Recently it was discovered that van der Waals bonded magnetic materials retain long range magnetic ordering down to a single layer, opening many avenues in fundamental physics and potential applications of these fascinating materials. One such material is $\mathrm{FeP}{\mathrm{S}}_3$, a large spin $\left(S=2\right)$ Mott insulator where the Fe atoms form a honeycomb lattice. In the bulk, $\mathrm{FeP}{\mathrm{S}}_3$ has been shown to be a quasi-two-dimensional-Ising antiferromagnet, with additional features in the Raman spectra emerging below the Néel temperature $\left(T_{\mathrm{N}}\right)$ of approximately 120 K. Using magneto-Raman spectroscopy as an optical probe of magnetic structure, we show that one of these Raman-active modes in the magnetically ordered state is actually a magnon with a frequency of $\approx 3.7\mathrm{THz}\left(122\mathrm{c}{\mathrm{m}}^{-1}\right)$. Contrary to previous work, which interpreted this feature as a phonon, our Raman data shows the expected frequency shifting and splitting of the antiferromagnetic magnon as a function of temperature and magnetic field, respectively, where we determine the $g$ factor to be $\approx 2$. In addition, the symmetry behavior of the magnon is studied by polarization-dependent Raman spectroscopy and explained using the magnetic point group of $\mathrm{FeP}{\mathrm{S}}_3$.

Frustrated quantum spin systems exhibit exotic physics induced by external magnetic field with anisotropic interactions. Here, we study the effect of nonuniform Dzyaloshinskii-Moriya (DM) interactions on a quasi-two-dimensional Shastry-Sutherland lattice using a matrix product state algorithm. We first recover the magnetization plateau structure present in this geometry and then we show that both interdimer and intradimer DM interactions significantly modify the plateaux. The non-number-conserving intradimer interaction smoothens the shape of the magnetization curve, while the number-conserving interdimer interaction induces different small plateaux, which are signatures of the finite size of the system. Interestingly, the interdimer DM interaction induces chirality in the system. We thus characterize these chiral phases with particular emphasis to their robustness against intradimer DM interactions.

${\mathrm{TbBaFe}}_2{\mathrm{O}}_{5+\delta }$ (TBFO) is a mixed valence compound with an antiferromagnetic order ($T_N=450\mathrm{K}$) that changes along with the crystal structure and conductivity upon cooling below a Verwey-like transition ($T_V=280\mathrm{K}$). This type of double-cell-layered perovskite is useful to study putative charge ordering in fractional valence systems because the crystal structure develops unique sites for the different valence states. While the evolution of the static magnetic structure in the different charge ordered states of TBFO is known, in this study we investigate the effect of charge ordering on the magnetic interactions. Dispersive magnetic excitations measured using single crystal inelastic neutron scattering experiments at $T=4\mathrm{K}<T_V$ and $T=320\mathrm{K}>T_V$ are modeled with linear-spin-wave theory to extract anisotropy and superexchange parameters. Below $T_V$, we observe a sizable magnon gap due to an easy-axis magnetic anisotropy that closes above $T_V$ due to the evolution of easy-plane anisotropy. The magnetic interactions are found to be highly three-dimensional and change across $T_V$. Above $T_V$, where the conductivity is greater, a damping term is required to model the spin waves that suggests a coupling to valence fluctuations.

We implement the Bogoliubov–de Gennes equation in a screened Korringa-Kohn-Rostoker method for solving, self-consistently, the superconducting state for three-dimensional crystals. This method combines the full complexity of the underlying electronic structure and Fermi surface geometry with a simple phenomenological parametrization for the superconductivity. We apply this theoretical framework to the known $s$-wave superconductors Nb, Pb, and ${\mathrm{MgB}}_2$. In these materials multiple distinct peaks at the gap in the density of states were observed, showing significant gap anisotropy which is in good agreement with experiment. Qualitatively, the results can be explained in terms of the $k$-dependent Fermi velocities on the Fermi surface sheets exploiting concepts from BCS theory.

