Using an experimental setup designed to scan a submicron sized light spot and collect the photogenerated current through larger electrodes, we map the photovoltaic response in ferroelectric $\mathrm{BiFe}{\mathrm{O}}_3$ single crystals. We study the effect produced by a unique 180° ferroelectric domain wall (DW) and show that the photocurrent maps are significantly affected by its presence and shape. The effect is large in its vicinity and in the Schottky barriers at the interface with the Au electrodes, but no extra photocurrent is observed when the illuminating spot touches the DW, indicating that this particular entity is not the heart of specific photoelectric properties. Using 3D modeling, we argue that the measured effect is due to the spatial distribution of internal fields which are significantly affected by the charge of the DW due to its distortion.

We have performed polarized and temperature-dependent Raman scattering measurements on extremely large magnetoresistance compounds ${\mathrm{TaAs}}_2$ and ${\mathrm{NbAs}}_2$. In both crystals, all the Raman active modes, including six $A_g$ modes and three $B_g$ modes, are clearly observed and well assigned with the combination of symmetry analysis and first-principles calculations. The well-resolved periodic intensity modulations of the observed modes with rotating crystal orientations verify the symmetry of each assigned mode and are fitted to experimentally determine the elements of Raman tensor matrices. The broadening of two $A_g$ modes seen in both compounds allows us to estimate the electron-phonon coupling constant, which suggests a relatively small electron-phonon coupling in the semimetals ${\mathrm{TaAs}}_2$ and ${\mathrm{NbAs}}_2$. The present study provides the fundamental lattice dynamics information on ${\mathrm{TaAs}}_2$ and ${\mathrm{NbAs}}_2$ and may shed light on the understanding of their extraordinary large magnetoresistance.

We have carried out ${}^{63,65}\mathrm{Cu}$ NMR spectra measurements in a magnetic field up to about 45 T on a single crystal of a multiferroic triangular antiferromagnet ${\mathrm{CuCrO}}_2$. The measurements were performed for magnetic fields aligned along the crystal $c$ axis. Field and temperature evolution of the spectral shape demonstrates a number of phase transitions. It was found that the 3D magnetic ordering takes place in the low field range ($H\lesssim 15$ T). At higher fields magnetic structures form within individual triangular planes whereas the spin directions of the magnetic ions from neighboring planes are not correlated. It is established that the 2D-3D transition is hysteretic in field and temperature. Line-shape analysis reveals several possible magnetic structures existing within individual planes for different phases of ${\mathrm{CuCrO}}_2$. Within certain regions on the magnetic $H-T$ phase diagram of ${\mathrm{CuCrO}}_2$ a 3D magnetic ordering with tensor order parameter is expected.

The magnetic structures of the ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Cr}}^{2+}$ moments in the nonsuperconducting parent compound ${\mathrm{EuCr}}_2{\mathrm{As}}_2$ have been determined by using neutron diffraction. While the ${\mathrm{Eu}}^{2+}$ moments order ferromagnetically with moments along the $\mathbf{c}$ direction at $T_{\text{C}}=21.0\left(1\right)$ K, the ordering temperature of the ${\mathrm{Cr}}^{2+}$ moments is found to be at very high temperature of 680(40) K by using magnetization measurements. The ${\mathrm{Cr}}^{2+}$ moments order in a $G$-type antiferromagnetic structure with moments along the $\mathbf{c}$ direction. According to this magnetic structure, the nearest-neighbor ${\mathrm{Cr}}^{2+}$ moments are antiferromagnetically aligned in the $a\text{−}b$ plane as well as in the $\mathbf{c}$ direction. The ordered magnetic moment of the ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Cr}}^{2+}$ amounts to $6.2\left(5\right){\mu }_{\mathrm{B}}$ and $1.7\left(4\right){\mu }_{\mathrm{B}}$, respectively, at $T=2$ K.

