arXiv subjects

Efstratios Manousakis

Publications and source records attributed to Efstratios Manousakis.

At least 19 recordsLinked to original sources

arXiv2026

Gradient expansion approximation of the inhomogeneous electron-gas revisited: Higher-order corrections

In our recently published work (our Ref. 1) we revisited the gradient expansion approximation (GEA) of the interacting electron gas, and recalculated the leading-order contribution$-$with respect to the Wigner-Seitz radius $r_s$$-$to the coefficient $B_{xc}[n]$ of the square of the gradient of the electron density in the high-density and slowly varying limits. That work resolved historical controversies regarding these coefficients and demonstrated that serious misconceptions have led to incorrect constraints being imposed on popular functionals within the generalized gradient approximation (GGA). In the present paper, we extend this calculation to obtain the coefficient of the next-to-leading term, which scales as $r_s \ln(r_s)$ relative to the leading order. First, we establish a systematic framework to evaluate the integral expressions for the $b_{xc}$ coefficient of the leading term ($\sim q^2$) of the density-density response function in the long-wavelength limit ($q \to 0$)$-$a prerequisite for computing $B_{xc}[n]$. The significance of the calculation stems from the proof that the coefficient of this $r_s \ln(r_s)$ term receives no corrections from higher-order diagrammatic expressions. Consequently, our derived value serves as an exact, definitive constraint for future GGA functional development; in the high-density slowly-varying limit, any valid functional must reproduce the exact constraints established in both our previous work and the present paper.

cond-mat.mtrl-sci↗

arXiv2026

Weyl excitonic condensation

We consider a half-filled two-dimensional Su-Schrieffer-Heeger lattice and examine the role of the long-range Coulomb electron-hole attractive interaction. We demonstrate that, under specific conditions, a rare interplay of topological and excitonic-collective behavior emerges as a novel state of matter. A unique Bose-Einstein condensate of excitons forms, exhibiting co-presence of pseudo-spin chiral texture. The emerging complex order-parameter, a particle-hole pairing-gap, has non-zero real and imaginary parts throughout the Brillouin zone (BZ) but vanish separately on two different nodal lines, which intersect at two Weyl points. The Weyl nodes possess opposite pseudo-spin chiralities, which act as source and drain of a Berry-flux associated with the particle-hole pairing-wavefunction, and are the cause of Bogoliubov-deGennes Fermi-arc edge-states. We self-consistently calculate the full momentum-dependence of the particle-hole pairing gap throughout the entire BZ. Near the Weyl points, the pairing gap exhibits the unconventional time-reversal-symmetry breaking $p_x+ip_y$ character. Finally, we discuss general potential experimental realizations of this novel state of matter.

cond-mat.str-el↗

arXiv2026

Gradient-expansion of the inhomogeneous electron-gas revisited

In the present work, we revisit the problem of the inhomogeneous electron gas under the influence of a weak external potential, which allows us to calculate the gradient corrections to the density functional within linear response, an approach known as the gradient expansion approximation. To obtain the exchange ($b_x$) and correlation ($b_{c}$) contributions to the coefficient $b_{xc}$, i.e., to the prefactor of the $q^2$ term of the proper-polarization function, we revisited all the previous calculations and expose misconceptions which led to incorrect conclusions. We used various ways to apply a necessary regularization to the singular Coulomb interaction potential. We found that the separate exchange ($b_x$) and correlation ($b_c$) contributions to the coefficient $b_{xc}$ have regularization-scheme dependent values even though the regulator is set to zero at the end of the calculation. This implies that it is impossible to define such a separation meaningfully. On the contrary, we found that when the regulator is set to zero at the end of the calculation, the combination $b_{xc}$ is regularization-scheme independent and, thus, has a unique value. We conclude that it is incorrect to separate those two terms when constructing a generalized-gradient-approximation (GGA) contribution to the density functional. This appears to be a common approach in most popular GGA functionals, where various constraints are applied to each contribution separately.

cond-mat.mtrl-sci↗

arXiv2026

Twisted bilayer graphene from first-principles: structural and electronic properties

