We consider theoretically an electronic Mach-Zehnder interferometer constructed from quantum Hall edge channels and quantum point contacts, fed with single electrons from a dynamic quantum dot source. By considering the energy dependence of the edge-channel guide centers, we give an account of the phase averaging in this setup that is particularly relevant for the short, high-energy wave packets injected by this type of electron source. We present both analytic and numerical results for the energy-dependent arrival time distributions of the electrons and also give an analysis of the delay times associated with the quantum point contacts and their effects on the interference patterns. A key finding is that, contrary to expectation, maximum visibility requires the interferometer arms to be different in length, with an offset of up to a micron for typical parameters. By designing interferometers that incorporate this asymmetry in their geometry, phase-averaging effects can be overcome such that visibility is only limited by other incoherent mechanisms.

We elaborate on the recently introduced concept of the reentrant Kondo effect in quantum impurities/dots coupled to hybrid metal-semiconductor contacts [G. Diniz et al., Phys. Rev. B 101, 125115 (2020)]. By noticing the equivalence of the originally suggested semiconducting arrangement to an analogous three-terminal quantum dot setup with a normal and two phase-biased superconducting leads introduced in our recent work [P. Zalom et al., Phys. Rev. B 103, 035419 (2021)], we put this effect into a new physical context, which gives us a fresh look on the problem. First, we identify the superconducting counterpart of the reentrant Kondo effect and, consequently, reveal its fragility with respect to an underlying doublet-singlet quantum phase transition induced by the particle-hole asymmetry of the reservoir densities of states. This is pertinent (even if previously unnoticed) also in the original as well as extended semiconducting setups, where it puts stringent conditions on the symmetry of the contact density of states. Furthermore, we analyze the experimental feasibility of observing the reentrant Kondo effect in its superconducting realization concluding that even present-day experiments might see the onset of the reentrant behavior, but fully developed Kondo features cannot be reached due to the required vast separation of energy/temperature scales.

We study translationally invariant spin chains where each unit cell contains an $n$-state projective representation of a ${\mathbb{Z}}_n\times {\mathbb{Z}}_n$ internal symmetry, generalizing the spin-1/2 XYZ chain. Such spin chains possess a generalized Lieb-Schultz-Mattis (LSM) constraint, and we demonstrate that certain $(n-1)$-component Luttinger liquids possess the correct anomalies to satisfy these LSM constraints. For $n=3$, using both numerical and analytical approaches, we find that such spin chains with nearest-neighbor interactions appear to be gapless for a wide range of microscopic parameters and described by a two-component conformally invariant Luttinger liquid. This implies the emergence of $n-1$ conserved U(1) charges from only discrete microscopic symmetries. Remarkably, the system remains gapless for an unusually large parameter regime despite the apparent existence of symmetry-allowed relevant operators in the field theory. This suggests that either these spin chains have hidden conserved quantities not previously identified, or the parameters of the field theory are simply unusual due to frustration effects of the lattice Hamiltonian. We argue that similar features are expected to occur in (1) ${\mathbb{Z}}_n\times {\mathbb{Z}}_n$ symmetric chains for $n$ odd and (2) ${\mathbb{S}}_n\times {\mathbb{Z}}_n$ symmetric chains for all $n>2$. Finally, we suggest the possibility of a lower bound growing with $n$ on the minimum central charge of field theories that possess such LSM anomalies.

We measure the Shubnikov–de Haas effect in thin-film ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ grown on an $({\mathrm{LaAlO}}_3{)}_{0.29}\text{−}({\mathrm{SrAl}}_{1/2}{\mathrm{Ta}}_{1/2}{\mathrm{O}}_3{)}_{0.71}$ substrate. We detect all three known Fermi surfaces and extract the Fermi surface volumes, cyclotron effective masses, and quantum lifetimes. We show that the electronic structure is nearly identical to that of single-crystal ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, and that the quasiparticle lifetime is consistent with the $T_{\mathrm{c}}$ of comparably clean, single-crystal ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$. Unlike single-crystal ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, where the quantum and transport lifetimes are roughly equal, we find that the transport lifetime is $1.3\pm 0.1$ times longer than the quantum lifetime. This may suggest that extended (rather than point) defects could be the dominant source of quasiparticle scattering in these films. To test this hypothesis, we perform cross-sectional scanning transmission electron microscopy and find that out-of-phase boundaries extending the entire thickness of the film occur with a density that is consistent with the quantum mean free path. The long quasiparticle lifetimes make these films ideal for studying the unconventional superconducting state in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ through the fabrication of devices—such as planar tunnel junctions and superconducting quantum interference devices.

