Uniaxial ferroelectric $\mathrm{BaT}{\mathrm{i}}_2{\mathrm{O}}_5$ with a Curie temperature $T_{\mathrm{C}}$ of 743 K was investigated to clarify its paraelectric-ferroelectric phase-transition behavior. The mechanism is discussed on the basis of the structure from short to long ranges determined by synchrotron x-ray diffraction and the lattice dynamics probed by Raman spectroscopy. $\mathrm{BaT}{\mathrm{i}}_2{\mathrm{O}}_5$ is regarded as a homogeneous system, and the lattice dynamics can be interpreted by the selection rules and tensor properties of the homogeneous structure. Angle-resolved polarized Raman spectroscopy clearly shows that an $A$-mode-type overdamped phonon plays the key role in the phase transition. Using a combination of experimental results and first-principles calculations, we explain the phase transition as follows: In one of three $\mathrm{Ti}{\mathrm{O}}_6$ octahedral units, Ti vibrates along the $b$ axis opposite an oxygen octahedral unit with large damping in the paraelectric phase, whereas this vibration is frozen in the ferroelectric phase, leading to a change in the space group from nonpolar $C2/m$ to polar $C2$.

Unconventional superconductivity typically emerges in the presence of quasidegenerate ground states, and the associated intense fluctuations are likely responsible for generating the superconducting state. Here we use polarized neutron scattering to study the spin space anisotropy of spin excitations in ${\mathrm{Fe}}_{1.07}\mathrm{Te}$ exhibiting bicollinear antiferromagnetic (AF) order, the parent compound of ${\mathrm{FeTe}}_{1-x}{\mathrm{Se}}_x$ superconductors. We confirm that the low-energy spin excitations are transverse spin waves, consistent with a local-moment origin of the bicollinear AF order. While the ordered moments lie in the $ab$ plane in ${\mathrm{Fe}}_{1.07}\mathrm{Te}$, it takes less energy for them to fluctuate out of plane, similar to ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ and NaFeAs. At energies above $E\gtrsim 20$ meV, we find magnetic scattering to be dominated by an isotropic continuum that persists up to at least 50 meV. Although the isotropic spin excitations cannot be ascribed to spin waves from a long-range-ordered local-moment antiferromagnet, the continuum can result from the bicollinear magnetic order ground state of ${\mathrm{Fe}}_{1.07}\mathrm{Te}$ being quasidegenerate with plaquette magnetic order.

Motivated by recent work on strain-induced pseudomagnetic fields in Dirac and Weyl semimetals, we analyze the possibility of analogous fields in two-dimensional nodal superconductors. We consider the prototypical case of a $d$-wave superconductor, a representative of the cuprate family, and find that the presence of weak, spatially varying strain leads to pseudomagnetic fields and Landau quantization of Bogoliubov quasiparticles in the low-energy sector. A similar effect is induced by the presence of generic, weak doping gradients. In contrast to genuine magnetic fields in superconductors, the strain- and doping-gradient-induced pseudomagnetic fields couple in a way that preserves time-reversal symmetry and is not subject to the screening associated with the Meissner effect. These effects can be probed by tuning weak applied supercurrents which lead to shifts in the energies of the Landau levels and hence to quantum oscillations in thermodynamic and transport quantities.

We present multiloop flow equations in the functional renormalization group (fRG) framework for the four-point vertex and self-energy, formulated for a general fermionic many-body problem. This generalizes the previously introduced vertex flow [F. B. Kugler and J. von Delft, Phys. Rev. Lett. 120, 057403 (2018)] and provides the necessary corrections to the self-energy flow in order to complete the derivative of all diagrams involved in the truncated fRG flow. Due to its iterative one-loop structure, the multiloop flow is well suited for numerical algorithms, enabling improvement of many fRG computations. We demonstrate its equivalence to a solution of the (first-order) parquet equations in conjunction with the Schwinger-Dyson equation for the self-energy.