Thorium is a chemical element that is beginning to attract attention because of its potential use as a nuclear fuel. It is not easy to carry out experiments because of its radioactive nature, and therefore theoretical works are highly appreciated. Thorium contains only a small number of the $5f$ states, and it is generally accepted that these states are itinerant, that they form a chemical bond, and that their nature does not need to be corrected with the Hubbard model. On the other hand, there is a well-known problem with the description of the $6p_{1/2}$ states when the spin-orbit coupling is added as the perturbation to a scalar-relativistic calculation. Electronic, elastic, phonon, and thermodynamic properties are analyzed in terms of the importance of the spin-orbit coupling acting on the $6d$ and $5f$ states. The importance of the spin-orbit coupling acting on the semicore $6p$ states is discussed. The same properties are analyzed for thorium monocarbide, and a difference caused by adding a carbon atom into the structure is discussed. Detailed analysis of the thermal conductivity (both phonon and electronic contributions) is also included. We have given extra attention to the thermal conductivity of ThC to explain why the optical phonon modes account only for approximately 6% of the phonon thermal conductivity.

Reproducing the electronic structure of $A{M}_4{X}_8$ lacunar spinels with a breathing pyrochlore lattice is a great theoretical challenge due to the interplay of various factors. The character of the $M_4{X}_4$ cluster orbitals is critically influenced by the Jahn-Teller instability, the spin-orbit interaction, and also by the magnetic state of the clusters. Consequently, to reproduce the narrow-gap semiconducting nature of these moderately correlated materials requires advanced approaches, since the strength of the intercluster hopping is strongly affected by the character of the cluster orbitals. In order to provide a solid experimental basis for theoretical studies, we performed broadband optical spectroscopy on a large set of lacunar spinels, with systematically changing ions at the A and M sites as well as the ligand (A = Ga, Ge, Al; M = V, Mo, Nb, Ta; X = S, Se). Our study covers the range of phonon excitations and also electronic transitions near the gap edge. In the phonon excitation spectrum a limited subset of the symmetry-allowed modes is observed in the cubic state, with a few additional modes emerging upon the symmetry-lowering structural transition. All the infrared-active modes are assigned to vibrations of the ligands and ions at the A sites, with no obvious contribution from the M-site ions. Concerning the electronic states, we found that all compounds are narrow-gap semiconductors ($E_{\mathrm{g}}=130–350$ meV) already in their room-temperature cubic state and their structural transitions induce weak, if any, changes in the band gap. The gap value is decreased when substituting S with Se and also when replacing $3d$ ions by $4d$ or $5d$ ions at the M sites.

Time-resolved x-ray diffraction experiments on the half-doped single-layered manganite $\mathrm{P}{\mathrm{r}}_{0.5}\mathrm{C}{\mathrm{a}}_{1.5}\mathrm{Mn}{\mathrm{O}}_4$ are used to monitor the ultrafast photoinduced dynamics of the structural distortion associated with the charge and orbital ordering (CO/OO). As in the nonlayered three-dimensional counterpart, the ordered phase melts in less than 100 fs after 800-nm photoexcitation and subsequently partially recovers due to thermal equilibration of electronic and vibrational systems. Photoexciting $\mathrm{P}{\mathrm{r}}_{0.5}\mathrm{C}{\mathrm{a}}_{1.5}\mathrm{Mn}{\mathrm{O}}_4$ below the transition temperature of a second structural phase transition that occurs around 146 K (deep inside the CO/OO phase) releases this structural transition, but progresses on a much slower timescale. This additional reduction of crystal symmetry, which we ascribe to a further tilt of the oxygen octahedra, can thus be considered to be only weakly coupled to CO/OO. Furthermore, static hard-x-ray and resonant soft-x-ray diffraction at the $\mathrm{Mn}{L}_{2,3}$ edges experiments identify correlations between structural distortions and electronic order in thermal equilibrium.