Semiconductor valence holes are known to have heavy and light effective masses, but the consequence of this mass difference on Coulomb scatterings has been considered intractable and thus ignored up to now. The reason is that the heavy/light index is quantized along the hole momentum that changes in a Coulomb scattering, so a heavy hole can turn light, depending on the scattering angle. This mass change has never been taken into account in many-body problems, and a single “average” hole mass has been used instead. To study the missed consequences of this crude approximation, the first necessary step is to determine the Coulomb scatterings with valence holes in a precise way. We here derive these scatterings from scratch, starting from the threefold valence-electron spatial level, all the way through the spin-orbit splitting, the Kohn-Luttinger effective Hamiltonian, its spherical approximation, and the phase factors that appear when turning from valence electron to hole operators, that is, all the points of semiconductor physics that render valence holes so different from a naïve positive charge.

The passivation of thin Bi(1 1 1) films with hydrogen and oxide capping layers is investigated from first principles. Considering termination-related changes of the crystal structure, we show how the bands and density of states are affected. In the context of the much-discussed semimetal-to-semiconductor transition and the band topology of the bulk material, we consider the effects of confinement in the whole Brillouin zone and go beyond standard density functional theory by including many-body interactions via the ${\mathrm{G}}_0{\mathrm{W}}_0$ approximation. The conductivity of unterminated films is calculated via the Boltzmann transport equation using the simple constant relaxation-time approximation and compared to experimental observations that have suggested a two-channel model.

We theoretically study the scattering behavior of spin waves (SWs) at the interface of an antiferromagnetically coupled (AFMC) heterojunction. It is shown that the SWs passing through the interface are evanescent and the incident waves are all reflected back, demonstrating a magnetization-dependent magnon blocking effect in this structure. We also analytically derive the expressions for the decay length of the evanescent waves (EWs). The theoretical result indicates that with the increase of the spin-wave (SW) frequency, the decay length decreases and the EWs are more concentrated at the interface, showing a magnonic skin effect (MSE) which is similar to the skin effect of electromagnetic waves. Furthermore, a positive magnonic Goos-Hänchen shift (MGHS) of the reflected waves is also predicted. It can be understood by an effective reflection interface shift induced by the nonzero decay length of the EWs. The results of micromagnetic simulations are consistent well with all the theoretical findings. Based on the above findings, we also propose a magnon valve without spacers, which shows 100% on-off ratio for magnons. Our work provides insights into SW transmissions in the system of AFMC heterostructures and will serve as a promising tool for future magnonic devices.

We consider the propagation of flexural-gravity waves in thin elastic plates floating atop nonviscous fluids, e.g., seawater, which are governed by a partial differential equation with Laplacian and tri-Laplacian terms. We investigate the effect of time modulation as well as spacetime modulation on thin floating elastic plates and show the peculiarity of the phenomena of the $k$-band gap and the rotated $k$-band gap in the context of flexural-gravity waves. This makes possible floating plates with nonreciprocal features and behaving as elastodynamic analogs of luminal electromagnetic metamaterials, with exotic applications in enhanced control of ocean waves, such as filtering devices, unidirectional acoustic propagation, and isolation effects and energy harvesting in maritime engineering.

We present a microscopic description of molecular magnets by the multiorbital Hubbard model, which includes the correlated hopping term, i.e., the dependence of the electron hopping amplitude between orbitals on the degree of their occupancy. In the limit of large Coulomb on-site interaction, we derive the spin Hamiltonian using the perturbation theory. We determine the magnetic coupling constant between two ions in two different ways: (a) from the expression obtained in the perturbation calculus and (b) from the analysis of distances between the lowest levels of the energy spectrum obtained by diagonalization of the multiorbital Hubbard model. The procedure we use can be applied to various nanomagnets, but we perform the final calculations for the molecular ring ${\text{Cr}}_8$. We show that the correlated hopping can reduce the antiferromagnetic exchange between ions, which is essential for a proper description of ${\mathrm{Cr}}_8$.

