We briefly review the electrical-optical response of materials to radiative forcing within the formalism of the Kramers-Kronig relations. A commensurate set of criteria is described that must be met by any frequency-domain model representing the time-domain response of a real (i.e., physically possible) material. The criteria are applied to the Brendel-Bormann (BB) oscillator, a model that was originally introduced for its fidelity at reproducing the non-Lorentzian peak broadening experimentally observed in the infrared absorption by thin amorphous films but has since been used for many other common materials. We show that the BB model fails to satisfy the established physical criteria. Taking an alternative approach to the model derivation, a physically consistent model is proposed. This model provides the appropriate line-shape broadening for modeling the infrared optical response of thin amorphous films while adhering strictly to the Kramers-Kronig criteria. Experimental data for amorphous alumina (${\mathrm{Al}}_2{\mathrm{O}}_3$) and amorphous quartz silica (${\mathrm{SiO}}_2$) are used to obtain model parametrizations for both the noncausal BB model and the proposed causal model. The proposed model satisfies consistency criteria required by the underlying physics and reproduces the experimental data with better fidelity (and often with fewer parameters) than previously proposed permittivity models.

We analyze the nodal points in multiorbital Fe-based superconductors from a topological perspective. We consider the $s^{+-}$ gap structure with accidental nodes, and the $d$-wave gap with nodes along the symmetry directions. In both cases, the nodal points can be moved by varying an external parameter, e.g., a degree of interpocket pairing. Eventually, the nodes merge and annihilate via a Lifshitz-type transition. We discuss the Lifshitz transition in Fe-based superconductors from a topological point of view. We show, both analytically and numerically, that the merging nodal points have winding numbers of opposite sign. This is consistent with the general reasoning that the total winding number is a conserved quantity in the Lifshitz transition.

The ground-state properties of quasi-one-dimensional (Q1D) Ising spin glass are investigated using an exact numerical approach and analytical arguments. A set of coupled recursive equations for the ground-state energy are introduced and solved numerically. For various types of coupling distribution, we obtain accurate results for magnetization, particularly in the presence of a weak external magnetic field. We show that in the weak magnetic field limit, similar to the 1D model, magnetization exhibits a singular power-law behavior with divergent susceptibility. Remarkably, the spectrum of magnetic exponents is markedly different from that of the 1D system even in the case of two coupled chains. The magnetic exponent makes a crossover from being dependent on a distribution function to a constant value independent of distribution. We provide an analytic theory for these observations by extending the Chen-Ma argument to the Q1D case. We derive an analytical formula for the exponent which is in perfect agreement with the numerical results.

Magnetic susceptibility and proton nuclear magnetic resonance $\left({}^1\mathrm{H}\text{−}\mathrm{NMR}\right)$ measurements were performed for the quasi-two-dimensional $\pi \text{−}d$ interacting system $\lambda \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{STF}\right)}_2{\mathrm{FeCl}}_4$ at ambient pressure. Magnetic susceptibility arising from the $3d$ spins of the ${\mathrm{FeCl}}_4$ anion show an anisotropy at low temperature and its temperature dependence for the external field parallel to the $c$ axis is described as a broad peak structure at 8 K. A sharp peak in the temperature dependence of $T_1^{-1}$ associated with the antiferromagnetic (AF) transition is observed at $T_{\mathrm{AF}}=16$ K, together with the drastic splitting of the NMR spectrum below $T_{\mathrm{AF}}$. The relation between the static susceptibility and the splitting of the NMR shift suggests the existence of the relatively strong $d\text{−}d$ AF interaction. These results can be explained by the model considering the AF-coupled $d$-spin system in the AF long-range-ordered $\pi$-spin system. We find that the AF phases in $\lambda$-type salts can be universally explained by this model.