We present a comprehensive first-principles study of twisted bilayer graphene (tBLG) for a wide range of twist angles, with a focus on structural and electronic properties. By employing density functional theory (DFT) with an optimized local basis set, we simulate tBLG, obtaining fully relaxed commensurate structures for twist angles down to 0.987{\deg}. For all angles the lattice relaxation agrees well with continuum elastic models. For angles accessible to plane-wave DFT (VASP), we provide a detailed comparison with our local basis DFT (SIESTA) calculations, demonstrating excellent agreement in both the atomic and electronic structure. The dependence of the Fermi velocity and band width on the twist angle shows qualitative agreement with results from an `exact' $\mathbf{k \cdot p}$ continuum model, but reveals a small twist angle offset. Additionally, we provide details of the low-energy wavefunction character, band inversion and symmetries. Our results provide an ab initio reference point for the microscopic structure and electronic properties of tBLG which will serve as the foundation for future studies incorporating many-body effects.

cond-mat.mes-hall↗

arXiv2025

Nonreciprocal Transport with Quantum Geometric Origin in Layered Hybrid Perovskite

Quantum geometry quantifies how the electron wavefunction evolves distinctly from conventional transport theory. In noncentrosymmetric materials, nonreciprocal transport with quantum geometric origin remains prominent with localized charge independent of vanished group velocity. The discovery of such nonreciprocal and nonlinear responses was realized by recent advances in two-dimensional materials. As a promising candidate, the electronic structure and symmetry of layered hybrid perovskites can be deliberately designed and manipulated by incorporating selected organic ligands. Despite the observation of exotic photogalvanic effects and chiral optical effects, the underlying mechanism how these nonlinear responses are enabled in the multi-quantum well structures remained unclear. Here we demonstrated the quantum geometric origin for interlayer spontaneous photocurrent in (PEA)2PbI4. Contrary to assumptions that charge transport across the 2D planes is limited, we observed a spontaneous photocurrent along this crystalline orientation. Theoretical analysis using a tight-binding model identifies shift current as the microscopic origin. This quantum geometric effect is enabled by ionic displacements from centrosymmetric coordinates and enhanced by multiband transition high-density bands of the layered hybrid crystal. We anticipate that such unique low-dimensional systems with structure can provide fertile ground for discovering novel optoelectronic functionalities.

cond-mat.mtrl-sci↗

arXiv2025

Wavefunction textures in twisted bilayer graphene from first principles

Motivated by recent experiments probing the wavefunctions of magic-angle twisted bilayer graphene (tBLG), we perform large-scale first-principles calculations of tBLG with full atomic relaxation across a wide range of twist angles down to $0.99^\circ$. Focusing on the magic angle, we compute wavefunctions of the low energy bands, resolving atomic-scale details and moir\'e-scale patterns that form triangular, honeycomb, and Kagome lattices. By tuning the interlayer interactions, we illustrate the formation of the flat bands from isolated monolayers and the emergence of the band inversion and fragile topology at a sufficiently large interaction strength. We identify strong indicators of a new phase transition with increasing interlayer interaction strength, achievable with external pressure or a decrease in the twist angle. When this transition occurs, the upper and lower flat bands exchange their wavefunction character and symmetry eigenvalues, which may be correlated with the appearance of superconductivity with electron doping below the magic angle. Our study demonstrates the feasibility of using first-principles wavefunctions to help interpret experimental signatures of topological and correlated phases in tBLG.

cond-mat.mes-hall↗

arXiv2024

QUANTUM ESPRESSO implementation of the RPA-based functional

We detail our implementation of the random-phase-approximation based functional (RPAF) derived in our previous publication [Phys. Rev. B 110, 195151 (2024)] for the QUANTUM ESPRESSO (QE) package. We also make available the source files required in order to apply this functional within QE. We also provide the corresponding RPAF projector augmented wave (PAW) and ultrasolf pseudopotentials for most elements. Lastly, we benchmark the performance of the RPAF by calculating the equilibrium lattice constant and bulk modulus of a set of the same 60 crystals used by other authors to benchmark other functionals for both PAW and ultrasoft pseudopotentials. We find that the RPAF performs better overall as compared to the other most popular functionals.

cond-mat.mtrl-sci↗

arXiv2024

Accurate electron correlation-energy functional: Expansion in an interaction renormalized by the random-phase approximation