We present a nonlocal spin transport theory for the coupled dynamics of magnetization and lattice vibrations in antiferromagnetic insulators. We find that magnon-polaron formation, i.e., coherently hybridized magnon and acoustic phonon modes, not only leads to anomalous features in the nonlocal spin current but also renormalizes the spin-flop transition field of the antiferromagnets. A length scale for the magnon-polaron formation below which the spin current is not affected by the lattice is also extracted from this nonlocal setup.

${\mathrm{GeCo}}_2{\mathrm{O}}_4$ is a unique system in the family of cobalt spinels $A{\mathrm{Co}}_2{\mathrm{O}}_4$ ($A$= Sn, Ti, Ru, Mn, Al, Zn, Fe, etc.) in which magnetic Co ions stabilize on the pyrochlore lattice exhibiting a large degree of orbital frustration. Due to the complexity of the low-temperature antiferromagnetic (AFM) ordering and long-range magnetic exchange interactions, the lattice dynamics and magnetic structure of a ${\mathrm{GeCo}}_2{\mathrm{O}}_4$ spinel have remained puzzling. To address this issue, here we present theoretical and experimental investigations of the highly frustrated magnetic structure, and the infrared (IR) and Raman-active phonon modes in the spinel ${\mathrm{GeCo}}_2{\mathrm{O}}_4$, which exhibits an AFM ordering below the Néel temperature $T_N\sim 21$ K and an associated cubic ($Fd\overline{3}m$) to tetragonal ($I{4}_1/amd$) structural phase transition whose location at $T_N$ vs at a lower $T_S\sim 16$ K is controversial. Our density functional theory ($\mathrm{DFT}+U$) calculations reveal that one needs to consider magnetic-exchange interactions up to the third-nearest neighbors to get an accurate description of the low-temperature AFM order in ${\mathrm{GeCo}}_2{\mathrm{O}}_4$. At room temperature, three distinct IR-active modes ($T_{1\mathrm{u}}$) are observed at frequencies 680, 413, and 325 ${\mathrm{cm}}^{-1}$ along with four Raman-active modes $A_{1\mathrm{g}}, T_{2\mathrm{g}}$(1), $T_{2\mathrm{g}}$(2), and $E_{\mathrm{g}}$ at frequencies 760, 647, 550, and 308 ${\mathrm{cm}}^{-1}$, respectively, which match reasonably well with our $\mathrm{DFT}+U$ calculated values. All the IR-active and Raman-active phonon modes exhibit signatures of moderate spin-phonon coupling. The temperature dependence of various parameters, such as the shift, width, and intensity, of the Raman-active modes is also discussed. Noticeable changes around $T_N\sim 21$ K and $T_S\sim 16$ K are observed in the Raman line parameters of the $E_{\mathrm{g}}$ and $T_{2\mathrm{g}}$(1) modes, which are associated with the modulation of the Co-O bonds in ${\mathrm{CoO}}_6$ octahedra during the excitations of these modes.

In the metallic pyrochlore ${\mathrm{Nd}}_2{\mathrm{Mo}}_2{\mathrm{O}}_7$, the conducting molybdenum sublattice adopts canted, yet nearly collinear, ferromagnetic order with nonzero scalar spin chirality. The chemical potential may be controlled by replacing ${\mathrm{Nd}}^{3+}$ with ${\mathrm{Ca}}^{2+}$, while introducing only minimal additional disorder to the conducting states. Here, we demonstrate the stability of the canted ferromagnetic state, including the tilting angle of molybdenum spins, in (${\mathrm{Nd}}_{1-x}{\mathrm{Ca}}_x$)${}_2{\mathrm{Mo}}_2{\mathrm{O}}_7$ with $x\le 0.15$ using magnetic susceptibility measurements. Mo-Mo and Mo-Nd magnetic couplings both change sign above $x=0.22$, where the canted ferromagnetic state gives way to a spin-glass metallic region. Contributions to the Curie-Weiss law from two magnetic species are separated systematically.