In this paper, the magnetic properties of $\mathrm{LiM}{\mathrm{n}}_2{\mathrm{O}}_4$ nanorods with an average diameter of $\sim 100\mathrm{nm}$ and length of $\sim 1\mu \mathrm{m}$ are investigated. The temperature dependences of dc and ac susceptibility measurements show that $\mathrm{LiM}{\mathrm{n}}_2{\mathrm{O}}_4$ nanorods experience multiple magnetic phase transitions upon cooling, i.e., paramagnetic (PM), antiferromagnetic (AFM), canted antiferromagnetic (CAFM), and cluster spin glass (SG). The coexistence between a long-range ordered AFM phase due to a $\mathrm{M}{\mathrm{n}}^{4+}\text{−}\mathrm{M}{\mathrm{n}}^{4+}$ interaction and a cluster SG phase originating from frozen AFM clusters at low temperature in $\mathrm{LiM}{\mathrm{n}}_2{\mathrm{O}}_4$ nanorods is elucidated. Field-cooled hysteresis loops (FC loops) and magnetic training effect (TE) measurements confirm the presence of an exchange-bias (EB) effect in $\mathrm{LiM}{\mathrm{n}}_2{\mathrm{O}}_4$ nanorods below the Néel temperature $(T_{\mathrm{N}}\sim 60\mathrm{K})$. Furthermore, by analyzing the TE, we conclude that the observed EB effect originates completely from an exchange coupling interaction at the interface between the AFM and cluster SG states. A phenomenological model based on phase coexistence is proposed to interpret the origin of the EB effect below 60 K in the present compound. In turn, the appearance of the EB effect further supports the coexistence of AFM order along with a cluster SG state in $\mathrm{LiM}{\mathrm{n}}_2{\mathrm{O}}_4$ nanorods.

Because of their complex Fermi surfaces, the identification of the physical phenomena contributing to electronic scattering in the Fe-based superconductors is a difficult task. Here, we report on the electrical resistivity, magnetoresistance, and Hall effect in two series of ${\mathrm{BaFe}}_{2-x}{T}_x{\mathrm{As}}_2(T=\mathrm{Co},\mathrm{Ni})$ crystals with different values of $x$. The $T$ contents were chosen so that the majority of the investigated samples present an intermediate magnetically ordered state and a superconducting ground state. We interpret the obtained results in terms of scattering of charge carriers by magnetic excitations instead of describing them as resulting uniquely from effects related to multiple-band conduction. Our samples are single crystals from the structural point of view and their overall magnetotransport properties are dominated by a single magnetic state.

Valence excitation spectra are computed for deep-center silicon-vacancy defects in 3C, 4H, and 6H silicon carbide (SiC), and comparisons are made with literature photoluminescence measurements. Optimizations of nuclear geometries surrounding the defect centers are performed within a Gaussian basis-set framework using many-body perturbation theory or density functional theory (DFT) methods, with computational expenses minimized by a QM/MM technique called SIMOMM. Vertical excitation energies are subsequently obtained by applying excitation-energy, electron-attached, and ionized equation-of-motion coupled-cluster (EOMCC) methods, where appropriate, as well as time-dependent (TD) DFT, to small models including only a few atoms adjacent to the defect center. We consider the relative quality of various EOMCC and TD-DFT methods for (i) energy-ordering potential ground states differing incrementally in charge and multiplicity, (ii) accurately reproducing experimentally measured photoluminescence peaks, and (iii) energy-ordering defects of different types occurring within a given polytype. The extensibility of this approach to transition-metal defects is also tested by applying it to silicon-substituted chromium defects in SiC and comparing with measurements. It is demonstrated that, when used in conjunction with SIMOMM-optimized geometries, EOMCC-based methods can provide a reliable prediction of the ground-state charge and multiplicity, while also giving a quantitative description of the photoluminescence spectra, accurate to within 0.1 eV of measurement for all cases considered.

Two-dimensional Lorentz gases formed by obstacles in the shape of circles, squares, and retroreflectors are reported to show a pronounced linear negative magnetoresistance at small magnetic fields. For circular obstacles at low number densities, our results agree with the predictions of a model based on classical retroreflection. In extension to the existing theoretical models, we find that the normalized magnetoresistance slope depends on the obstacle shape and increases as the number density of the obstacles is increased. The peaks are furthermore suppressed by in-plane magnetic fields as well as by elevated temperatures. These results suggest that classical retroreflection can form a significant contribution to the magnetoresistivity of two-dimensional Lorentz gases, while contributions from weak localization cannot be excluded, in particular for large obstacle densities.