Insertion of disorder in thermal interacting quantum systems decreases the amount of level repulsion and can lead to many-body localization. In this paper we use the many-body picture to perturbatively study the effect of level repulsion in the localized phase. We find that most eigenstates can be described accurately in an approximate way, including many with rare resonances. A classification of the rare resonances shows that most types are exponentially rare and requires exponential fine tuning in an approximate description. The classification confirms that no rare thermal eigenstates exist in a fully localized phase and we argue that all types of resonances need to become common if a continuous transition into a thermal phase should occur.

Topological photonic crystals are designed based on the concept of Zak's phase rather than the topological invariants such as the Chern number and spin Chern number, which rely on the existence of a nonvanishing Berry curvature. Our photonic crystals (PCs) are made of pure dielectrics and sit on a square lattice obeying the $C_{4v}$ point-group symmetry. Two varieties of PCs are considered: one closely resembles the electronic two-dimensional Su-Schrieffer-Heeger model, and the other continues as an extension of this analogy. In both cases, the topological transitions are induced by adjusting the lattice constants. Topological edge modes (TEMs) are shown to exist within the nontrivial photonic band gaps on the termination of those PCs. The high efficiency of these TEMs transferring electromagnetic energy against several types of disorders has been demonstrated using the finite-element method.

We consider mesoscopic four-terminal Josephson junctions and study emergent topological properties of the Andreev subgap bands. We use symmetry-constrained analysis for Wigner-Dyson classes of scattering matrices to derive band dispersions. When the scattering matrix of the normal region connecting superconducting leads is energy independent, the determinant formula for Andreev spectrum can be reduced to a palindromic equation that admits a complete analytical solution. Band topology manifests with an appearance of the Weyl nodes which serve as monopoles of finite Berry curvature. The corresponding fluxes are quantified by Chern numbers that translate into a quantized nonlocal conductance that we compute explicitly for the time-reversal-symmetric scattering matrix. The topological regime can also be identified by supercurrents as Josephson current-phase relationships exhibit pronounced nonanalytic behavior and discontinuities near Weyl points that can be controllably accessed in experiments.

Despite numerous studies, coupled spin and valley physics is currently limited to two-dimensional (2D) transition-metal dichalcogenides (TMDCs). Here, we predict an exceptional 2D valleytronic material associated with the spin-valley coupling phenomena beyond 2D TMDCs—single-layer (SL) H-$\mathrm{T}{\mathrm{l}}_2\mathrm{O}$. It displays large valley spin splitting (VSS), significantly larger than that of 2D TMDCs, and a finite band gap, which are both critically attractive for the integration of valleytronics and spintronics. More importantly, in sharp contrast to all the experimentally confirmed 2D valleytronic materials, where the strong valence-band VSS (0.15–0.46 eV) supports the spin-valley coupling, the VSS in SL H-$\mathrm{T}{\mathrm{l}}_2\mathrm{O}$ is pronounced in its conduction band (0.61 eV), but negligibly small in its valence band (21 meV), thus opening a way for manipulating the coupled spin and valley physics. Moreover, SL H-$\mathrm{T}{\mathrm{l}}_2\mathrm{O}$ possesses extremely high carrier mobility, as large as $9.8\times {10}^3\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$.

An intense terahertz laser field is shown to have a profound effect on the electronic and optical properties of quantum rings where the isotropic and anisotropic quantum rings can now be treated on equal footing. We have demonstrated that in isotropic quantum rings the laser field creates unusual Aharonov-Bohm oscillations that are usually expected in anisotropic rings. Furthermore, we have shown that intense laser fields can restore the isotropic physical properties in anisotropic quantum rings. In principle, all types of anisotropies (structural, effective masses, defects, etc.) can evolve as in isotropic rings in our present approach. Most importantly, we have found a continuous evolution of the energy spectra and intraband optical characteristics of structurally anisotropic quantum rings to those of isotropic rings in a controlled manner with the help of a laser field.

