We theoretically demonstrate that, in a ferromagnet/paramagnet bilayer, a magnon instability accompanied by a gigahertz microwave emission can be driven simply by means of a temperature bias. Employing many-body theory for investigating the effects of a phonon heat current on the magnon lifetime, we show that the magnon instability occurs upon the suppression of the umklapp scattering at low temperatures, leading to microwave emission. The present finding provides crucial information about the interplay of spin current and heat current.

We report the observation of field-dependent magnetization of $\mathrm{BiFe}{\mathrm{O}}_3$ (BFO) in an ultrathin $\mathrm{BFO}/\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ (LSMO) superlattice using polarized neutron reflectivity (PNR). Our PNR results indicate parallel alignment of magnetization across BFO/LSMO interfaces. The study showed an increase in average magnetization of the BFO layer on increasing applied magnetic field at 10 K, similar to the ferromagnetic nature of LSMO. We obtained a saturation magnetization of $110\pm 15\mathrm{kA}/\mathrm{m}\left(\sim 0.8{\mu }_{\mathrm{B}}/\mathrm{Fe}\right)$ for an ultrathin BFO layer (∼2 unit cells) sandwiched between ultrathin LSMO layers (∼2 unit cells), which is much higher than the canted moment $(0.03{\mu }_{\mathrm{B}}/\mathrm{Fe})$ in the bulk BFO.

Nematicity has emerged as a key feature of cuprate superconductors, but its link to other fundamental properties such as superconductivity, charge order, and the pseudogap remains unclear. Here we use measurements of transport anisotropy in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_y$ to distinguish two types of nematicity. The first is associated with short-range charge-density-wave modulations in a doping region near $p=0.12$. It is detected in the Nernst coefficient, but not in the resistivity. The second type prevails at lower doping, where there are spin modulations but no charge modulations. In this case, the onset of in-plane anisotropy—detected in both the Nernst coefficient and the resistivity—follows a line in the temperature-doping phase diagram that tracks the pseudogap energy. We discuss two possible scenarios for the latter nematicity.

We derive a compact matrix expression for the linear thermal expansion coefficients (TECs) for a general orthorhombic system which relates elastic properties and integrated quantities based on deformation and mode dependent Grüneisen parameters and mode dependent heat capacities. The density of Grüneisen parameters $\mathrm{\Gamma }\left(\nu \right)$ as a function of frequency $\nu$, weighted by the number of phonon modes, is introduced and found to be illuminating in interpreting the TEC results. Using density functional perturbation theory and Grüneisen formalism for thermal expansion, we illustrate the general usefulness of this method by calculating the linear and volumetric TECs of a low-symmetry orthorhombic compound antimony sulfide $\left({\mathrm{Sb}}_2{\mathrm{S}}_3\right)$, which belongs to a large class of technologically and fundamentally important materials. Even though negative Grüneisen parameters are found for deformations in all three crystal directions, the $\mathrm{\Gamma }\left(\nu \right)$ data rule out the occurrences of negative TECs at all temperatures. ${\mathrm{Sb}}_2{\mathrm{S}}_3$ exhibits a large thermal expansion anisotropy where the TEC in the $b$ direction can reach as high as $13\times {10}^{-6} {\mathrm{K}}^{-1}$ at high temperatures, about two and seven times larger than the TECs in the $c$ and $a$ direction, respectively. Our work suggests a general and practical first-principles approach to calculate the thermal properties of other complicated low-symmetry systems.

We report quite exotic thermoelectric power $S$ in ${\mathrm{UBe}}_{13}$. At 0 T, the negative $S/T$ continues to strongly enhance down to the superconducting transition temperature with no Fermi-liquid behavior. $|S/T|$ is dramatically suppressed and becomes rather modest with increasing field. We have also obtained precise field dependencies of (i) an anomaly in $S$ due to an exotic Kondo effect and (ii) a field-induced anomaly in $S/T$ associated with the anomalous upward $H_{\mathrm{c}2}\left(T\right)$. In contrast to the field-sensitive transport property, the normal-state specific heat is magnetically robust, indicating that the largeness of the $5f$ density of states remains in high fields. This unusual behavior in ${\mathrm{UBe}}_{13}$ can be explained by a considerable change in the energy derivative of the conduction-electron lifetime ${\tau }_{\mathrm{c}}\left(\epsilon \right)$ at the Fermi level under magnetic fields.

