Chemical composition and pressure dependencies of the electronic structures of ${\text{Bi}}_{2-x}{\text{Sb}}_x{\text{Te}}_{3-y}{\text{Se}}_y$ ($y\sim 1.2$) have been studied by the high-resolution x-ray absorption spectroscopy. We find a shift of the Bi-$L_3$ absorption edge due to Sb substitution, suggesting a change in the Bi charge state. In the pressure dependence, the electronic structures of ${\text{Bi}}_2{\text{Te}}_2\text{Se}$ and ${\text{Bi}}_{1.5}{\text{Sb}}_{0.5}{\text{Te}}_2\text{Se}$ start to change below the pressure of the first structural phase transition where the emergence of the superconductivity was observed and then show a large change just around that pressure. This is in contrast to the behavior observed in ${\text{Bi}}_2{\text{Se}}_3$-based compounds where the structural phase transition was necessary for the onset of superconductivity. We performed density functional theory calculations using the experimentally determined structures for ${\text{Bi}}_2{\text{Te}}_2\text{Se}$ and ${\text{Bi}}_{1.5}{\text{Sb}}_{0.5}{\text{Te}}_2\text{Se}$. We show that both compounds become metallic within the rhombohedral phase below the pressure of the first structural transition and we thus corroborate the experimental observation. The experimental and calculated results show that closing the gap and increasing the density of states in the rhombohedral phase are the triggers to induce superconductivity.

All-inorganic perovskite quantum dots films have received significant research interest for photovoltaic applications because of their better mechanical durability than bulk film and the various tunable properties that perovskite quantum dots exhibit. Here we develop a simple, almost analytic approach to calculating the electronic states in self-organized perovskite quantum dot systems. We present an extension to the tight-binding method, where the role of atoms is now played by quantum dots. This generalized tight-binding approach is applied to assess the feasibility of achieving miniband formation through phenyl-${\mathrm{C}}_{60}$-butyric acid methyl ester (PCBM)/${\mathrm{CsPbI}}_3$ quantum dots films. Type-II band alignment in these hybrid heterostructures led to the development of a nearly-free-electron model combined with a Hartree variational approach for describing conduction-band states. By implementing our approach, we reveal that PCBM/${\mathrm{CsPbI}}_3$ quantum dot films are indirect-band-gap systems. A 41-meV bandwidth for the hole ground miniband is found due to the coupling between dots. Energy-gap values consistent with the experiment are obtained. This approach opens the way for calculating relevant optical properties in a broad class of perovskite quantum dot systems, such as the absorption coefficient.

Joint measurements of two-Pauli observables are powerful tools for both the control and protection of quantum information. By following a simple recipe for measurement choices, single- and two- qubit rotations using two-Pauli parity and single qubit measurements are guaranteed to be unitary while requiring only a single ancilla qubit. This language for measurement based quantum computing is shown to be directly applicable to encoded double quantum dot singlet-triplet spin qubits, by measuring spin-parity between dots from neighboring qubits. Along with exchange interaction, a complete, leakage free, measurement based gate set can be shown, up to a known Pauli correction. Both theoretically exact spin-parity measurements and experimentally demonstrated asymmetric spin-parity measurements are shown to be viable for achieving the proposed measurement based scheme, provided some extra leakage mitigating measurement steps. This method of spin qubit control offers a leakage suppressed, low resource overhead implementation of a measurement-based control that is viable on current spin qubit processor devices.

The Rashba interaction is known to yield unusual effects in two-dimensional electron solids. One of these is the magnetotransport under the combined influence of an in-plane electric field and a magnetic field. The result is an anisotropic magnetoresistance that characterizes the band propagation concomitant with the spintronic influence of the Rashba coupling and the Zeeman interaction with the applied magnetic field. With the recent experimental results in mind, we present here a fully microscopic treatment of the underlying relaxation behavior using the methods of dissipative quantum statistical mechanics. The employed Kubo formula for the conductivity helps to make connection with particle diffusivity of the two-dimensional electronic solid and also under certain limits, with the semiclassical Boltzmann transport equation for the conductivity. The presented theory is in good agreement with recent data.

