Microscopic Theory Research Articles

Conventional type-II superconductivity in locally noncentrosymmetric LaRh2As2 single crystals

We report on the observation of superconductivity in $\mathrm{La}{\mathrm{Rh}}_{2}{\mathrm{As}}_{2}$, which is the analog without $f$ electrons of the heavy-fermion system with two superconducting phases $\mathrm{Ce}{\mathrm{Rh}}_{2}{\mathrm{As}}_{2}$ [S. Khim et al., Science 373, 1012 (2021)]. A zero-resistivity transition, a specific-heat jump, and a drop in magnetic ac susceptibility consistently point to a superconducting transition at a temperature of ${T}_{c}=0.28$ K. The magnetic-field temperature superconducting phase diagrams determined from field-dependent ac-susceptibility measurements reveal small upper critical fields ${\ensuremath{\mu}}_{0}{H}_{c2}\ensuremath{\approx}12$ mT for $H\ensuremath{\parallel}ab$ and ${\ensuremath{\mu}}_{0}{H}_{c2}\ensuremath{\approx}9$ mT for $H\ensuremath{\parallel}c$. The observed ${H}_{c2}$ is larger than the estimated thermodynamic critical-field ${H}_{c}$ derived from the heat-capacity data, suggesting that $\mathrm{La}{\mathrm{Rh}}_{2}{\mathrm{As}}_{2}$ is a type-II superconductor with Ginzburg-Landau parameters ${\ensuremath{\kappa}}_{GL}^{ab}\ensuremath{\approx}1.9$ and ${\ensuremath{\kappa}}_{GL}^{c}\ensuremath{\approx}2.7$. The microscopic Eliashberg theory indicates superconductivity to be in the weak-coupling regime with an electron-phonon-coupling constant ${\ensuremath{\lambda}}_{e\ensuremath{-}ph}\ensuremath{\approx}0.4$. Despite a similar ${T}_{c}$ and the same crystal structure as the Ce compound, $\mathrm{La}{\mathrm{Rh}}_{2}{\mathrm{As}}_{2}$ displays conventional superconductivity, corroborating the substantial role of the $4f$ electrons for the extraordinary superconducting state in $\mathrm{Ce}{\mathrm{Rh}}_{2}{\mathrm{As}}_{2}$.

Momentum, angular momentum, and spin of waves in an isotropic collisionless plasma

We examine the momentum and angular momentum (including spin) properties of linear waves, both longitudinal (Langmuir) and transverse (electromagnetic), in an isotropic nonrelativistic collisionless electron plasma. We focus on conserved quantities associated with the translational and rotational invariance of the wave fields with respect to the homogeneous medium; these are sometimes called pseudomomenta. There are two types of the momentum and angular momentum densities: (i) the kinetic ones associated with the energy flux density and the symmetrized (Belinfante) energy-momentum tensor and (ii) the canonical ones associated with the conserved Noether currents and canonical energy-momentum tensor. We find that the canonical momentum and spin densities of Langmuir waves are similar to those of sound waves in fluids or gases; they are naturally expressed via the electron velocity field. In turn, the momentum and spin densities of electromagnetic waves can be written either in the forms known for free-space electromagnetic fields, involving only the electric field, or in the dual-symmetric forms involving both electric and magnetic fields, as well as the effective permittivity of plasma. We derive these properties both within the phenomenological macroscopic approach and microscopic Lagrangian field theory for the coupled electromagnetic fields and electrons. Finally, we explore implications of the canonical momentum and spin densities in transport and electrodynamic phenomena: the Stokes drift, the wave-induced magnetization (inverse Faraday effect), etc.

Unique Signatures of Topological Phases in Two-Dimensional THz Spectroscopy

We develop a microscopic theory for the two-dimensional (2D) spectroscopy of one-dimensional topological superconductors. We consider a ring geometry of an archetypal topological superconductor with periodic boundary conditions, bypassing energy-specific differences caused by topologically protected or trivial boundary modes that are hard to distinguish. We show numerically and analytically that the cross-peak structure of the 2D spectra carries unique signatures of the topological phases of the chain. Our work reveals how 2D spectroscopy can identify topological phases in bulk properties.