Atomically uniform Sb(111) films are fabricated by the method of molecular beam epitaxy on an optimized Si(111) surface. Two-dimensional quantum well states and topological surface states in these films are well resolved as measured by angle-resolved photoemission spectroscopy. We observe an evolution of direct transition resonances by varying the excitation photon energy (and thus the perpendicular crystal momentum). The experimental results are reproduced in a comprehensive model calculation taking into account first-principles calculated initial states and time-reversed low-energy-electron-diffraction final states in the photoexcitation process. The resonant behavior illustrates that the topological surface states and the quantum well states are analytically connected in momentum space in all three dimensions.

We present a general scheme to model an energy for analyzing interfaces between crystalline solids, quantitatively including the effects of varying configurations and lattice strain. This scheme is successfully applied to the modeling of likely interface geometries of several solid state battery materials including Li metal, ${\mathrm{Li}}_3{\mathrm{PO}}_4, {\mathrm{Li}}_3{\mathrm{PS}}_4, {\mathrm{Li}}_2\mathrm{O}$, and ${\mathrm{Li}}_2\mathrm{S}$. Our formalism, together with a partial density of states analysis, allows us to characterize the thickness, stability, and transport properties of these interfaces. We find that all of the interfaces in this study are stable with the exception of ${\mathrm{Li}}_3{\mathrm{PS}}_4/\mathrm{Li}$. For this chemically unstable interface, the partial density of states helps to identify mechanisms associated with the interface reactions. Our energetic measure of interfaces and our analysis of the band alignment between interface materials indicate multiple factors, which may be predictors of interface stability, an important property of solid electrolyte systems.

We investigate the persistent currents, spin-polarized local density of states, and spectral functions of topological superconductors constructed by placing ferromagnetic impurities on top of an $s$-wave superconductor with Rashba spin-orbit interaction. We solve self-consistently for the superconducting order parameter and investigate both two-dimensional blocks and one-dimensional wires of ferromagnetic impurities, with the magnetic moments pointing both perpendicular and parallel to the surface. We find that the topologically protected edge states of ferromagnetic blocks give rise to spin-polarized edge currents, but that the total persistent current flows in opposite direction to what is expected from the dispersion relation of the edge states. We also show that the Majorana fermions at the end points of one-dimensional wires are spin polarized, which can be directly related to the spin polarization of the edge currents of two-dimensional blocks. Connections are also made to the physics of the Yu-Shiba-Rusinov states for zero-dimensional impurities.

A unified treatment of coupled optical and acoustic phonons in piezoelectric cubic materials is presented whereby the lattice displacement vector and the internal ionic displacement vector are found simultaneously. It is shown that phonon couplings exist in pairs only; either between the electric potential and the lattice displacement coordinate perpendicular to the phonon wave vector or between the two other lattice displacement components. The former leads to coupled acousto-optical phonons by virtue of the piezoelectric effect. We then establish three new conjectures that entirely stem from piezoelectricity in a cubic structured material slab. First, it is shown that isolated optical phonon modes generally cannot exist in piezoelectric cubic slabs. Second, we prove that confined acousto-optical phonon modes only exist for a discrete set of in-plane wave numbers in piezoelectric cubic slabs. Third, it is shown that coupled acousto-optical phonons do not exist at the longitudinal-optical (LO) phonon frequency where the dielectric constant vanishes.

The diffusion coefficients of interstitial hydrogen in bulk Fe and Ni crystals have been calculated over a wide range of temperatures employing first-principles methods based on density functional theory. Quantum mechanical effects have been included by means of the semiclassical transition state theory and the small-polaron model of Flynn and Stoneham. Our results show that to include such effects is crucial for a quantitative simulation of H diffusion in bcc Fe even at room temperature, while in the case of fcc Ni this is less important. The comparison with other theoretical approaches as well as with experimental studies emphasizes the main advantages of the present approach: it is quantitatively accurate and computationally efficient.