We derive spin-orbital Hamiltonians for cubic double perovskite $A_{2}M{M}^{\prime }{\mathrm{O}}_6$ Mott insulators with $5d^1$ or $5d^2$ magnetic $M^{\prime }$ ions and nonmagnetic $M$ ions. We find that with strong spin-orbit coupling, exchange and intersite Coulomb repulsion lead to orbital order at a temperature $T_o$ higher than the magnetic $T_c$. Orbital order produces a $T$-dependent moment below $T_o$ and stabilizes canted ferromagnetic (FM) and noncollinear antiferromagnetic (AFM) states below $T_c$. We explain many experimental puzzles including loss of entropy above $T_c$, deviations from a Curie-Weiss susceptibility, a negative Curie-Weiss intercept for FM systems, and why FM order is common in cubic $5d^1$ materials but all $5d^2$ systems are AFM.

We report the importance of anisotropic Coulomb interactions in $\mathrm{DFT}+U$ calculations of the electronic and magnetic properties of ${\mathrm{Mn}}_3{\mathrm{O}}_4$. The effects of anisotropic interactions in ${\mathrm{Mn}}^{2+}$ and ${\mathrm{Mn}}^{3+}$ are examined separately by defining two different sets of Hubbard parameters: $U^{2+}$ and $J^{2+}$ for ${\mathrm{Mn}}^{2+}$ and $U^{3+}$ and $J^{3+}$ for ${\mathrm{Mn}}^{3+}$. The anisotropic interaction in ${\mathrm{Mn}}^{3+}$ has significant effects on the physical properties of ${\mathrm{Mn}}_3{\mathrm{O}}_4$ including local magnetic moments, canted angle, spontaneous magnetic moment, and superexchange coupling, but that in ${\mathrm{Mn}}^{2+}$ has no noticeable effect. Weak ferromagnetic interchain superexchange observed experimentally is predicted only if a sizable anisotropic interaction is considered in ${\mathrm{Mn}}^{3+}$. By analyzing the eigenoccupations of the on-site Mn density matrix, we find that the spin channel involving ${\mathrm{Mn}}^{3+}{d}_{x^2-y^2}$ orbitals, which governs the ${90}^{\circ }$ correlation superexchange, is directly controlled by the anisotropic interactions. These findings indicate that the exchange correction $J$ for the intraorbital Coulomb potential is critical for the first-principles description of Mn oxides containing ${\mathrm{Mn}}^{3+}$ or ${\mathrm{Mn}}^{4+}$.

The electronic structure and related properties of perovskites ${\mathrm{ABO}}_3$ are strongly affected by even small modifications in their crystalline structure. In the case of ${\mathrm{BiFeO}}_3$, variations in the octahedral rotations and ionic displacements lead to significant changes in the band gap. This effect can possibly explain the wide range of values (2.5–3.1 eV) reported in the literature, obtained from samples of varied structural qualities, including polycrystalline films, epitaxial films grown by pulsed-laser deposition and molecular beam epitaxy, nanowires, nanotubes, and bulk single crystals. Using hybrid density-functional calculations, we investigate the dependence of the electronic structure on the crystal lattice distortions of the ferroelectric-antiferromagnetic ${\mathrm{BiFeO}}_3$, disentangling the effects of the ferroelectric ionic displacements and the antiferrodistortive octahedral rotations on the band gap and the band-edge positions. The band gap is shown to vary from 3.39 eV for the rhombohedral ground-state (R3c) structure down to 1.58 eV for the perfect cubic (Pm$\overline{3}$m) structure, with changes in the conduction band being much more prominent than in the valence band. The gap varies linearly with the ferroelectric ionic displacements, but nonlinearly with the octahedral rotations around the pseudocubic [111${\mathrm{]}}_c$ axis, and this is explained in terms of the different interactions between Bi $6s, 6p$, Fe $3d$, and O $2p$ bands. We argue that such large variation of the band gap with structural changes may well explain the large scattering of the reported values, especially if significant deviations from the equilibrium crystal structure are found near domain boundaries, extended defects, or grain boundaries in polycrystalline films.