Microscopic theory of ionic motion in solids

Drag and diffusion of mobile ions in solids are of interest for both purely theoretical and applied scientific communities. This article proposes a theoretical description of ion drag in solids that can be used to estimate ionic conductivities in crystals, and forms a basis for the rational design of solid electrolyte materials. Starting with a general solid-state Hamiltonian, we employ the nonequilibrium path integral formalism to develop a microscopic theory of ionic transport in solids in the presence of thermal fluctuations. As required by the fluctuation-dissipation theorem, we obtain a relation between the variance of the random force and friction. Because of the crystalline nature of the system, however, the two quantities are tensorial. We use the drag tensor to write down the formula for ionic mobility, determined by the potential profile generated by the crystal's ions.

Crack tip cavitation in metallic glasses

Metallic glasses (MGs) are potential engineering materials, however their ductility is currently a limiting factor. Despite significant attempts to improve their fracture toughness, the fundamental physics of crack propagation remains poorly understood. The most concerning issue is the development of nano-corrugations (NCs), a distinctive fracture pattern found in brittle MGs. We offer a proof that NCs may be properly understood using the stress-driven cavitation process. The findings might provide insight on the genesis of dynamic fracture propagation in amorphous materials, which is a fiercely contested issue. The combination of shear and dilation strengths is shown to be critical in forecasting subsequent damage development. Finally, we apply what we have learned here to the debate over the macro and microscopic amorphous plasticity theories.

Microscopic theory of a magnetic-field-tuned sweet spot of exchange interactions in multielectron quantum-dot systems

The exchange interaction in a singlet-triplet qubit defined by two-electron states in the double-quantum-dot system (``two-electron singlet-triplet qubit'') typically varies monotonically with the exchange interaction and thus carries no sweet spot. Here we study a singlet-triplet qubit defined by four-electron states in the double-quantum-dot system (``four-electron singlet-triplet qubit''). We demonstrate, using configuration-interaction calculations, that in the four-electron singlet-triplet qubit the exchange energy as a function of detuning can be nonmonotonic, suggesting existence of sweet spots. We further show that the tuning of the sweet spot and the corresponding exchange energy by perpendicular magnetic field can be related to the variation of orbital splitting. Our results suggest that a singlet-triplet qubit with more than two electrons can have advantages in the realization of quantum computing.

Multiple magnetic orders in LaFeAs1-xPxO uncover universality of iron-pnictide superconductors

The iron-pnictide superconductors have generated tremendous excitement as the competition between magnetism and superconductivity has allowed unique in-roads towards elucidating a microscopic theory of unconventional high-temperature superconductivity. In addition to the stripe spin density wave ({C}_{2M}^{a}) phase observed in the parent compounds of all iron-pnictide superconductors, two novel magnetic orders have recently been discovered in different parent structures: an out-of-plane collinear double-Q ({C}_{4M}^{c}) structure in the hole-doped (Ca, Sr, Ba)1-x(Na)xFe2As2 and Ba1-xKxFe2As2 families, and a spin vortex crystal “hedgehog” ({C}_{4M}^{{ab}}) structure in the CaKFe4As4 family. Using neutron diffraction, we demonstrate that LaFeAs1-xPxO contains all three magnetic orders within a single-phase diagram as a function of substitution, all of which compete strongly with superconductivity. Our experimental observations combined with theoretical modeling demonstrate how the reduction in electronic correlations by chemical substitution results in larger Fermi surfaces and the sequential stabilization of multiple magnetic anisotropies. Our work presents a unified narrative for the competing magnetic and superconducting phases observed in various iron-pnictide systems with different crystal structures and chemistry.

Theory of superconductivity in doped quantum paraelectrics

Recent experiments on Nb-doped SrTiO3 have shown that the superconducting energy gap to the transition temperature ratio maintains the Bardeen–Cooper–Schrieffer (BCS) value throughout its superconducting dome. Motivated by these and related studies, we show that the Cooper pairing mediated by a single soft transverse-optical phonon is the most natural mechanism for such a superconducting dome given experimental constraints, and present the microscopic theory for this pairing mechanism. Furthermore, we show that this mechanism is consistent with the T2 resistivity in the normal state. Lastly, we discuss what physical insights SrTiO3 provides for superconductivity in other quantum paraelectrics such as KTaO3.