Scandium trifluoride $\left({\mathrm{ScF}}_3\right)$ is a rare example of a material with a simple structure exhibiting a large negative thermal expansion over a wide range of temperatures. In the present work, the thermal behavior of this material has been studied by means of density-functional molecular dynamics in the isothermal-isobaric ensemble. Our simulations reproduce the experimentally observed trends: thermal expansion is negative at low temperatures, approaches zero at $\approx 1000$ K, and is positive at higher temperatures. The simulations permit us to develop an atomistic scenario of the observed phenomena, which arises from the correlated dynamics of ${\mathrm{ScF}}_6$ octahedra. The relation between the cubic-to-rhombohedral transformation under a very modest compression and the thermal behavior of ${\mathrm{ScF}}_3$ is demonstrated.

We have measured the temperature dependence of the spin-lattice relaxation and dephasing of the two nuclear quadrupole transitions in the ${}^7{F}_0$ ground state of ${}^{151}\mathrm{Eu}$ in yttrium orthosilicate up to 21 K. Spin-lattice relaxation $\left(T_1\right)$ is dominated by an Orbach process and decoherence comes from ${}^{87}\mathrm{Y}$ nuclear spin fluctuations at low temperatures and is $T_1$ limited at higher temperatures. The different contributions to relaxation and dephasing are evaluated.

We demonstrate that the current-voltage $\left(I\text{−}V\right)$ characteristics of resistively and capacitively shunted Josephson junctions (RCSJs) hosting localized subgap Majorana states provide a phase-sensitive method for their detection. The $I\text{−}V$ characteristics of such RCSJs, in contrast to their resistively shunted counterparts, exhibit subharmonic odd Shapiro steps. These steps, owing to their subharmonic nature, exhibit qualitatively different properties compared to harmonic odd steps of conventional junctions. In addition, the RCSJs hosting Majorana bound states also display an additional sequence of steps in the devil's staircase structure seen in their $I\text{−}V$ characteristics; such a sequence of steps makes their $I\text{−}V$ characteristics qualitatively distinct from that of their conventional counterparts. A similar study for RCSJs with graphene superconducting junctions hosting Dirac-like quasiparticles reveals that the Shapiro step width in their $I\text{−}V$ curves bears a signature of the transmission resonance phenomenon of their underlying Dirac quasiparticles; consequently, these step widths exhibit a $\pi$ periodic oscillatory behavior with variation of the junction barrier potential. We discuss experiments which can test our theory.

The Hall effect resistivity ${\rho }_{xy}$ of high-quality films of the itinerant ferromagnet ${\mathrm{SrRuO}}_3$ was measured at low temperatures. ${\mathrm{SrRuO}}_3$ films have large uniaxial magnetocrystalline anisotropy, and in the relevant temperature range the easy axis is ${30}^{\circ }$ degrees from the film normal. We measure changes in ${\rho }_{xy}$ associated with changes in the orientation of the magnetization (M) for the same applied magnetic field B $\left[\mathrm{\Delta }{\rho }_{xy}(\mathbf{B},\mathrm{\Delta }\mathbf{M})\right]$, and we find that with increasing B, $\mathrm{\Delta }{\rho }_{xy}(\mathbf{B},\mathrm{\Delta }\mathbf{M})$ changes its sign and its magnitude becomes more than an order of magnitude larger than the remanent (spontaneous) anomalous Hall effect. Furthermore, it appears that ${\rho }_{xy}$ cannot be described as a sum of a term that depends on B and a term that depends on M, that is, the ordinary Hall effect and the anomalous Hall effect are effectively intermixed. We address qualitatively a possible link between the observed behavior of ${\rho }_{xy}$ and the predicted existence of Weyl nodes in ${\mathrm{SrRuO}}_3$.

Ab initio calculations have been performed on hexagonal layers of $M_2{\mathrm{O}}_3$ ($M$ being several transition metals of the $5d$ series) sandwiched by a band insulator such as ${\mathrm{Al}}_2{\mathrm{O}}_3$ that provides the honeycomb lattice where the $5d$ electrons reside. This corundum-structure-based superlattice is a way to design a honeycomb lattice with transition metal cations avoiding the use of largely polar surfaces. We find that this system supports the presence of Dirac cones at the Fermi level that open up with the introduction of spin-orbit coupling at various fillings of the $5d$ band. The DFT calculations performed in this paper show that the $5d^5$ situation is always a trivial insulator, whereas the $5d^8$ filling presents topological insulating configurations which evolve into a trivial state with increasing tensile strain or on-site Coulomb potential $U$. However, LDA + $U$ calculations show a stable antiferromagnetic solution for the $5d^8$ case at every $U$ value, which would break time reversal symmetry and could affect the topological properties of the system. We also discuss the similarities with the buckled honeycomb lattice obtained using perovskite (111) bilayers previously studied in literature, in particular for the $5d^5$ and $5d^8$ configurations. This paper provides some clues on the stability of topological phases using metal oxides in general.