Both ${\mathrm{Nb}}_3{\mathrm{Pd}}_x{\mathrm{Se}}_7$ and ${\mathrm{Ta}}_4{\mathrm{Pd}}_3{\mathrm{Te}}_{16}$ crystallize in a monoclinic point group while exhibiting superconducting transition temperatures as high as $T_c\sim 3.5$ and $\sim 4.7$ K, respectively. Disorder was claimed to lead to the extremely large upper critical fields ($H_{c2}$) observed in related compounds. Despite the presence of disorder and heavier elements, $H_{c2}\mathrm{s}$ in ${\mathrm{Ta}}_4{\mathrm{Pd}}_3{\mathrm{Te}}_{16}$ are found to be considerably smaller than those of ${\mathrm{Nb}}_3{\mathrm{Pd}}_x{\mathrm{Se}}_7$ while displaying an anomalous, nonsaturating linear dependence on temperature $T$ for fields along all three crystallographic axes. In contrast, crystals of the latter compound displaying the highest $T_c\mathrm{s}$ display $H_{c2}\propto {(1-T/T_c)}^{1/2}$, which in monolayers of transition metal dichalcogenides is claimed to be evidence for an Ising paired superconducting state resulting from strong spin-orbit coupling. This anomalous $T$ dependence indicates that the superconducting state of ${\mathrm{Nb}}_3{\mathrm{Pd}}_x{\mathrm{Se}}_7$ is quasi-two-dimensional in nature. This is further supported by a nearly divergent anisotropy in upper-critical fields, i.e., $\gamma =H_{c2}^b/H_{c2}^{a^{\prime }}$, upon approaching $T_c$. Hence, in ${\mathrm{Nb}}_3{\mathrm{Pd}}_x{\mathrm{Se}}_7$ the increase of $T_c$ correlates with a marked reduction in electronic dimensionality as observed, for example, in intercalated FeSe. For the Nb compound, Density functional theory (DFT) calculations indicate that an increase in the external field produces an anisotropic orbital response, with especially strong polarization at the Pd sites when the field is perpendicular to their square planar environment. The field also produces an anisotropic spin moment at both Pd sites. Therefore, DFT suggests the field-induced pinning of the spin to the lattice as a possible mechanism for decoupling the superconducting planes.

Diffusionless displacive lattice rearrangements, which include martensitic transformations, are in real materials often accompanied by a displacive drag of interstitials. The interplay of both processes leads to a particular atomistic arrangement of the interstitials in the product phase, which is decisive for its performance. An archetype example is the martensitic transformation in Fe-C alloys. One of the puzzles for this system is that the deviation from the cubic symmetry (i.e., the tetragonality) in the martensite resulting from this interplay is lower than what thermodynamics dictates. In our ab initio approach, the relative motion of C in the transforming lattice is studied with the nudged elastic band method. We prove that an atomic shearlike shuffle mechanism of adjacent $\left(11\overline{2}\right)$ Fe layers along the $\pm {\left[111\right]}_{\mathrm{bcc}}$ directions is essential to achieve a redistribution of C atoms during the fcc $\to$ bcc transition, which fully explains the abnormal behavior. Furthermore, the good agreement with experiment validates our method to treat a diffusionless redistribution of interstitials and a displacive rearrangement of the host lattice simultaneously.

It is shown that a generic form of an antiferromagnetic wave function opens strong electron-phonon coupling channels in the iron-based superconductors. In the nonmagnetic state these channels exist locally on a single iron atom but are canceled out between the two iron atoms in the primitive unit cell. Our findings are based on symmetry and the presence of an $xz/yz$ Fermi surface near the M point and thus should be relevant for the known iron-based pnictide or chalcogenide superconductors.

By coupling one quantum dot to a normal metal, the Kondo structure is constructed, and then its change is investigated in the case of the dot connected with a DIII-class topological superconductor supporting Majorana doublets at its ends. It is observed that Kondo correlations occur between the localized state in the dot and two continuum states simultaneously, i.e., the continuum state in the metal and the continuum Andreev reflection state between the metal and topological superconductor. This causes the Kondo-model Hamiltonian to be topologically protected by the $\text{SU}{\left(2\right)}_s⋊Z_2^T$ symmetry. Besides, the system Kondo temperature can be altered via adjusting the structural parameters. This work clarifies the special influence of Majorana doublet on the Kondo effect.

We study the effect of strong electron-phonon interactions on the damping of the Higgs amplitude mode in superconductors by means of nonequilibrium dynamical mean-field simulations of the Holstein model. In contrast to the BCS dynamics, we find that the damping of the Higgs mode strongly depends on the temperature, becoming faster as the system approaches the transition temperature. The damping at low temperatures is well described by a power law, while near the transition temperature the damping shows exponential-like behavior. We explain this crossover in terms of a temperature-dependent quasiparticle lifetime caused by the strong electron-phonon coupling, which smears the superconducting gap edge and makes the relaxation of the Higgs mode into quasiparticles more efficient at elevated temperatures. We also reveal that the phonon dynamics can soften the Higgs mode, which results in a slower damping.

Dimer models have long been a fruitful playground for understanding topological physics. Here, we introduce a class, termed Majorana-dimer models, wherein bosonic dimers are decorated with pairs of Majorana modes. We find that the simplest examples of such systems realize an intriguing, intrinsically fermionic phase of matter that can be viewed as the product of a chiral Ising theory, which hosts deconfined non-Abelian quasiparticles, and a topological $p_x-i{p}_y$ superconductor. While the bulk anyons are described by a single copy of the Ising theory, the edge remains fully gapped. Consequently, this phase can arise in exactly solvable, frustration-free models. We describe two parent Hamiltonians: one generalizes the well-known dimer model on the triangular lattice, while the other is most naturally understood as a model of decorated fluctuating loops on a honeycomb lattice. Using modular transformations, we show that the ground-state manifold of the latter model unambiguously exhibits all properties of the $\text{Ising}\times (p_x-i{p}_y)$ theory. We also discuss generalizations with more than one Majorana mode per site, which realize phases related to Kitaev's 16-fold way in a similar fashion.