Strengthening and converse strengthening effects of polyurea layer on polyurea–steel composite structure subjected to combined actions of blast and fragments

Coating polyurea can significantly enhance the blast and impact resistance of thin-walled metal structures. However, most current research focuses only on the condition of single damage factor, such as blast or penetration loading. In this paper, the protective performance of polyurea-coated steel plates under the combined actions of blast and high-speed fragments was studied by means of field tests. Test results showed that under the combined actions of blast and high-speed fragments, when the coating thickness was within a certain range, polyurea had a converse strengthening effect on the protective performance of steel plate no matter which configuration was adopted. Only when the coating thickness exceeded a certain value did polyurea play a strengthening role. In other words, an obvious inflection point effect occurred in the influence of polyurea thickness on the protective performance of steel plate. These phenomena seem to be related to the characteristics of combined actions, which was different from that of single load. On the basis of tests, the strengthening and converse strengthening effects of polyurea layer was explained through microscopic analysis, numerical calculation and stress wave propagation theory. The significance of this research was to reexamine the practical application method of polyurea in improving the protective performance of metal thin-walled structures under specific circumstances to provide technical guidance for engineering application.

Low‐Frequency Phonon Modes in Layered Silver‐Bismuth Double Perovskites: Symmetry, Polarity, and Relation to Phase Transitions

AbstractMetal‐halide perovskites (PSKs) are emergent materials for a large range of applications, and the layered double PSK architectures vastly enrich the opportunities to design their composition, structural properties, and optoelectronic behavior. The stability, crystal phase, and electronic bandgap depend strongly on the bonds and distortions of the octahedra lattice that are at the origin of the vibrational spectrum of these materials. This work investigates the structural dynamics of flakes of exfoliated layered Ag‐Bi bromide double PSKs by angle‐dependent polarized Raman spectroscopy and density functional theory modeling. The well‐defined orientation of the inorganic octahedra lattice with respect to the light polarization allows to correlate the angle‐dependent intensity of the Raman signal to the directionality and symmetry of the phonon modes. Low‐frequency vibrations are revealed for which a detailed microscopic and group theory assignment of the Raman modes is provided. The temperature‐dependent measurements across the phase transitions show marked changes in the phonon frequencies, reveal soft modes, and help to distinguish first from second‐order transitions as well as to determine their transition temperature. This provides highly valuable insights to improve the properties of this class of Pb‐free PSKs for applications in energy harvesting and optoelectronics.

Constraining neutron-star matter with microscopic and macroscopic collisions

Interpreting high-energy, astrophysical phenomena, such as supernova explosions or neutron-star collisions, requires a robust understanding of matter at supranuclear densities. However, our knowledge about dense matter explored in the cores of neutron stars remains limited. Fortunately, dense matter is not probed only in astrophysical observations, but also in terrestrial heavy-ion collision experiments. Here we use Bayesian inference to combine data from astrophysical multi-messenger observations of neutron stars1–9 and from heavy-ion collisions of gold nuclei at relativistic energies10,11 with microscopic nuclear theory calculations12–17 to improve our understanding of dense matter. We find that the inclusion of heavy-ion collision data indicates an increase in the pressure in dense matter relative to previous analyses, shifting neutron-star radii towards larger values, consistent with recent observations by the Neutron Star Interior Composition Explorer mission5–8,18. Our findings show that constraints from heavy-ion collision experiments show a remarkable consistency with multi-messenger observations and provide complementary information on nuclear matter at intermediate densities. This work combines nuclear theory, nuclear experiment and astrophysical observations, and shows how joint analyses can shed light on the properties of neutron-rich supranuclear matter over the density range probed in neutron stars.

Brownian motion with time-dependent friction and single-particle dynamics in liquids.