We compute the phonon dispersion, density of states, and the Grüneisen parameters of bulk palladium in the combined density functional theory (DFT) and dynamical mean-field theory. We find good agreement with experimental results for ground-state properties (equilibrium lattice parameter and bulk modulus) and the experimentally measured phonon spectra. We demonstrate that at temperatures $T\lesssim 20\mathrm{K}$ the phonon frequency in the vicinity of the Kohn anomaly, ${\omega }_{T1}\left({\mathbf{q}}_K\right)$, strongly decreases. This is in contrast to DFT where this frequency remains essentially constant in the whole temperature range. Apparently, correlation effects reduce the restoring force of the ionic displacements at low temperatures, leading to a mode softening.

The crystal lattice of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ is investigated with synchrotron x-ray powder diffraction under hydrostatic pressures up to $P=43$ GPa and temperatures down to 20 K. The tetragonal unit cell is maintained over the whole investigated pressure range, within our resolution and sensitivity. The $c$-axis compressibility ${\kappa }_c(P,T)\equiv -(1/c)(dc/dP)$ presents an anomaly with pressure at $P_1=17$ GPa at fixed $T=20$ K that is not observed at $T=300$ K, whereas ${\kappa }_a(P,T)$ is nearly temperature independent and shows a linear behavior with $P$. The anomaly in ${\kappa }_c(P,T)$ is associated with the onset of long-range magnetic order, as evidenced by an analysis of the temperature dependence of the lattice parameters at fixed $P=13.7\pm 0.5$ GPa. At fixed $T=20$ K, the tetragonal elongation $c/a(P,T)$ shows a gradual increment with pressure and a depletion above $P_2=30$ GPa that indicates an orbital transition and possibly marks the collapse of the $J_{\text{eff}}=1/2$ spin-orbit-entangled state. Our results support pressure-induced phase transitions or crossovers between electronic ground states that are sensed, and therefore can be probed, by the crystal lattice at low temperatures in this prototype spin-orbit Mott insulator.

Cluster perturbation theory (CPT) is a computationally economic method commonly used to estimate the momentum- and energy-resolved single-particle Green's function. It has been used extensively in direct comparisons with experiments that effectively measure the single-particle Green's function, e.g., angle-resolved photoemission spectroscopy. However, many experimental observables are given by two-particle correlation functions. CPT can be extended to compute two-particle correlation functions by approximately solving the Bethe-Salpeter equation. We implement this method and focus on the transverse spin susceptibility, measurable via inelastic neutron scattering or with optical probes of atomic gases in optical lattices. We benchmark the method with the one-dimensional Fermi-Hubbard model by comparing with known results.

${\mathrm{BaAg}}_2{\mathrm{As}}_2$, a sibling compound of ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ with a nonmagnetic phase transition around 150 K, is studied by the comprehensive measurements of angle-resolved photoemission spectroscopy, synchrotron x-ray diffraction, and resonant soft x-ray scattering. The Fermi surfaces and electronic structure of ${\mathrm{BaAg}}_2{\mathrm{As}}_2$ are revealed, with strong $k_z$ dispersion, consistent with the strongly contracted $c/a$ ratio in ${\mathrm{BaAg}}_2{\mathrm{As}}_2$. Across the phase transition, splitting of [101] Bragg peak is observed, showing a structural distortion with the in-plane distortion magnitude $\delta =|a-b|/(a+b)=0.0052$. Although the nesting condition is satisfied in some parallel Fermi-surface sectors, there is no signature of charge density wave order at the nesting wave vector. Moreover, neither a charge density wave gap opening nor band reconstruction are observed across the phase transition. Instead, an enhancement on the spectral weight of dispersive bands is observed across the structural phase transition, which can explain the sharp drop of resistivity below the phase transition temperature. These studies could enrich the understanding of the variable and common features of the structural transition in transition metal pnictide layered materials.