We explore a scheme for engineering a one-dimensional spinless $p$-wave superconductor hosting unpaired Majorana zero-energy modes, using an all-electric setup with a spin-orbit-coupled quantum wire in proximity to an $s$-wave superconductor. The required crossing of the Fermi level by a single spin-split energy band is ensured by employing a periodically modulated Rashba interaction, which, assisted by electron-electron interactions and a uniform Dresselhaus interaction, opens a gap at two of the spin-orbit shifted Fermi points. While an implementation in a hybrid superconductor-semiconductor device requires improvements upon present-day capabilities, a variant of our scheme where spin-orbit-coupled cold fermions are effectively proximity coupled to a BEC reservoir of Feshbach molecules may provide a ready-to-use platform.

When treating the exchange interaction of Wannier excitons, usually only the leading terms of the analytic and the nonanalytic exchange interaction are considered. However, higher-order terms can lead to a splitting of exciton states, for which reason a splitting of the $1S$ exciton in cuprous oxide $\left({\mathrm{Cu}}_2\mathrm{O}\right)$ depending on its total momentum $\hslash K$ has been attributed to a $K$-dependent analytic exchange interaction by Dasbach et al. [Phys. Rev. Lett. 91, 107401 (2003)]. Going beyond the common treatment of the exchange interaction, we derive the correct expressions for these $K$-dependent higher-order terms using $\mathit{k}·\mathit{p}$ perturbation theory. We prove that the appearance of a $K$-dependent exchange interaction is inseparably connected with a $K$-independent exchange interaction of $P$ and $D$ excitons. We estimate the magnitude of these terms for ${\mathrm{Cu}}_2\mathrm{O}$ from microscopic calculations and show that they are far too small to explain the observed $K$-dependent splitting. Instead, this splitting has to be treated in terms of the dispersion of the excitons. Furthermore, we prove the occurrence of a coupling between longitudinal and transverse excitons in ${\mathrm{Cu}}_2\mathrm{O}$ due to the $K$-dependent nonanalytic exchange interaction.

The zone-center phonons are calculated for rhombohedral $\mathrm{CsGe}{X}_3$ compounds with $X=\mathrm{I}$, Br, Cl using density functional perturbation theory in a plane-wave pseudopotential method. The infrared absorption and reflection spectra are simulated and show that the absorption has a strong contribution from the LO as well as TO modes. Both the lowest and highest IR active modes have strong oscillator strengths for each symmetry ($A_1$ and $E$) and correspond to motions of either the Cs or Ge ions relative to the halogen ions. respectively. The intermediate modes have low oscillator strength because their mode pattern shows less clearly a dipole pattern. The polarization dependent Raman spectra for various backscattering configurations are simulated. Only the highest modes of each symmetry have a strong Raman intensity. The results for the Raman spectra are found to be in good agreement with the available experimental data for polycrystalline films when averaging over directions. The Born effective charges and high and low frequency dielectric tensors as well as the second-order nonlinear optical coefficients are calculated. The ratio of the static to the high-frequency dielectric constants are extremely high and analyzed in terms of the contribution of each mode to the Lyddane-Sachs-Teller relation. The nonlinear optical coefficients decrease strongly from I to Br to Cl in reverse order of the band gaps and are found to be consistent with available experimental results.

Two-dimensional electron gases (2DEGs) at surfaces and interfaces of semiconductors are described straightforwardly with a one-dimensional (1D) self-consistent Poisson-Schrödinger scheme. However, their band energies have not been modeled correctly in this way. Using angle-resolved photoelectron spectroscopy we study the band structures of 2DEGs formed at sulfur-passivated surfaces of InAs(001) as a model system. Electronic properties of these surfaces are tuned by changing the S coverage, while keeping a high-quality interface, free of defects and with a constant doping density. In contrast to earlier studies we show that the Poisson-Schrödinger scheme predicts the 2DEG band energies correctly but it is indispensable to take into account the existence of the physical surface. The surface substantially influences the band energies beyond simple electrostatics, by setting nontrivial boundary conditions for 2DEG wave functions.