Graphene quantum dots (GQD's) have optical properties which are very different from those of an extended graphene sheet. In this paper, we explore how the size, shape, and edge structure of a GQD affect its optical conductivity. Using representation theory, we derive optical selection rules for regular-shaped dots, starting from the symmetry properties of the current operator. We find that, where the $x$ and $y$ components of the current operator transform with the same irreducible representation (irrep) of the point group (for example in triangular or hexagonal GQD's), the optical conductivity is independent of the polarization of the light. On the other hand, where these components transform with different irreps (for example in rectangular GQD's), the optical conductivity depends on the polarization of light. We carry out explicit calculations of the optical conductivity of GQD's described by a simple tight-binding model and, for dots of intermediate size, find an absorption peak in the low-frequency range of the spectrum which allows us to distinguish between dots with zigzag and armchair edges. We also clarify the one-dimensional nature of states at the Van Hove singularity in graphene, providing a possible explanation for very high exciton-binding energies. Finally, we discuss the role of atomic vacancies and shape asymmetry.

The magneto-optical response of single-walled carbon nanotubes (CNTs) and graphene nanoribbons (GNRs) is studied theoretically, including excitonic effects. Both diagonal and nondiagonal response functions are obtained and employed to compute Faraday rotation spectra. For single-walled CNTs in a parallel field, the results show field-dependent splitting of the exciton absorption peaks caused by brightening a dark exciton state. Similarly, for GNRs in a perpendicular magnetic field, we observe a field-dependent shift of the exciton peaks and the emergence of an absorption peak above the energy gap. Results show that excitonic effects play a significant role in the optical response of both materials, particularly for the off-diagonal tensor elements.

Motivated by the recent experiments on the triangular lattice spin-liquid candidate ${\mathrm{YbMgGaO}}_4$, we explore the effect of spin-orbit coupling on the effective-spin correlation of the Yb local moments. We point out that the anisotropic interaction between the effective spins on the nearest-neighbor bonds is sufficient to reproduce the spin-wave dispersion of the fully polarized state in the presence of strong magnetic field normal to the triangular plane. We further evaluate the effective-spin correlation at zero magnetic field within the mean-field spherical approximation. We explicitly demonstrate that the nearest-neighbor anisotropic effective-spin interaction, originating from the strong spin-orbit coupling, enhances the effective-spin correlation at the M points in the Brillouin zone. We identify these results as strong evidence for the anisotropic interaction and strong spin-orbit coupling in ${\mathrm{YbMgGaO}}_4$.

The shock Hugoniot for full-density and porous ${\mathrm{CeO}}_2$ was investigated in the liquid regime using ab initio molecular dynamics (AIMD) simulations with Erpenbeck's approach based on the Rankine-Hugoniot jump conditions. The phase space was sampled by carrying out NVT simulations for isotherms between 6000 and 100 000 K and densities ranging from $\rho =2.5$ to $20\mathrm{g}/{\mathrm{cm}}^3$. The impact of on-site Coulomb interaction corrections $+U$ on the equation of state (EOS) obtained from AIMD simulations was assessed by direct comparison with results from standard density functional theory simulations. Classical molecular dynamics (CMD) simulations were also performed to model atomic-scale shock compression of larger porous ${\mathrm{CeO}}_2$ models. Results from AIMD and CMD compression simulations compare favorably with Z-machine shock data to 525 GPa and gas-gun data to 109 GPa for porous ${\mathrm{CeO}}_2$ samples. Using results from AIMD simulations, an accurate liquid-regime Mie-Grüneisen EOS was built for ${\mathrm{CeO}}_2$. In addition, a revised multiphase SESAME-type EOS was constrained using AIMD results and experimental data generated in this work. This study demonstrates the necessity of acquiring data in the porous regime to increase the reliability of existing analytical EOS models.