A microscopic theory of molecular motion in classical monatomic liquids, proposed by Glass and Rice [Phys. Rev. 176, 239 (1968)10.1103/PhysRev.176.239], is revisited and extended to incorporate the dynamic friction in the Brownian description of the atomic diffusion in a mean-time-dependent harmonic force field. A modified, non-Markovian Langevin equation is utilized to derive an equation of motion for the velocity autocorrelation function with time-dependent friction coefficient. Numerical solution of the equation gives an excellent account of the velocity autocorrelation function in Lennard-Jones liquids, liquid alkali, and transition metals over a broad range of density and temperature. Derivation of the equation of motion leads to a self-consistent expression for the time dependence of friction coefficient. Our results demonstrate that the nature of time dependence of the friction coefficient changes dramatically with the liquid density. At low and moderate densities, the dynamic friction decays exponentially whereas it increases exponentially at high liquid densities. Our findings provide an opportunity for a different outlook of the Brownian description of atomic dynamics in liquids.

Meniscus osculation and adsorption on geometrically structured walls.

We study the adsorption of simple fluids at smoothly structured, completely wet walls and show that a meniscus osculation transition occurs when the Laplace and geometrical radii of curvature of locally parabolic regions coincide. Macroscopically, the osculation transition is of fractional, 7/2, order and separates regimes in which the adsorption is microscopic, containing only a thin wetting layer, and mesoscopic, in which a meniscus exists. We develop a scaling theory for the rounding of the transition due to thin wetting layers and derive critical exponent relations that determine how the interfacial height scales with the geometrical radius of curvature. Connection with the general geometric construction proposed by Rascón and Parry is made. Our predictions are supported by a microscopic model density functional theory for drying at a sinusoidally shaped hard wall where we confirm the order of the transition and also an exact sum rule for the generalized contact theorem due to Upton. We show that as bulk coexistence is approached the adsorption isotherm separates into three regimes: A preosculation regime where it is microscopic, containing only a thin wetting layer; a mesoscopic regime, in which a meniscus sits within the troughs; and finally another microscopic regime where the liquid-gas interface unbinds from the crests of the substrate.

Fast prediction of methane adsorption in shale nanopores using kinetic theory and machine learning algorithm

Understanding the gas adsorption behavior in shale nanopores is essential for the reservoir estimation and performance prediction of shale gas, which is still unclear considering the complexity of geological environment and nanoporous structure of shale. In general, traditional methods based on experiments and molecular dynamics (MD) simulations are always expensive and time consuming. In this work, a machine learning (ML) framework to predict the methane adsorption behavior in shale nanopores is constructed from the microscopic and kinetic theory perspectives, where three novel parameters related to potential energy distribution (PED) are proposed to represent the methane adsorption characteristics of shale slit nanopores. Machine learning algorithm based on the uniformly constructed dataset is introduced to realize fast and accurate prediction of methane adsorption behavior, which is well validated by typical inorganic and organic models. Moreover, the application of the proposed ML model to predict the adsorption behavior of methane in different geological conditions (e.g., pressure and temperature) is performed, indicating its feasibility to predict the gas adsorption behavior in shale nanopores with ultra-fast computation speed, that would be beneficial for the exploitation and development of shale gas reservoirs. The insights gained in this work is also instructive for the prediction of nano-confined fluid behavior under complex environmental factors.

Linear causality and stability of third-order relativistic dissipative fluid dynamics

We analyze the linear causality and stability of third-order fluid dynamics considering perturbations around a global equilibrium state. We investigate the formulation derived from kinetic theory, using the Chapman-Enskog expansion, in [Phys. Rev. C 88, 021903 (2013)] which was shown to be in excellent agreement with solutions of the microscopic theory. From this analysis, we demonstrate that this theory is linearly acausal and unstable and that such instabilities cannot be corrected by tuning the transport coefficients. We then propose a modification of this theory, valid only in the linear regime, that can be constructed to be linearly causal and stable and obtain the conditions the transport coefficients must satisfy in order for this to be the case.

Microscopic study of compound-nucleus formation in cold-fusion reactions

The understanding of the fusion probability is of particular importance to reveal the mechanism of producing superheavy elements. We present a microscopic study of the compound nucleus formation by combining time-dependent density functional theory, coupled-channels approach, and dynamical diffusion models. The fusion probability and compound nucleus formation cross sections for cold-fusion reactions $^{48}$Ca+$^{208}$Pb, $^{50}$Ti+$^{208}$Pb, and $^{54}$Cr+$^{208}$Pb are investigated and it is found that the deduced capture barriers, capture cross sections for these reactions are consistent with experimental data. Above the capture barrier, our calculations reproduce the measured fusion probability reasonably well. Our studies demonstrate that the restrictions from the microscopic dynamic theory improve the predictive power of the coupled-channels and diffusion calculations.