We present an accurate local density-functional for electronic-structure calculations within the density functional theory (DFT). The functional is derived by analyzing the structure of the standard perturbative expansion of the correlation energy of the interacting uniform electron gas. Then, the expansion is partially re-summed and reorganized as a self-consistent series in powers of a renormalized electron-electron interaction vertex based on the screened frequency-momentum dependent dielectric matrix given by the well-known random-phase approximation. First, we demonstrate that the range of $r_s$, where this reorganized and renormalized series converges, contains and is significantly larger than the average range realized in real crystalline materials. Using a combination of analytical, numerical, and stochastic integration techniques we are able to calculate all the diagrams which have contribution up to the same leading order. We benchmarked the functional using the Quantum ESPRESSO implementation of the DFT applied to the same list of materials, selected previously by other authors, in its entirety without any modification of the list. We find that for ground-state properties in general, such as, equilibrium atomic distances and bulk moduli, the functional presented here is more accurate than the currently available most popular one.

cond-mat.mtrl-sci↗

arXiv2024

Lattice-mismatched and twisted multi-layered materials for efficient solar cells

We argue that alternating-layer structures of lattice mismatched or misaligned (twisted) atomically-thin layers should be expected to be more efficient absorbers of the broad-spectrum of solar radiation than the bulk material of each individual layer. In such mismatched layer-structures the conduction and valence bands of the bulk material, split into multiple minibands separated by minigaps confined to a small-size emerging Brillouin zone due to band-folding. We extended the Shockley-Queisser approach to calculate the photovoltaic efficiency for a band split into minibands of bandwidth $\Delta E$ and mini-gaps $\delta G$ to model the case when such structures are used as solar cells. We find a significant efficiency enhancement due to impact ionization processes, especially in the limit of small but non-zero $\delta G$, and a dramatic increase when fully concentrated sun-light is used.

cond-mat.mtrl-sci↗

arXiv2024

Excitonic-trion population in two-dimensional halide perovskites

There are many reports of a surprisingly high charge-carrier density with sizable mobility in photo-excited two-dimensional (2D) halide perovskites despite their unusually high exciton binding-energy. In this work we study the thermodynamic quasi-equilibrium of the relative population of photoexcited free quasi-electron/quasi-hole pairs, neutral-excitons and excitonic trions, in 2D materials that support such excitonic complexes with large binding energy. We derive and solve the general Saha equations which describe the detailed balance of such a system of photo-excited electronic degrees of freedom forming a multi-component fluid of excitations in thermodynamic quasi-equilibrium.The solution to these equations, for the special case of 2D perovskites where the reported exciton and excitonic trion binding-energies are of the order of 0.3-0.4 eV for the former and 30-40 meV for the latter, reveals that while the charge-neutral excitonic population dominates all other excitations, at room temperature and below, the excitonic trion component can be the dominant population among charge carriers. We also argue that trionic hopping can take place via a tunneling mechanism which is speculated to play a role in a novel charge-transport mechanism.

cond-mat.mtrl-sci↗

arXiv2023

Quantum Weyl-Heisenberg antiferromagnet

Beginning from the conventional square-lattice nearest-neighbor antiferromagnetic Heisenberg model, we allow the $J_x$ and $J_y$ couplings to be anisotropic, with their values depending on the bond orientation. The emergence of anisotropic, bond-dependent, couplings should be expected to occur naturally in most antiferromagnetic compounds which undergo structural transitions that reduce the point-group symmetry at lower temperature. Using the spin-wave approximation, we study the model in several parameter regimes by diagonalizing the reduced Hamiltonian exactly, and computing the edge spectrum and Berry connection vector, which show clear evidence of localized topological charges. We discover phases that exhibit Weyl-type spin-wave dispersion, characterized by pairs of degenerate points and edge states, as well as phases supporting lines of degeneracy. We also identify a parameter regime in which there is an exotic state hosting gapless linear spin-wave dispersions with different longitudinal and transverse spin-wave velocities. Author email info: prosenberg15@gmail.com and manousakis@gmail.com

cond-mat.str-el↗

arXiv2023

Towards understanding the electronic structure of the simpler members of two-dimensional halide-perovskites