We construct a set of lattice models of noninteracting topological insulators with chiral symmetry in three dimensions. We build a model of the topological insulators in the class AIII by coupling lower dimensional models of $\mathbb{Z}$ classes. By coupling the two AIII models related by time-reversal symmetry we construct other chiral symmetric topological insulators that may also possess additional symmetries (the time-reversal and/or particle-hole). There are two different chiral symmetry operators for the coupled model that correspond to two distinct ways of defining the sublattices. The integer topological invariant (the winding number) in case of weak coupling can be either the sum or difference of indices of the basic building blocks, dependent on the preserved chiral symmetry operator. The value of the topological index in case of weak coupling is determined by the chiral symmetry only and does not depend on the presence of other symmetries. For $\mathbb{Z}$ topological classes AIII, DIII, and CI with chiral symmetry are topologically equivalent, it implies that a smooth transition between the classes can be achieved if it connects the topological sectors with the same winding number. We demonstrate this explicitly by proving that the gapless surface states remain robust in $\mathbb{Z}$ classes as long as the chiral symmetry is preserved, and the coupling does not close the gap in the bulk. By studying the surface states in ${\mathbb{Z}}_2$ topological classes, we show that class CII and AII are distinct, and cannot be adiabatically connected.

In this paper, we report the successful synthesis of a Fe-based highly spin-polarized quaternary Heusler alloy FeMnVGa and its structural, magnetic, and transport properties probed through different experimental methods and theoretical techniques. Density functional theory (DFT) calculations performed on different types of structures reveal that the structure with Ga at $4a$, V at $4b$, Mn at $4c$, and Fe at $4d$ (space group $F\overline{4}3m$) possess minimum energy among all the ordered variants. Ab initio simulations in the most stable ordered structure show that the compound is a ferromagnet having a large spin-polarization (89.9%). Neutron diffraction reveals that the compound crystallizes in disordered type-2 structure (space group $Fm\overline{3}m$) in which Ga occupies at $4a$, V $4b$ and Fe/Mn occupy $4c/4d$ sites with 50:50 proportions. The structural disorder is further confirmed by x-ray diffraction, extended x-ray absorption fine structure, ${}^{57}\mathrm{Fe}$ Mössbauer spectrometry results, and DFT calculations. Magnetization studies suggest that the compound orders ferromagnetically below $T_{\mathrm{C}}\sim 293$ K and the saturation magnetization follows the Slater-Pauling rule. Mössbauer spectrometry, along with neutron diffraction, suggest that Mn is the major contributor to the total magnetism in the compound, consistent with the theoretical calculations, which also indicates that spin polarization remains high (81.3%), even in the presence of such large atomic disorder. The robustness of the half-metallic ferromagnetic (HMF) property in the presence of disorder is a quite unique characteristic over other reported HMF in literature and makes this compound quite promising for spintronics applications.

We have studied the evolution of the topological properties of a band-engineered, AB-stacked bilayer honeycomb structure in the presence of a Haldane flux. Without a Haldane flux, band engineering makes the band touching points (the so-called Dirac points) move towards each other and eventually merge into one at an intermediate $\mathbf{M}$ point in the Brillouin zone. Here the dispersion is linear along one direction and quadratic along the other. In the presence of a Haldane flux, the system acquires topological properties, and finite Chern numbers can be associated with the pairs of the conduction and the valence bands. The valence band closer to the Fermi level ($E_F$) possesses Chern numbers equal to $\pm 2$ and $\pm 1$, while the one further away from $E_F$ corresponds to Chern numbers $\pm 1$. The conduction bands are associated with similar properties, except their signs are reversed. The Chern lobes shrink in the band-engineered model, and we find evidence of multiple topological phase transitions, where the Chern numbers discontinuously jump from $\pm 2$ to $\mp 2, \pm 1$ to $\mp 1, \pm 1$ to 0 to $\pm 2,$ and $\pm 2$ to $\pm 1$. These transitions are supported by the presence or absence of the chiral edge modes in a nanoribbon bilayer geometry and the vanishing of the plateau in the anomalous Hall conductivity. Different phases are further computed for different hopping amplitudes across the layers, which shows shrinking of the Chern lobes for large interlayer tunneling.