In-plane resistivity ${\rho }_{\parallel }$ and out-of-plane resistivity ${\rho }_{\perp }$ were investigated across the pressure-induced Mott transition in molecular Mott insulators $Z{\left[\mathrm{Pd}{\left(\mathrm{dmit}\right)}_2\right]}_2$ ($Z={\mathrm{Et}}_2{\mathrm{Me}}_2\mathrm{As}$, ${\mathrm{Me}}_4\mathrm{N}$, and ${\mathrm{EtMe}}_3\mathrm{P}$) having a triangular lattice. All three compounds exhibit superconducting transition with $T_c=5.5–7.0$ K in the metallic phase near the Mott insulating phase. For the ${\beta }^{\prime }\text{−}{\mathrm{Et}}_2{\mathrm{Me}}_2\mathrm{As}$ salt, the anisotropy ${\rho }_{\perp }/{\rho }_{\parallel }$ exceeds ${10}^3$ at low temperatures, indicating a highly two-dimensional electronic state with incoherent interlayer hopping. The $\beta \text{−}{\mathrm{Me}}_4\mathrm{N}$ salt has a smaller ${\rho }_{\perp }/{\rho }_{\parallel }$ exhibiting a weak interlayer coupling. The resistivity is dominated by electron-electron scattering in the metallic state for these two compounds, as expected in a correlated Fermi liquid. On the other hand, the ${\mathrm{EtMe}}_3\mathrm{P}$ salt with a valence bond order state becomes a nearly three-dimensional metal across the Mott transition, in which the electron correlation is strongly suppressed. Instead, the electron-phonon scattering plays a significant role in the resistivity. The different interlayer coherence is quantitatively explained by the calculated interlayer transfer integrals between $\mathrm{Pd}{\left(\mathrm{dmit}\right)}_2$ molecules. These results suggest that the dimensionality governs the nature of electron correlations in the Fermi liquid state.

Polarization conversion of light reflected from quantum wells governed by both magnetic field and light propagation direction is observed. We demonstrate that the polarization conversion is caused by the magnetospatial dispersion in quantum wells which manifests itself in the reflection coefficient contribution bilinear in the in-plane components of the magnetic field and the light wave vector. The magnetospatial dispersion is shown to arise due to structure inversion asymmetry of the quantum wells. The effect is resonantly enhanced in the vicinity of the heavy-hole exciton. We show that microscopically the magnetospatial dispersion is caused by the mixing of heavy- and light-hole states in the quantum well due to both orbital effect of the magnetic field and the in-plane hole motion. The degree of the structure inversion asymmetry is determined for GaAs/AlGaAs and CdTe quantum wells.

We demonstrate that a key elastic parameter of a suspended crystalline membrane—the Poisson ratio (PR) $\nu$—is a nontrivial function of the applied stress $\sigma$ and of the system size $L$, i.e., $\nu ={\nu }_L\left(\sigma \right)$. We consider a generic two-dimensional membrane embedded into space of dimensionality $2+d_c$. (The physical situation corresponds to $d_c=1$.) A particularly important application of our results is to freestanding graphene. We find that at a very low stress, when the membrane exhibits linear response, the PR ${\nu }_L\left(0\right)$ decreases with increasing system size $L$ and saturates for $L\to \infty$ at a value which depends on the boundary conditions and is essentially different from the value $\nu =-1/3$ previously predicted by the membrane theory within a self-consistent scaling analysis. By increasing $\sigma$, one drives a sufficiently large membrane (with the length $L$ much larger than the Ginzburg length) into a nonlinear regime characterized by a universal value of PR that depends solely on $d_c$, in close connection with the critical index $\eta$ controlling the renormalization of bending rigidity. This universal nonlinear PR acquires its minimum value ${\nu }_{\min }=-1$ in the limit $d_c\to \infty$, when $\eta \to 0$. With the further increase of $\sigma$, the PR changes sign and finally saturates at a positive nonuniversal value prescribed by the conventional elasticity theory. We also show that one should distinguish between the absolute and differential PR ($\nu$ and ${\nu }^{\mathrm{diff}}$, respectively). While coinciding in the limits of very low and very high stress, they differ in general: $\nu \ne {\nu }^{\mathrm{diff}}$. In particular, in the nonlinear universal regime, ${\nu }^{\mathrm{diff}}$ takes a universal value which, similarly to the absolute PR, is a function solely of $d_c$ (or, equivalently, of $\eta$) but is different from the universal value of $\nu$. In the limit of infinite dimensionality of the embedding space, $d_c\to \infty$ (i.e., $\eta \to 0$), the universal value of ${\nu }^{\mathrm{diff}}$ tends to $-1/3$, at variance with the limiting value $-1$ of $\nu$. Finally, we briefly discuss generalization of these results to a disordered membrane.

