation and competition of superconductivity and anti-ferromagnetism. In addition to particle tunneling and on-site interaction, a bosonic excitation mediated attractive interaction is also included in the model. We assume that the attractive interaction is mediated by spin fluctuations and excitations of Bose-Einstein condensation (BEC) in electronic systems and Bose-Fermi mixtures on optical lattices, respectively. Using an effective mean-field theory to treat both superconductivity and anti-ferromagnetism at equal footing, we study the model within the Landau energy functional approach and a linearized theory. Within our approaches, we find possible co-existence of superconductivity and anti-ferromagnetism for both electronic and cold-atomic models. Our linearized theory shows while spin fluctuations favor d-wave superconductivity and BEC excitations favor s-wave superconductivity.

cond-mat.supr-con↗

arXiv2015

Phase diagram of strongly attractive $p$-orbital fermions on optical lattices

We examine a system of doubly degenerate $p$-orbital polarized fermions on a two-dimensional square lattice with a strong on-site interaction. We consider the system density at the half filling limit and tackle the strong attractive interaction using a perturbation theory. We treat the four-site square plaquette interaction term generated from the directional tunneling dependence of $p$-orbitals using the fourth order in perturbation theory. We map the strong coupling particle Hamiltonian into an effective spin-Hamiltonian and then use a variational mean field approach and a linear spin-wave theory to study the phase diagram. Further, we discuss the experimental signatures of these phases within the context of current cold-atom experimental techniques.

cond-mat.quant-gas↗

arXiv2015

Polarization-induced phase separation and re-entrant transition of two component-fermions in a one-dimensional lattice

By investigating the compressibility of one-dimensional lattice fermions at various filling factors, we study the phase separation and re-entrant transition within the framework of the Bethe ansatz method. We model the system using the repulsive Hubbard model and calculate compressibility as a function of polarization for arbitrary values of chemical potential, temperature, and interaction strength. For filling factors $ 0 < n < 1$, compressibility is a non-monotonic function of polarization at all thermodynamic parameters. The compressibility reveals a phase transition into a phase-separated state for both low and intermediate temperatures at intermediate interactions as one increases the polarization. For certain filling factors, we find the re-entrant transition into the mixed phase at a higher polarization.

cond-mat.quant-gas↗

arXiv2014

Evidence for the breakdown of momentum independent many-body t-matrix approximation in the normal phase of Bosons

We revisit the momentum independent many-body t-matrix approach for boson systems developed by Shi and Griffin~\cite{sg} and Bijlsma and Stoof~\cite{bs}. Despite its popularity, simplicity, and expected advantage of being its applicability to both normal and superfluid phases, we find that the theory breaks down in the normal phase of bosons. We conjecture that this failure is due to neglecting of momentum dependence on the t-matrix.

cond-mat.quant-gas↗

arXiv2013

Phase diagram of two-component dipolar fermions in one-dimensional optical lattices

We theoretically map out the ground state phase diagram of interacting dipolar fermions in one-dimensional lattice. Using a bosonization theory in the weak coupling limit at half filing, we show that one can construct a rich phase diagram by changing the angle between the lattice orientation and the polarization direction of the dipoles. In the strong coupling limit, at a general filing factor, we employ a variational approach and find that the emergence of a Wigner crystal phases. The structure factor provides clear signatures of the particle ordering in the Wigner crystal phases.

cond-mat.quant-gas↗

arXiv2012

Thermodynamic Properties of Universal Fermi Gases

We develop a simple, mean-field-like theory for the normal phase of a unitary Fermi gas by deriving a self-consistent equation for its self-energy via a momentum-dependent coupling constant for both attractive and repulsive universal fermions. For attractive universal fermions in the lower branch of a Feshbach resonance, we use zero-temperature Monte Carlo results as a starting point for one-step iteration in order to derive an analytical expression for the momentum-dependent self-energy. For repulsive universal fermions in the upper branch of a Feshbach resonance, we iteratively calculate the momentum-dependent self-energy via our self-consistent equation. Lastly, for the case of population imbalance, we propose an ansatz for higher order virial expansion coefficents. Overall, we find that our theory is in good agreement with currently available, high temperature experimental data.

cond-mat.quant-gas↗

arXiv2012

Spin diffusion of lattice fermions in one dimension

We study long-time spin diffusion of harmonically trapped lattice fermions in one dimension. Combining thermodynamic Bethe ansatz approach and local density approximation, we calculate spin current and spin diffusion coefficient driven by the population imbalance. We find spin current is driven by susceptibility effects rather than typical diffusion where magnetization would transport from regions of high magnetization to low. As expected, spin transport is zero through insulating regions and are only present in the metallic regions. In the weak coupling limit, the local spin diffusion coefficient shows maxima at all the insulating regions. Further, we estimate damping rate of diffusion modes in the weak coupling limit within the lower metallic portion of the cloud. The predicted spin current pattern can be probed via currently available experimental techniques.

cond-mat.quant-gas↗