We study a non-Hermitian variant of the $(2+1)$-dimensional Dirac wave equation that hosts a real energy spectrum with pairwise-orthogonal eigenstates. In the spatially uniform case, the Hamiltonian's non-Hermitian symmetries allow its eigenstates to be mapped to a pair of Hermitian Dirac subsystems. When a wave is transmitted across an interface between two spatially uniform domains with different parameters, an anomalous form of Klein tunneling occurs, whereby reflection is suppressed while the transmittance is substantially higher or lower than unity. The interface can even function as a simultaneous laser and coherent perfect absorber. Remarkably, the violation of flux conservation occurs entirely at the interface, with no wave amplification or damping taking place in the bulk. Moreover, at energies within the Dirac mass gaps, the interface can support exponentially localized boundary states with real energies. These features of the continuum model can be reproduced in non-Hermitian lattice models.

Circularly polarized light fails to generate currents in inversion-symmetric Weyl semimetals with degenerate Weyl nodes. While each node generates current with the direction depending on its chirality, the two currents in the two degenerate nodes of opposite chirality cancel each other. By extension, it is also generally expected that the currents generated at the same Weyl node by the fields of opposite helicity should also observe mirror symmetry and cancel. Surprisingly, here we find that this is not the case. The origin of this effect lies in the nonlinear energy dispersion, which manifests strongly already very close to the Weyl nodes, where linear dispersion is expected to hold and the Weyl fermions are thus expected to be massless. A scheme based on using a trefoil field composed of a counterrotating fundamental and its second harmonic is proposed to control the induced asymmetry at a chiral node from positive to negative, including zero.

Thermal transport simulations have attracted wide attention in recent years, and one standard approach is to use the Green-Kubo method based on machine-learning interatomic potentials and equilibrium molecular dynamics (GK-MLIP-EMD). In this work, we focus on the lattice thermal conductivities ${\kappa }_L\mathrm{s}$ for solids with atomic diffusion by taking $\beta \text{−}\mathrm{C}{\mathrm{u}}_{2\text{−}x}\mathrm{Se}$ ($0\le x\le 0.05$) as an example. Surprisingly, the GK-MLIP-EMD approach fails in the evaluation of ${\kappa }_L\mathrm{s}$ for $\beta \text{−}\mathrm{C}{\mathrm{u}}_{1.95}\mathrm{Se}$, whereas the direct method based on nonequilibrium molecular dynamics reliably predicts these values instead. The failure of GK-MLIP-EMD for $\beta \text{−}\mathrm{C}{\mathrm{u}}_{1.95}\mathrm{Se}$ could be attributed to the ambiguous projection of the local atomic potential energy $U_i$ in MLIPs, exacerbated by the Cu diffusion at elevated temperatures. The Cu diffusion in $\beta \text{−}\mathrm{C}{\mathrm{u}}_{1.95}\mathrm{Se}$ greatly increases the ratio of the convective term and the uncertainty of the conductive term. These influences are considered negligible in crystalline solids. Our findings imply that the ambiguous definition of $U_i$ in MLIPs breaks down the applicability of the GK-MLIP-EMD approach to ${\kappa }_L$ prediction for solids with severe atomic diffusion.

Nodal-line semimetals provide the opportunity to explore exotic properties owing to their nontrivial band structures. Here, the structural and electronic properties of nodal-line semimetal candidate CaCdGe were systematically investigated under pressure up to 53.3 GPa through synchrotron x-ray diffraction, Raman scattering, electrical transport measurements, and theoretical calculations. Upon compression, although no trace of evident structural transition is detected, a maximum value of lattice ratio $c/a$ and abnormal changes in bond angles around $P_{\mathrm{C}}\sim 12.5\mathrm{GPa}$ are observed from our x-ray diffraction data. Meanwhile, a new Raman peak is observed in the vicinity of $P_{\mathrm{C}}$. Furthermore, the resistance parameters ($n, A$, and ${\mathrm{\Theta }}_R$) all show the abrupt changes above $P_{\mathrm{C}}$. Since the electronic topological transition is absent within 30.0 GPa based on band structure calculations, our results may demonstrate the occurrence of a subtle isostructural transition around $P_{\mathrm{C}}$ in pressurized CaCdGe, due to the distortion of $\mathrm{Cd}{\mathrm{Ge}}_4$ tetrahedra.