We propose a method to calculate the charge dynamical structure factors for the ground states of correlated electron systems based on the variational Monte Carlo method. Our benchmarks for the one- and two-dimensional Hubbard models show that inclusion of composite-fermion excitations in the basis set greatly improves the accuracy, in reference to the exact charge dynamical structure factors for clusters. Together with examination for larger systems beyond tractable sizes by the exact diagonalization, our results indicate that the variational Monte Carlo method is a promising way for studies on the nature of charge dynamics in correlated materials such as the copper oxide superconductors if the composite-fermion excitations are properly included in the restricted Hilbert space of intermediate states in the linear response theory. Our results are consistent with the particle-hole excitations inferred from the single-particle spectral function $A(\mathit{k},\omega )$ in the literature. We also discuss the importance of incorporating nonlocal composite fermion for a more accurate description. Future issues for further improvements are also discussed.

We study transport properties of topological Weyl nodal-line semimetals (NLSs). Starting from a minimal lattice model with a single nodal loop, and by focusing on a normal metal–NLS–normal metal junction, we investigate the dependence of the novel transport behavior on the orientation of the nodal loop. When the loop is parallel to the junction interfaces, the transmitted current is found to be nearly fully spin polarized. Correspondingly, there exists a spin orientation along which the incident electrons would be totally reflected. An unusual resonance of half transmission with the participation of surface states also occurs for a pair of incident electrons with opposite spin orientations. All these phenomena have been shown to originate from the existence of a single forward-propagating mode in the NLS of the junction and argued to survive in more generic multiband Weyl NLSs.

The point of zero gradient of the electronic band structure, the critical point, generally induces the singularity in the density of states (DOS), but no isolated critical point yields strict divergence of the DOS in three dimensions, differently from the lower-dimensional cases. In view of the band structure as a smooth hypersurface on the reciprocal space, we discuss the minimal deformation of the band structure that yields nondivergent but large sharp DOS peaks in three dimensions. By “pushing down” the energy level at the second-order saddle point (maximum), a continuous closed loop of saddle points (sphere of maxima) encircling the original position of the saddle point (maximum) emerges, with which the DOS peak is formed. Being high-dimensional features, the saddle loop and extremum shell thus formed are difficult to locate with standard band structure analysis on linear $\mathbf{k}$-point paths. The Lifshitz transition occurring over a linear or planar manifold is discussed as an indicator of such features. We also find that the celebrated DOS peak in the recently discovered superconducting hydride ${\mathrm{H}}_3\mathrm{S}$ originates from the saddle loop. On this basis, we successfully extract the minimal model that explains how the DOS peak is formed. Our theory characterizes a large class of DOS peaks sometimes found in the three-dimensional electronic structures, building a basis for profound understanding of their origins.

Based on the combination of the variational polaron transformation and the Green-Kubo formalism, a method toward the thermopower calculations in organic crystals is proposed, which covers a broad regime of electron-phonon interaction strengths and successfully recovers the small-polaron theory in the narrow-band limit and the modified Mott formula in the coherent limit, respectively. The application to a molecular chain reveals a crossover of thermopower with respect to the chemical potential and exhibits abnormal regions where the sign of thermopower is opposite to the sign of the carrier charge. This abnormal property may lead to a sign-inversion phenomenon of thermopower in terms of the temperature. It is also found that the incomplete dressing of the electron by the phonon cloud destroys the particle-hole symmetry and results in nonzero thermopower in a half-filled band.

Organic semiconductors have attracted increasing interest as thermoelectric converters in recent years due to their intrinsically low thermal conductivity compared to inorganic materials. This boom has led to encouraging practical results in which the thermal conductivity has predominantly been treated as an empirical number. However, in an optimized thermoelectric material, the electronic component can dominate the thermal conductivity, in which case the figure of merit $ZT$ becomes a function of thermopower and Lorentz factor only. Hence the design of effective organic thermoelectric materials requires understanding the Lorenz number. Here, analytical modeling and kinetic Monte Carlo simulations are combined to study the effect of energetic disorder and length scales on the correlation of electrical and thermal conductivity in organic semiconductor thermoelectrics. We show that a Lorenz factor up to a factor $\sim 5$ below the Sommerfeld value can be obtained for weakly disordered systems, in contrast with what has been observed for materials with band transport. Although the electronic contribution dominates the thermal conductivity within the application-relevant parameter space, reaching $ZT>1$ would require minimization of both the energetic disorder and also the lattice thermal conductivity to values below ${\kappa }_{\text{lat}}<0.2\mathrm{W}/\mathrm{mK}$.