We calculate, within the single-loop or equivalently the Hartree-Fock approximation (HFA), the finite-temperature interacting compressibility for three-dimensional (3D) Dirac materials and renormalized quasiparticle velocities for 3D and two-dimensional (2D) Dirac materials. We find that in the extrinsic (i.e., doped) system, the inverse compressibility (incompressibility) and renormalized quasiparticle velocity at $k=0$ show nonmonotonic dependences on temperature. At low temperatures, the incompressibility initially decreases to a shallow minimum with a $T^{2}lnT$ dependence. As the temperature increases further, the incompressibility rises to a maximum and beyond that it decreases with increasing temperature. On the other hand, the renormalized quasiparticle velocity at $k=0$ for both 2D and 3D Dirac materials first increases with $T^2$, rises to a maximum, and after reaching the maximum it decreases with increasing temperature. We also find that within the HFA, the leading-order temperature correction to the low-temperature renormalized extrinsic Fermi velocity for both 2D and 3D doped Dirac materials is $ln(1/T)$.

The family of binary compounds including TaAs, TaP, NbAs, and NbP was recently discovered as the first realization of Weyl semimetals. In order to develop a comprehensive description of the charge carriers in these Weyl semimetals, we performed detailed and systematic electronic band structure calculations which reveal the nature of Fermi surfaces and their complex interconnectivity in TaAs, TaP, NbAs, and NbP. Our work reports a comparative and comprehensive study of Fermi surface topology and band structure details of all known members of the Weyl semimetal family and hence provides the fundamental knowledge for realizing the many predicted exotic topological quantum physics of Weyl semimetals based on the TaAs class of materials.

We investigate a method for entangling two singlet-triplet qubits in adjacent double quantum dots via capacitive interactions. In contrast to prior work, here we focus on a regime with strong interactions between the qubits. The interplay of the interaction energy and simultaneous large detunings for both double dots gives rise to the “double charge resonant” regime, in which the unpolarized (1111) and fully polarized (0202) four-electron states in the absence of interqubit tunneling are near degeneracy, while being energetically well separated from the partially polarized (0211 and 1102) states. A rapid controlled-phase gate may be realized by combining time evolution in this regime in the presence of intraqubit tunneling and the interqubit Coulomb interaction with refocusing $\pi$ pulses that swap the singly occupied singlet and triplet states of the two qubits via, e.g., magnetic gradients. We calculate the fidelity of this entangling gate, incorporating models for two types of noise—charge fluctuations in the single-qubit detunings and charge relaxation within the low-energy subspace via electron-phonon interaction—and identify parameter regimes that optimize the fidelity. The rates of phonon-induced decay for pairs of GaAs or Si double quantum dots vary with the sizes of the dipolar and quadrupolar contributions and are several orders of magnitude smaller for Si, leading to high theoretical gate fidelities for coupled singlet-triplet qubits in Si dots. We also consider the dependence of the capacitive coupling on the relative orientation of the double dots and find that a linear geometry provides the fastest potential gate.

Entangling two quantum bits by letting them interact is the crucial requirement for building a quantum processor. For qubits based on the spin of the electron, these two qubit gates are typically performed by exchange interaction of the electrons captured in two nearby quantum dots. Since the exchange interaction relies on tunneling of the electrons, the range of interaction for conventional approaches is severely limited as the tunneling amplitude decays exponentially with the length of the tunneling barrier. Here, we present an approach to couple two spin qubits via a superconducting coupler. In essence, the superconducting coupler provides a tunneling barrier for the electrons which can be tuned with exquisite precision. We show that as a result exchange couplings over a distance of several microns become realistic, thus enabling flexible designs of multiqubit systems.