The Dallenbach layer is composed of an absorbing magnetic-dielectric layer attached to a perfect electric conductor (PEC) sheet. Under linearity and time invariance (LTI) assumptions, Rozanov has established analytically a sum-rule tradeoff between the absorption efficacy over a predefined bandwidth and the thickness of the layer, which is the so called Rozanov bound. In recent years, several proposals have been introduced to bypass this bound by using non-LTI absorbers. However, in practice, their implementation may be challenging. Here, we expose additional hidden assumptions in Rozanov's derivation, and thus we introduce several sum rules for LTI layer absorbers that are not covered by the original Rozanov criterion, and they give rise to more relaxed constraints on the absorption limit. We demonstrate practical LTI designs of absorbing thin layers that provide absorption beyond Rozanov's bound. These designs are based on the replacement of the original PEC boundary by various types of penetrable impedance sheets.

We present the exact dimer ground state of a quantum antiferromagnet defined on a quasicrystal constructed from the bronze-mean hexagonal quasicrystal. A coupling isotropy on the first- and second-neighbor bonds is sufficient to stabilize a product state of singlets on the third-neighbor bonds. We also provide a systematic approach for constructing additional crystals, quasicrystals, and amorphous structures that can sustain an exact dimer ground state.

The magnetic, thermal, and transport properties of the noncentrosymmetric compound ${\mathrm{Eu}}_2{\mathrm{Pd}}_2\mathrm{Sn}$ are reexamined with the inclusion of detailed measurements. In its paramagnetic phase, the outstanding feature of this compound is the formation of ${\mathrm{Eu}}^{2+}$ dimers, which allows us to understand the deviation of the magnetic susceptibility $\chi \left(T\right)$ from the Curie-Weiss law below about 70 K, the field-dependent magnetization $M\left(B\right)$ below $\approx 80\mathrm{K}$, and the reduced entropy at the ordering temperature $S\left(T_N\right)=0.64Rln\left(8\right)$. A significant change in the exchange interactions occurs between $T\approx 70\mathrm{K}$ (where ${\theta }_P=18\mathrm{K})$ and $T_N=13.3\mathrm{K}$ (to ${\theta }_P=-4.5\mathrm{K})$. The strong electronic overlap, arising from the reduced Eu-Eu spacing (compared with that in pure ${\mathrm{Eu}}^{2+})$ is expected to favor dimer quasiparticles formation, leading to a significant change in the magnetic structure. From the analysis of the derivatives of the magnetic parameters, $\partial \chi /\partial T$ and $\partial M/\partial B$, as well as the field dependence of the specific heat and mangnetoresistance, a comprehensive magnetic phase diagram is obtained. Two critical points are recognized and a tentative description of the magnetic structures is proposed. The possible formation of a skyrmion lattice, arising from the presence of magnetically frustrated pockets in the phase diagram, is suggested by theoretical studies on hexagonal structures that exhibit similar interaction patterns.

Multiorbital quantum materials with strong interactions can host a variety of novel phases. In this work we study the possibility of interaction-driven superconductivity in the iridate compound ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ under strain and doping. We find numerous regimes of strain-induced superconductivity in which the pairing structure depends on model parameters. Spin-fluctuation mediated superconductivity is modeled by a Hubbard-Kanamori model with an effective particle-particle interaction, calculated via the random phase approximation. Magnetic orders are found using the Stoner criterion. The most likely superconducting order we find has $d$-wave pairing, predominantly in the total angular momentum $J=\frac{1}{2}$ states. Moreover, an $s_{\pm }$ order which mixes different bands is found at high Hund's coupling, and at high strain anisotropic $s$- and $d$-wave orders emerge. Finally, we show that in a fine-tuned region of parameters a spin-triplet $p$-wave order exists. The combination of strong spin-orbit coupling, interactions, and a sensitivity of the band structure to strain proves a fruitful avenue for engineering new quantum phases.