Carrier transport across polar $n$-type InGaN/GaN multiple quantum wells (MQWs) has been studied by time-resolved photoluminescence (PL) using an optical marker technique. Efficiency of the hole transfer into the marker well experienced a nonmonotonous temperature dependence. First, as the temperature was lowered below room temperature, the number of transferred holes decreased because of the decreased efficiency of the thermionic emission. However, when the temperature was lowered below ∼80 K, the number of transferred holes experienced a significant rise. In addition, the low-temperature hole transport across the MQW structure was very fast, <3 ps. These features indicate that the low-temperature hole transport across the MQWs is ballistic or quasiballistic. Comparison of PL data for structures with different MQW parameters suggests that at low temperatures the hole mean-free path is about 10 nm. Probably, hole transport via light hole and split-off valence bands contributes to this high value.

An extended Drude model, termed the Gurzhi model, which takes into account the electron-phonon and electron-electron interactions, is applied to calculate the Casimir force between two metallic plates. It is shown that although the dielectric permittivity of the Gurzhi model has a first-order pole in the upper half-plane of complex frequencies and, thus, violates the causality principle, it can be used in a restricted frequency interval in combination with the experimental permittivity determined by the optical data for the complex index of refraction. The imaginary part of the Gurzhi dielectric permittivity of Au at low frequencies demonstrates better agreement with the permittivity given by the optical data than the simple Drude model. The Casimir pressure between two Au plates is computed using the Gurzhi, Drude, and plasma model approaches, taking into account the optical data, as well as with the simple Drude and plasma models. The contribution of the electron-electron scattering to the Casimir pressure is estimated. Although a comparison with the measurement data of two precise experiments shows that the Gurzhi model does not resolve the Casimir puzzle, the obtained results suggest further clarification of this fundamental problem.

Majorana zero modes (MZM-s) occurring at the edges of a one-dimensional (1D), $p$-wave, spinless superconductor, in the absence of fluctuations of the phase of the superconducting order parameter, are quintessential examples of topologically protected zero-energy modes occurring at the edges of 1D symmetry-protected topological phases. In this work, we numerically investigate the fate of the topological phase in the presence of phase fluctuations using the density matrix renormalization group (DMRG) technique. To that end, we consider a one-dimensional array of MZM-s on mesoscopic superconducting islands at zero temperature. Cooper-pair and MZM-assisted single-electron tunneling, together with finite charging energy of the mesoscopic islands, give rise to a rich phase diagram of this model. We show that the system can be in either a Mott-insulating phase, a Luttinger liquid (LL) phase of Cooper pairs, or a second gapless phase. In contrast to the LL of Cooper pairs, this second phase is characterized by nonlocal string correlation functions which decay algebraically due to gapless charge-$e$ excitations. The three phases are separated from each other by phase transitions of either Kosterlitz-Thouless or Ising type. Using a Jordan-Wigner transformation, we map the system to a generalized Bose-Hubbard model with two types of hopping and use DMRG to analyze the different phases and the phase transitions.

We investigate nonlinear dispersive mode coupling between the flexural in- and out-of-plane modes of two doubly clamped, nanomechanical silicon nitride string resonators. As the amplitude of one mode transitions from the linear response regime into the nonlinear regime, we find a frequency shift of two other modes. The resonators are strongly elastically coupled via a shared clamping point and can be tuned in and out of resonance dielectrically, giving rise to multimode avoided crossings. When the modes start hybridizing, their polarization changes. This affects the nonlinear dispersive coupling in a nontrivial way. We propose a theoretical model to describe the dependence of the dispersive coupling on the mode hybridization.