Coupled hydrodynamics in dipole-conserving quantum systems

We investigate the coupled dynamics of charge and energy in interacting lattice models with dipole conservation. We formulate a generic hydrodynamic theory for this combination of fractonic constraints and numerically verify its applicability to the late-time dynamics of a specific bosonic quantum system by developing a microscopic non-equilibrium quantum field theory. Employing a self-consistent $1/N$ approximation in the number of field components, we extract all entries of a generalized diffusion matrix and determine their dependence on microscopic model parameters. We discuss the relation of our results to experiments in ultracold atom quantum simulators.

Microscopic theory of photon-induced energy, momentum, and angular momentum transport in the nonequilibrium regime

We set up a general microscopic theory for the transfer of energy, momentum, and angular momentum mediated by photons. Using the nonequilibrium Green's function method, we propose a unified Meir-Wingreen formalism for the energy emitted, force experienced, and torque experienced by the objects due to the fluctuating electromagnetic field. Our theory does not require the local thermal equilibrium that is the central assumption of the conventional theory of fluctuational electrodynamics (FE). The obtained formulas are valid for arbitrary objects as well as the environment without the requirement of reciprocity. To show the capability of our microscopic theory, we apply the general formulas to transport problems of graphene edges in both equilibrium and nonequilibrium situations. We show the local equilibrium energy radiation of graphene obeys the well-known $T^4$ law with a converged theoretical emissivity of 2.058$\%$. In the ballistic nonequilibrium situation driven by chemical potential biases, we observe nonzero results for force and torque from the graphene edges, which go beyond the predictive ability of the FE theory. Our method is general and efficient for large systems, which paves the way for studying more complex transport phenomena in the nonequilibrium regime.

Strategy to extract Kitaev interaction using symmetry in honeycomb Mott insulators

The Kitaev spin liquid, a ground state of the bond-dependent Kitaev model in a honeycomb lattice has been a center of attraction, since a microscopic theory to realize such an interaction in solid-state materials was discovered. A challenge in real materials though is the presence of the Heisenberg and another bond-dependent Gamma interactions detrimental to the Kitaev spin liquid, and there have been many debates on their relative strengths. Here we offer a strategy to extract the Kitaev interaction out of a full microscopic model by utilizing the symmetries of the Hamiltonian. Two tilted magnetic field directions related by a two-fold rotational symmetry generate distinct spin excitations originated from a specific combination of the Kitaev and Gamma interactions. Together with the in- and out-of-plane magnetic anisotropy, one can determine the Kitaev and Gamma interactions separately. Dynamic spin structure factors are presented to motivate future experiments. The proposed setups will advance the search for Kitaev materials.

Three-dimensional energy gap and origin of charge-density wave in kagome superconductor KV3Sb5

Kagome lattices offer a fertile ground to explore exotic quantum phenomena associated with electron correlation and band topology. The recent discovery of superconductivity coexisting with charge-density wave (CDW) in the kagome metals KV3Sb5, RbV3Sb5, and CsV3Sb5 suggests an intriguing entanglement of electronic order and superconductivity. However, the microscopic origin of CDW, a key to understanding the superconducting mechanism and its possible topological nature, remains elusive. Here, we report angle-resolved photoemission spectroscopy of KV3Sb5 and demonstrate a substantial reconstruction of Fermi surface in the CDW state that accompanies the formation of small three-dimensional pockets. The CDW gap exhibits a periodicity of undistorted Brillouin zone along the out-of-plane wave vector, signifying a dominant role of the in-plane inter-saddle-point scattering to the mechanism of CDW. The characteristics of experimental band dispersion can be captured by first-principles calculations with the inverse star-of-David structural distortion. The present result indicates a direct link between the low-energy excitations and CDW, and puts constraints on the microscopic theory of superconductivity in alkali-metal kagome lattices.