We theoretically study a Josephson junction based on a semiconducting nanowire subject to a time-dependent flux bias. We establish a general density-matrix approach for the dynamical response of the Majorana junction and calculate the resulting flux-dependent susceptibility using both microscopic and effective low-energy descriptions for the nanowire. We find that the diagonal component of the susceptibility, associated with the dynamics of the Majorana state populations, dominates over the standard Kubo contribution for a wide range of experimentally relevant parameters. The diagonal term, explored, in this Rapid Communication, in the context of Majorana physics, allows probing accurately the presence of Majorana bound states in the junction.

The tunneling Hamiltonian describes a particle transfer from one region to another. Although there is no particle storage in the tunneling region itself, it has an associated amount of energy. The corresponding energy flux was named reactance since, such as an electrical reactance, it manifests itself in time-dependent transport only. We show here that the existence of the energy reactance leads to the universal response of a mesoscopic thermometer, a floating contact coupled to an adiabatically driven quantum dot.

We develop a theory of quantum oscillations in insulators with an emergent Fermi sea of neutral fermions minimally coupled to an emergent $\text{U}\left(1\right)$ gauge field. As pointed out by Motrunich [Phys. Rev. B 73, 155115 (2006)], in the presence of a physical magnetic field the emergent magnetic field develops a nonzero value leading to Landau quantization for the neutral fermions. We focus on the magnetic field and temperature dependence of the analog of the de Haas–van Alphen effect in two and three dimensions. At temperatures above the effective cyclotron energy, the magnetization oscillations behave similarly to those of an ordinary metal, albeit in a field of a strength that differs from the physical magnetic field. At low temperatures, the oscillations evolve into a series of phase transitions. We provide analytical expressions for the amplitude and period of the oscillations in both of these regimes and simple extrapolations that capture well their crossover. We also describe oscillations in the electrical resistivity of these systems that are expected to be superimposed with the activated temperature behavior characteristic of their insulating nature and discuss suitable experimental conditions for the observation of these effects in mixed-valence insulators and triangular lattice organic materials.

Results of optical spectroscopy studies on bulk $2H-{\mathrm{WS}}_2$ at energies close to its direct band gap are presented. Reflectance and absorption measurements at low temperature show only one dominant feature due to the $A$ exciton of bulk ${\mathrm{WS}}_2$ at $\sim 2.02$ eV. However, a laser-modulated photoreflectance spectrum looks quite different, revealing a second even stronger feature $A^{*}\sim 62$ meV above $A$. The relative intensity of these two features is shown to change significantly in a lateral electroreflectance measurement with electric field applied perpendicular to the c axis of ${\mathrm{WS}}_2$. The experimental results are analyzed by comparison with many-body perturbation theory calculations, including the solutions of the Bethe-Salpeter equation. $A^{*}$ is identified as the first excited state of the $A$ exciton, that is, $A(n=2)$. The anomalous behavior of $A^{*}$ is explained by its distinct wave function spread along the c axis, the direction of weak van der Waals bonding, which makes it more susceptible to perturbations. Our ab initio calculations suggest that the $A$ exciton in the ground state has a two-dimensional (2D) nature with a large binding energy $E_{\text{b}}$, in fair agreement with $E_{\text{b}}\sim 90\pm 20$ meV estimated from a temperature-dependent reflectance study. The applicability of the 2D hydrogenic Wannier-Mott model for the exciton spectrum of a layered semiconductor like bulk ${\mathrm{WS}}_2$ is discussed.

We consider a graphene sheet encapsulated in a two-dimensional (2D) metallic grating and a substrate (${\mathrm{Al}}_2{\mathrm{O}}_3$) and subjected to an external magnetic field (in Faraday configuration). The grating consists of a thin perfectly conducting metal film perforated with a 2D periodic array of square holes. According to our calculations, significant changes in the spectra of the Faraday rotation angle of the transmitted wave and of the magnetic circular dichroism should be expected in this situation compared to bare graphene. We explain this enhancement by the excitation of graphene magnetoplasmons that accompanies the transmission of the electromagnetic wave through the structure. The results can be interesting for applications in THz photonics, such as switchable rotating polarizer and optical isolator.