In this paper we analyze the band-structure of two-dimensional (2D) halide perovskites by considering structures related to the simpler case of the series, (BA)$_2$PbI$_4$, in which PbI$_4$ layers are intercalated with butylammonium (BA=CH$_3$(CH$_2$)$_3$NH$_3$) organic ligands. We use density-functional-theory (DFT) based calculations and tight-binding (TB) models aiming to discover a simple description of the bands in the vicinity of the valence-band maximum and the conduction-band minimum. We find that the atomic orbitals of the butylammonium chains have negligible contribution to the Bloch states which form the conduction and valence bands in near the Fermi energy. Our calculations reveal a rather universal, i.e., independent of the intercalating BA, rigid-band picture characteristic of the layered perovskite ``matrix''. Besides demonstrating the above conclusion, the main goal of this paper is to find accurate TB models which capture the essential features of the DFT bands near the Fermi energy. First, we ignore electron hopping along the $c$-axis and the octahedral distortions and this increased symmetry halves the Bravais-lattice unit-cell size and the Brillouin zone unfolds to a 45$^{\circ}$ rotated square and this allows some analytical handling of the 2D TB-Hamiltonian. The Pb $6s$ and I $5s$ orbitals are far away from the Fermi level and, thus, we integrate them out to obtain an effective model which only includes hybridized Pb $6p$ and I $5p$ states. Our TB-based treatment a) provides a good quantitative description of the DFT band-structure, b) helps us conceptualize the complex electronic structure in the family of these materials in a simple way and c) yields the one-body part to be combined with appropriately screened electron interaction to describe many-body effects, such as excitonic bound-states.

cond-mat.mtrl-sci↗

arXiv2023

Transition to an excitonic insulator from a two-dimensional conventional insulator

In this paper, first, we present a general formulation to investigate the ground-state and elementary excitations of an excitonic insulator (EI) in real materials. In addition, we discuss the out-of-equilibrium state induced (albeit transiently) by high-intensity light illumination of a conventional two-dimensional (2D) insulator. We then, present various band-structure models which allow us to study the transition from a conventional insulator to an EI in 2D materials as a function of the dielectric constant, the conventional insulator gap (and chemical potential), the bandwidths of the conduction and valence bands and the Bravais lattice unit-cell size. One of the goals of this investigation is to determine which range of these experimentally determined parameters to consider in order to find the best candidate materials to realize the excitonic insulator. The numerical solution to the EI gap equation for various band-structures shows a significant and interesting momentum-dependence of the EI gap function $Δ(\vec k)$ and of the zero-temperature electron and hole momentum-distribution across the Brillouin zone. Last, we discuss that these features can be detected by tunneling microscopy

cond-mat.str-el↗

arXiv2022

On the origin of matter-antimatter asymmetry in the Universe

In order to investigate the origin of matter-antimatter asymmetry in the Universe, we adopt a theoretical framework where the standard model emerges as a Poincaré invariant field theory localized at a domain-antidomain wall (DW-aDW) brane pair of a theory which lives on a higher dimensional bulk. We argue that such a system of a parallel DW-aDW pair could have been created at a very early epoch of the cosmological evolution when the Universe was still of microscopic size because its creation is topologically possible as compared to a single DW creation when the perpendicular extra dimension is compact. The conservation laws, such as of charge and chirality, are not violated in vacuum fluctuations of the combined DW-aDW system, but as we show their simultaneous conservation for each wall separately may not be favorable in certain processes. In particular, as expansion of spacetime occurs in the higher dimensional bulk, the distance d between the DW and the aDW increases as a function of time. We show that during the early stages of the cosmological evolution, when d was of microscopic size, the leading mechanism for pair-creation from fluctuations of the gauge-field with polarization perpendicular to the DWs (i.e., polarization along the extra dimension) was one in which the particle and the antiparticle were created on the opposite domain-walls hosting opposite chirality fermions. This mechanism allows for a matter-antimatter asymmetry to appear separately in the DW and the aDW, while in the combined DW-aDW system no such asymmetry was allowed. In this scenario, at a later and the present stage of the cosmological evolution, where d is macroscopically large, the probability to violate these conservation laws on a single DW, as a function of the inter-wall distance, is exponentially suppressed.