We applied single-crystal x-ray diffraction and Raman spectroscopy in a diamond-anvil cell up to 36 GPa and first-principles theoretical calculations to study the molecular dissociation of solid iodine at high pressure. Unlike previously reported, we find that the familiar $Cmce$ molecular phase transforms to a $Cmc{2}_1$ molecular structure at 16 GPa, and then to an incommensurate polymeric $Fmmm\left(00\gamma \right)s00$ structure, which can be viewed as a mixture of molecular ${\mathrm{I}}_2$ and zigzag chains of three iodine atoms, at 20 GPa. The available data are consistent with metallization of iodine at the boundary between the commensurate $Cmc{2}_1$ structure and the incommensurate $Fmmm\left(00\gamma \right)s00$ structure at 20 GPa.

Phase separation accompanied by domain growth and coarsening is a phenomenon common to a broad variety of dynamical systems. In this context, controlling such processes represents a relevant interdisciplinary problem. Through numerical modeling, we demonstrate two complementary approaches of coarsening control in bistable systems based on the example of a spatially extended model describing an optically addressed spatial light modulator with two-color illumination subject to optical feedback. The first method implies varying system parameters such that the system evolves as the pitchfork or saddle-node normal forms. The second method leverages noise, whose intensity serves as an additional system control parameter. Both deterministic and stochastic schemes allow us to control the direction and speed of the fronts separating spatial domains. The considered stochastic control represents a particular case of noise-sustained front propagation in bistable systems and involves the properties of the optical system under study. In contrast, the proposed deterministic control technique can generally be applied to bistable systems of different natures.

Optical conductivity measurements may provide access to distinct signatures of Floquet electronic phases, described theoretically by their quasienergy band structures. In this paper we characterize experimental observables of the Floquet graphene antidot lattice (FGAL), which we introduced previously [Phys. Rev. B 104, 174304 (2021)]. On the basis of Floquet linear response theory, the real and imaginary parts of the longitudinal and Hall optical conductivity are computed as a function of probe frequency. We find that the number and positions of peaks in the response function are distinctive of the different Floquet electronic phases, and identify multiple properties with no equilibrium analog. First, for several intervals of probe frequencies, the real part of the conductivity becomes negative. We argue this is indicative of a subversion of the usual Joule heating mechanism: The Floquet drive causes the material to amplify the power of the probe, resulting in gain. Moreover, while the Hall response vanishes at equilibrium, the real and imaginary parts of the Floquet Hall conductivity are nonzero and can be as large as the longitudinal components. Lastly, driving-induced localization tends to reduce the overall magnitude of and to flatten out the optical conductivity signal. From an implementation standpoint, a major advantage of the FGAL is that the above-bandwidth driving limit is reached with photon energies that are at least twenty times lower than that required for the intrinsic material, allowing for significant band renormalization at orders-of-magnitude smaller intensities. Our paper provides essential tools for experimentalists to map reflectance data to particular Floquet phases in this novel material.

We demonstrate that the topological Hall effect (THE) in a $\mathrm{Pd}/{\mathrm{Tm}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ (TmIG) bilayer can be delicately manipulated by ${\mathrm{H}}_2$ with a maximum $100\%$ tunability in the reversible manner. This phenomenon originates from the variation of the Dzyaloshinskii-Moriya interaction (DMI) at the Pd/TmIG interface, which can be effectively tuned by the absorption/desorption of ${\mathrm{H}}_2$ in Pd. Furthermore, we show that the THE in a Pd/W/TmIG trilayer barely changes by applying ${\mathrm{H}}_2$ even the W layer is only 1-nm-thick, which indicates that the DMI generated by the W/TmIG interface is responsible for the THE, and the change in the net spin accumulation at the W/TmIG interface has no effect on the THE. Finally, we show that the Cu/TmIG interface can generate sufficient interfacial DMI to induce THE, even Cu is a light metal with weak spin-orbit coupling. Our study provides a simple approach to delicately manipulate the THE in spintronic devices and paves a practical way for developing more sensitive hydrogen sensors based on the spintronic technology.