We study the existence of guided acoustic modes in layered structures whose phase velocity is higher than that of bulk waves in a solid substrate or an adjacent fluid half space, which belong to the class of bound states in the radiation continuum (BICs). We demonstrate that in contrast to the electromagnetic case, non-symmetry-protected BICs exist in isotropic layered systems without periodic structures. Two systems supporting non-symmetry-protected sagittally polarized BICs have been identified: (i) a supported solid layer yields BICs whose phase velocity is higher than the transverse velocity of the substrate but lower than the longitudinal velocity; (ii) a supported solid layer loaded by a fluid half space supports BICs whose velocity is higher that the bulk velocity of the fluid but lower than acoustic velocities of the substrate. The latter case is a unique example of BICs in the sense that it does not involve an evanescent field in the fluid half space providing the radiation continuum. In either case, BICs are represented by isolated points in the dispersion relations located within “leaky” branches. We show that these BICs are robust with respect to small perturbations of the system parameters. Numerical results are provided for realistic materials combinations. We also show that no BICs exist in all-fluid layered structures, whereas in solid layered structures there are no shear horizontal BICs and no sagittally polarized BICs whose velocity exceeds the longitudinal velocity of the substrate.

Shock wave interactions with defects, such as pores, are known to play a key role in the chemical initiation of energetic materials. The shock response of hexanitrostilbene is studied through a combination of large-scale reactive molecular dynamics and mesoscale hydrodynamic simulations. In order to extend our simulation capability at the mesoscale to include weak shock conditions ($<6$ GPa), atomistic simulations of pore collapse are used to define a strain-rate-dependent strength model. Comparing these simulation methods allows us to impose physically reasonable constraints on the mesoscale model parameters. In doing so, we have been able to study shock waves interacting with pores as a function of this viscoplastic material response. We find that the pore collapse behavior of weak shocks is characteristically different than that of strong shocks.

The problem of the theoretical description of the critical temperature $T_c$ of a Fermi superfluid dates back to the work by Gorkov and Melik-Barkhudarov (GMB), who addressed it for a weakly coupled (dilute) superfluid in what would today be referred to as the (extreme) BCS (weak-coupling) limit of the BCS-BEC crossover. The point made in this context by GMB was that particle-particle (pairing) excitations, which are responsible for superfluidity to occur below $T_c$, and particle-hole excitations, which give rise to screening also in a normal system, get effectively disentangled from each other in the BCS limit, thus yielding a reduction by a factor of 2.2 of the value of $T_c$ obtained when neglecting screening effects. Subsequent work on this topic, that was aimed at extending the original GMB argument away from the BCS limit with diagrammatic methods, has tout court kept this disentangling between pairing and screening throughout the BCS-BEC crossover, without realizing that the conditions for it to be valid are soon violated away from the BCS limit. Here, we reconsider this problem from a more general perspective and argue that pairing and screening are intrinsically entangled with each other along the whole BCS-BEC crossover but for the BCS limit considered by GMB, with the particle-hole excitations soon transmuting into particle-particle excitations away from this limit. We substantiate our argument by performing a detailed numerical calculation of the GMB diagrammatic contribution suitably extended to the whole BCS-BEC crossover, where the full wave-vector and frequency dependence occurring in the repeated in-medium two-particle scattering is duly taken into account. Our numerical calculations are tested against analytic results available in both the BCS and BEC limits, and the contribution of the GMB diagrammatic term to the scattering length of composite bosons in the BEC limit is highlighted. We calculate $T_c$ throughout the BCS-BEC crossover and find that it agrees quite well with quantum Monte Carlo calculations and experimental data available in the unitarity regime.