We study nonlinear optical effects in electron systems with and without inversion symmetry in a Fabry-Perot cavity. General photon up- and down-conversion processes are modeled by the coupling of a noninteracting lattice model to two modes of the quantized light field. Effective descriptions retaining the most relevant states are devised via downfolding and a generalized Householder transformation. These models are used to relate the transition amplitudes for even order photon-conversion processes to the shift vector, a topological quantity describing the difference in polarization between the valence and conduction band in noncentrosymmetric systems. We also demonstrate that the truncated models, despite their small Hilbert space, capture correlation effects induced by the photons in the electronic subsystem.

Colossal electroresistance (CER) in manganites, i.e., a large change in electrical resistance as a function of varying applied electric field or applied electric current, has often been described as complimentary to the colossal magnetoresistance (CMR) effect. Mixed valence vanadates with active $t_{2g}$ and empty $e_g$ orbitals, unlike manganites, have not naturally been discussed in this context, as double exchange based CMR is not realizable in them. However, presence of coupled spin and orbital degrees of freedom, metal-insulator transition (MIT) accompanied by orbital order-disorder transition, still make the vanadates important. Here we probe a Fe-doped hollandite lead vanadate $\mathrm{Pb}{\mathrm{Fe}}_{1.75}{\mathrm{V}}_{4.25}{\mathrm{O}}_{11}$ (PFVO), which exhibits a clear MIT as a function of temperature. Most importantly, a giant fall in the resistivity, indicative of a CER, as well as a systematic shift in the MIT towards higher temperatures are observed with increasing applied current. Detailed structural, magnetic, thermodynamic, and transport studies point towards a complex interplay between the structural distortion, orbital order/disorder effect, and the resultant MIT and magnetic ordering in this system.

The primary goal of this paper is to study topological invariants in two-dimensional twofold rotation and time-reversal symmetric spinful systems. In this paper, first we build a new homotopy invariant based on the lifting of the Wilson loop to the universal covering group of the special orthogonal group. Furthermore, we prove that the invariant we built agrees with the K theory invariant. We go beyond the previous understanding of the Wilson loop unwinding in more than two occupied bands by finding an obstruction of such unwinding. Then, within this formalism, we show two examples that have the same Wilson loop spectrum but belong to different topological classes. Finally, we present a tight-binding model realizing the nontrivial phase.

In the last five years a large number of new high-temperature superconductors have been predicted and experimentally discovered among hydrogen-rich crystals, at pressures, which are way too high to meet any practical application. In this paper, we report the computational prediction of a hydride superconductor, $\mathrm{La}{\mathrm{BH}}_8$, with a $T_{\text{c}}$ of 126 K at a pressure of 50 GPa, thermodynamically stable above 100 GPa, and dynamically stable down to 40 GPa, an unprecedentedly low pressure for high-$T_{\text{c}}$ hydrides. $\mathrm{La}{\mathrm{BH}}_8$ can be seen as a ternary sodalite-like hydride, in which a metallic hydrogen sublattice is stabilized by the chemical pressure exerted by the La-B scaffolding, which achieves a more efficient packing of atoms than in binary sodalite hydrides thanks to the combination of elements with very different sizes. The proposed aufbau principle may be exploited to design high-$T_{\text{c}}$ hydrides that survive at even lower pressure, through a careful choice of the elements.

Although ferromagnets are found in all kinds of technological applications, only a few substances are known to be intrinsically ferromagnetic at room temperature. In the past 20 years, a plethora of new artificial ferromagnetic materials have been found by introducing defects into nonmagnetic host materials. In contrast to the intrinsic ferromagnetic materials, they offer an outstanding degree of material engineering freedom, provided one finds a type of defect to functionalize every possible host material to add magnetism to its intrinsic properties. Still, one controversial question remains: Are these materials really technologically relevant ferromagnets? To answer this question, in this work the emergence of a ferromagnetic phase upon ion irradiation is systematically investigated both theoretically and experimentally. Quantitative predictions are validated against experimental data from the literature of SiC hosts irradiated with high-energy Ne ions and own experiments on low-energy Ar ion irradiation of ${\mathrm{TiO}}_2$ hosts. In the high-energy regime, a bulk magnetic phase emerges, which is limited by host lattice amorphization, whereas at low ion energies an ultrathin magnetic layer forms at the surface and evolves into full magnetic percolation. Lowering the ion energy, the magnetic layer thickness reduces down to a bilayer, where a perpendicular magnetic anisotropy appears due to magnetic surface states.