hep-th↗

arXiv2022

Topological Superconductivity in a two-dimensional Weyl SSH model

We study the emergence of topological superconductivity in a two-dimensional (2D) Weyl system, composed of stacked Su-Schrieffer-Heeger (SSH) chains. A previous analysis of the model showed that the addition of an attractive Hubbard interaction between spinful electrons leads to a superconducting state that has an intricate pairing structure, but is topologically trivial. Here we consider a pairing interaction that couples spinless fermions on opposite sublattices within the same unit cell. We observe that this physically motivated, momentum-independent pairing interaction induces a topological superconducting state, characterized by a gap function with a non-trivial phase, as well as Majorana and Fermi arc edge states along the edge perpendicular to the direction of the SSH dimerization. In addition, we observe a transition as a function of pairing interaction strength and chemical potential, indicated by a change in the sign of the topological charge carried by each of the four Bogoliubov-Weyl nodes.

cond-mat.supr-con↗

arXiv2022

Gradient Descent Optimization of Fermion Nodes in Diffusion Monte Carlo

We present a method for optimizing the location of the fermion ground-state nodes using a combination of diffusion Monte Carlo (DMC) and projected gradient descent (PGD). A PGD iteration shifts the parameters of an arbitrary node-fixing trial function in the opposite direction of the DMC energy gradient, while maintaining the cusp condition for atomic electrons. The energy gradient is calculated from DMC walker distributions by one of three methods we derive from an exact analytical expression. We combine our energy gradient calculation methods with different gradient descent algorithms and a projection operator that maintains the cusp condition. We apply this stochastic PGD method to trial functions of Be, Li$_2$, and Ne, all consisting of a single Slater determinant with randomized parameters, and find that the nodes dramatically improve to the same DMC energy as nodes optimized by variational Monte Carlo. Our method, therefore, departs from the standard procedure of optimizing the nodes with a non-DMC scheme such as variational Monte Carlo, Density function theory, or configuration interaction based calculation, which do not directly minimize the DMC energy.

physics.comp-ph↗

arXiv2021

Weyl Nodal-Ring Semimetallic Behavior and Topological Superconductivity in Crystalline Forms of Su-Schrieffer-Heeger Chains

We consider a three-dimensional model of coupled Su-Schrieffer-Heeger (SSH) chains. The analytically soluble model discussed here reliably reproduces the features of the band structure of crystalline polyacetylene as obtained from density-functional theory. We show that when a certain inter-chain coupling is sufficiently increased, the system develops a ring of Weyl nodes. We argue that such an increase could be achieved experimentally by intercalation or extreme pressure. With the addition of a simple intra-orbital pairing term we find that the system supports an exotic superconducting state with drumhead surface states and annular Majorana states localized on the surface. In addition to suggesting a novel real material realization of a nodal ring semimetal and possibly topological superconductivity, our results provide a new perspective on the SSH model, demonstrating that a simple extension of this broadly-impacting model can once again provide fundamental insights on the topological behavior of condensed matter systems.

cond-mat.str-el↗

arXiv2020

Bulk Fermi surfaces of the Dirac type-II semimetallic candidate NiTe2

Here, we present a study on the Fermi-surface of the Dirac type-II semi-metallic candidate NiTe$_2$ via the temperature and angular dependence of the de Haas-van Alphen (dHvA) effect measured in single-crystals grown through Te flux. In contrast to its isostructural compounds like PtSe$_2$, band structure calculations predict NiTe$_2$ to display a tilted Dirac node very close to its Fermi level that is located along the $Γ$ to A high symmetry direction within its first Brillouin zone (FBZ). The angular dependence of the dHvA frequencies is found to be in agreement with the first-principle calculations when the electronic bands are slightly shifted with respect to the Fermi level ($\varepsilon_F$), and therefore provide support for the existence of a Dirac type-II node in NiTe$_2$. Nevertheless, we observed mild disagreements between experimental observations and density Functional theory calculations as, for example, nearly isotropic and light experimental effective masses. This indicates that the dispersion of the bands is not well captured by DFT. Despite the coexistence of Dirac-like fermions with topologically trivial carriers, samples of the highest quality display an anomalous and large, either linear or sub-linear magnetoresistivity. This suggests that Lorentz invariance breaking Dirac-like quasiparticles dominate the carrier transport in this compound.

cond-mat.mtrl-sci↗