This is a numerical study of thermoelectric properties of ballistic bilayer graphene in the presence of a trigonal warping term in the effective Hamiltonian. We find, in the mesoscopic samples of the length $L>10\mu \mathrm{m}$ at sub-Kelvin temperatures, that both the Seebeck coefficient and the Lorentz number show anomalies (the additional maximum and minimum, respectively) when the electrochemical potential is close to the Lifshitz energy, which can be attributed to the presence of the van Hove singularity in a bulk density of states. At higher temperatures the anomalies vanish, but measurable quantities characterizing the remaining maximum of the Seebeck coefficient still unveil the presence of massless Dirac fermions and make it possible to determine the trigonal warping strength. Behavior of the thermoelectric figure of merit ($ZT$) is also discussed.

We report the physical properties of ${\mathrm{Tb}}_2{\mathrm{Hf}}_2{\mathrm{O}}_7$ based on ac magnetic susceptibility ${\chi }_{\mathrm{ac}}\left(T\right)$, dc magnetic susceptibility $\chi \left(T\right)$, isothermal magnetization $M\left(H\right)$, and heat capacity $C_{\mathrm{p}}\left(T\right)$ measurements combined with muon spin relaxation ($\mu \mathrm{SR}$) and neutron powder diffraction measurements. No evidence for long-range magnetic order is found down to 0.1 K. However, ${\chi }_{\mathrm{ac}}\left(T\right)$ data present a frequency-dependent broad peak (near 0.9 K at 16 Hz) indicating slow spin dynamics. The slow spin dynamics is further evidenced from the $\mu \mathrm{SR}$ data (characterized by a stretched exponential behavior) which show persistent spin fluctuations down to 0.3 K. The neutron powder diffraction data collected at 0.1 K show a broad peak of magnetic origin (diffuse scattering) but no magnetic Bragg peaks. The analysis of the diffuse scattering data reveals a dominant antiferromagnetic interaction in agreement with the negative Weiss temperature. The absence of long-range magnetic order and the presence of slow spin dynamics and persistent spin fluctuations together reflect a dynamical ground state in ${\mathrm{Tb}}_2{\mathrm{Hf}}_2{\mathrm{O}}_7$.

We investigate a bilayer Kitaev model in which two honeycomb layers are coupled by the Heisenberg interactions to discuss the effects of interlayer coupling on Kitaev quantum spin liquids (QSLs). In this model, there exists a local conserved quantity which results in no long-range spin correlations in the system. Using the exact diagonalization, bond-operator mean-field theory, and cluster expansion techniques, we study ground-state properties in the system. The obtained results suggest the existence of a first-order quantum phase transition between the Kitaev QSL and dimer-singlet states. We find that a one-triplet excitation from the dimer-singlet ground state is localized owing to the existence of the local conserved quantity. To examine finite-temperature properties, we make use of the thermal pure quantum state approach. We clarify that the double-peak structure in the specific heat inherent in the Kitaev QSL is maintained even above the quantum phase transition. The present results suggest that the Kitaev QSL is stable against the interlayer interference. Magnetic properties of multilayer Kitaev models are also addressed.

We predict that biaxial strain of several percent has a colossal effect on the magnetic anisotropy of ultrathin $\mathrm{Fe}\text{/}X{\mathrm{TiO}}_3$ $(X=\mathrm{Sr},\mathrm{Ba})$ bilayers grown epitaxially on appropriate substrates. We demonstrate that under large compressive biaxial strain the Fe film undergoes an in-plane to out-of-plane spin reorientation via ferroelectric polarization switching, where the critical strain depends on the Fe film thickness. The underlying mechanism is the interplay between the strain-enhanced magnetoelectric coupling associated with the enhanced polarization in the ferroelectric substrate and the strain-reduced magnetic anisotropy energy of the Fe overlayer. These findings open interesting prospects for exploiting stain engineering to harvest higher electric field efficiency of magnetic anisotropy for the next generation of magnetoelectric random access memory devices.