The Quantum anomalous Hall (QAH) effect in the films with nontrivial band structure accompanies the ferromagnetic transition in the system of magnetic dopants. Experimentally, the QAH transition manifests itself as a jump in the dependence of longitudinal resistivity on a weak external magnetic field. Microscopically, this jump originates from the emergence of a chiral edge mode on one side of the ferromagnetic transition. We study analytically the effect of an extended confinement on the structure of the edge modes. We employ the simplest model of the extended confinement in the form of a potential step next to the hard wall. It is shown that, unlike the conventional quantum Hall effect, where all edge channels are chiral, in the QAH effect, a complex structure of the boundary leads to nonchiral edge modes which are present on both sides of the ferromagnetic transition. Wave functions of nonchiral modes are different above and below the transition: on the “topological” side, where the chiral edge mode is supported, nonchiral modes are “repelled” from the boundary; i.e., they are much less localized than on the “trivial” side. Thus, the disorder-induced scattering into these modes will boost the extension of the chiral edge mode. The prime experimental manifestation of nonchiral modes is that, by contributing to longitudinal resistance, they smear the QAH transition.

We carry out a theoretical analysis of a prototypical Majorana system, which demonstrates the existence of a Majorana-mediated many-body state and an associated intermediate low-energy fixed point. Starting from two Majorana bound states, hosted by a Coulomb-blockaded topological superconductor and each coupled to a separate lead, we derive an effective low-energy Hamiltonian, which displays a Kondo-like character. However, in contrast to the Kondo model which tends to a strong- or weak-coupling limit under renormalization, we show that this effective Hamiltonian scales to an intermediate fixed point, whose existence is contingent upon teleportation via the Majorana modes. We conclude by determining experimental signatures of this fixed point, as well as the exotic many-body state associated with it.

We report the experimental evidence of directional Fano resonances at the scattering of a linearly polarized electromagnetic plane wave by a homogeneous dielectric sphere with a high refractive index and low losses. We observe a typical asymmetric Fano profile for the intensity scattered in practically any given direction, while the overall extinction cross section remains Lorentzian. The phenomenon originates in the interference of the selectively excited electric dipolar and quadrupolar modes. The selectivity of the excitation is achieved by the proper choice of the frequency of the incident wave. Owing to the scaling invariance of the Maxwell equations, in these experiments we mimic the scattering of the visible and near IR radiation by a nanoparticle made of common semiconductor materials (Si, Ge, GaAs, GaP) by the equivalent scattering of a spherical particle of 18 mm in diameter in the microwave range. The theory developed to explain the experiments extends the conventional Fano approach to the case when both interfering partitions are resonant. A perfect agreement between the experiment and the theory is demonstrated.

The neutral fermionic edge mode is essential to the non-Abelian topological property and its experimental detection in $Z_k$ fractional quantum Hall (FQH) state for $k>1$. Usually, the identification of the edge modes in a finite size system is difficult, especially near the region of the edge reconstruction, due to mixing with the bosonic edge mode and the bulk states as well. We study the edge-mode excitations of the Moore-Read (MR) and Read-Rezayi (RR) states by using Jack polynomials in the truncated subspace. It is found that the electron density, as a detector, has marked different behaviors between the bosonic and fermionic edge modes. As an application, it helps us to identify them near the edge reconstruction and in the RR edge spectrum. On the other hand, we systematically study the edge excitations for the RR state, extrapolating the edge velocities and their related coherence length and temperature in the interferometer experiments.

Recently, it was shown that by using special artificial materials it is possible to ensure that all electromagnetic modes of free space are conjugately matched to the modes of a material body and, thus, all modes deliver power to the body in the most effective way. Such a fascinating feature is acquired because the conjugate matching does not concern only the propagating modes but, most importantly, is applied to all evanescent modes; in this way, all the possible ways of transferring the electromagnetic energy to the material body can be optimally exploited. However, coupling to higher-order (mostly evanescent) modes is weak and totally disappears in the limit of an infinite planar boundary. Here, we show that by properly perturbing the surface of the receiving or emitting body with, for example, randomly distributed small particles, we can open up channels for super-radiation into the far zone. The currents induced in the small particles act as secondary sources (radiation “vessels”) which send the energy to travel far away from the surface and, reciprocally, receive power from far-located sources. For a particular example, we theoretically predict about 20-fold power transfer enhancement between the conjugately matched power-receiving body (as compared with the ideal black body) and far-zone sources. Reciprocally, the proposed structure radiates about 20 times more power into the far zone as compared with the same source over a perfect reflector.

We report interlayer electronic transport in ${\mathrm{CaMnBi}}_2$ single crystals. Quantum oscillations and angular magnetoresistance suggest coherent electronic conduction and valley polarized conduction of Dirac states. The small cyclotron mass, high mobility of carriers, and nontrivial Berry's phase are consistent with the presence of Dirac fermions on the side wall of the warped cylindrical Fermi surface. Similarly to ${\mathrm{SrMnBi}}_2$, which features an anisotropic Dirac cone, our results suggest that magnetic-field-induced changes in interlayer conduction are also present in layered bismuth-based materials with a zero-energy line in momentum space created by the staggered alkaline earth atoms.