Inconsistencies in estimates of the spin Hall angle (${\theta }_{SH}$) and spin diffusion length (${\lambda }_{SD}$) of nonmagnetic (NM) layers using the spin transfer torque ferromagnetic resonance (ST-FMR) in ferromagnetic FM/NM bilayer structures are attributed to the inverse spin Hall effect (ISHE) and interfacial parameter contributions, interface spin transparency, interfacial anisotropic magnetoresistance, and effective spin-mixing conductance. These contributions in Fe(10 nm)/Pd(2–8 nm) bilayer structures have been probed employing the simultaneous detection of ST-FMR and ISHE in conjunction with in-plane FMR measurements. The interfacial contributions are found to increase with an increase in Pd layer thickness ($t_{NM}$), which can be linked to the spin pumping effect in conjunction with spin backflow. Correcting the $t_{NM}$ dependence of the ST-FMR spectra for the interfacial and ISHE contributions prior to estimating ${\theta }_{SH}$ and ${\lambda }_{SD}$ of the Pd layer, the estimated values are found to be $0.10\pm 0.03$ and $5.4\pm 1.2\mathrm{nm}$, respectively.

We develop a method to estimate the spin-spin interactions in the Hamiltonian from the observed magnetization curve by machine learning based on Bayesian inference. In our method, plausible spin-spin interactions are determined by maximizing the posterior distribution, which is the conditional probability of the spin-spin interactions in the Hamiltonian for a given magnetization curve with observation noise. The conditional probability is obtained with the Markov chain Monte Carlo simulations combined with an exchange Monte Carlo method. The efficiency of our method is tested using synthetic magnetization curve data, and the results show that spin-spin interactions are estimated with a high accuracy. In particular, the relevant terms of the spin-spin interactions are successfully selected from the redundant interaction candidates by the $l_1$ regularization in the prior distribution.

Magnetic properties of $A\mathrm{MoOP}{\mathrm{O}}_4\mathrm{Cl}$ $\left(A=\mathrm{K},\mathrm{Rb}\right)$ with $\mathrm{M}{\mathrm{o}}^{5+}$ ions in the $4d^1$ electronic configuration are investigated by magnetization, heat capacity, and nuclear magnetic resonance (NMR) measurements on single crystals, combined with powder neutron diffraction experiments. The magnetization measurements reveal that they are good model compounds for the spin-1/$2J_1\text{−}{J}_2$ square-lattice magnet with the first and second nearest-neighbor interactions. Magnetic transitions are observed at around 6 and 8 K in the K and Rb compounds, respectively. In contrast to the normal Néel-type antiferromagnetic order, the NMR and neutron diffraction experiments find a columnar antiferromagnetic order for each compound, which is stabilized by a dominant antiferromagnetic $J_2$. Both compounds realize the unusual case of two interpenetrating $J_2$ square lattices weakly coupled to each other by $J_1$.

We study the fermionic boundary modes (Andreev bound states) in a time-reversal invariant one-dimensional superconductor. In the presence of a substrate, spatial inversion symmetry is broken and the electronic properties are strongly affected by an antisymmetric spin-orbit coupling. We assume an arbitrary even number of nondegenerate bands crossing the Fermi level. We show that there is only one possible pairing symmetry in one dimension, an analog of $s$-wave pairing. The zero-energy Andreev bound states are present if the sign of the gap function in an odd number of the bands is different from all other bands.

Using dynamic cantilever magnetometry and experimentally determining the cantilever's vibrational mode shape, we precisely measured the magnetic moment of a lithographically defined micron-sized superconducting Nb ring, a key element for the previously proposed subpiconewton force standard. The magnetic moments due to individual magnetic fluxoids and a diamagnetic response were independently determined at $T=4.3$ K, with a subfemtoampere-square-meter resolution. The results show good agreement with the theoretical estimation yielded by the Brandt and Clem model within the spring constant determination accuracy.

The temperature dependence of $ab$-plane optical conductivity of ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$ has been measured below and above its superconducting transition temperature $T_c\simeq 35.5$ K. In the normal state, analysis with the two-Drude model reveals a $T$-linear scattering rate for the coherent response, which suggests strong spin-fluctuation scattering. Below the superconducting transition, the optical conductivity below 120 ${\mathrm{cm}}^{-1}$ vanishes, indicating nodeless gap(s). The Mattis-Bardeen fitting in the superconducting state gives two gaps of ${\mathrm{\Delta }}_1\simeq 9$ meV and ${\mathrm{\Delta }}_2\simeq 14$ meV, in good agreement with recent angle-resolved photoemission spectroscopy (ARPES) results. In addition, around 255 ${\mathrm{cm}}^{-1}$, we observe two different infrared-active Fe-As modes with obvious asymmetric lineshape, originating from strong coupling between lattice vibrations and spin or charge excitations. Considering a moderate Hund's rule coupling determined from spectral weight analysis, we propose that the strong fluctuations induced by the coupling between itinerant carriers and local moments may affect the phonon mode, and the electron-phonon coupling through the spin channel is likely to play an important role in the unconventional pairing in iron-based superconductors.

We used density functional theory (DFT) calculations to study the interaction of hydrogen with amorphous hafnia ($\mathrm{a}\text{−}{\mathrm{HfO}}_2$) using a hybrid exchange-correlation functional. Injection of atomic hydrogen, its diffusion towards electrodes, and ionization can be seen as key processes underlying charge instability of high-permittivity amorphous hafnia layers in many applications. Hydrogen in many wide band gap crystalline oxides exhibits negative-U behavior (+1 and $-1$ charged states are thermodynamically more stable than the neutral state) . Our results show that in $\mathrm{a}\text{−}{\mathrm{HfO}}_2$ hydrogen is also negative-U, with charged states being the most thermodynamically stable at all Fermi level positions. However, metastable atomic hydrogen can share an electron with intrinsic electron trapping precursor sites [Phys. Rev. B 94, 020103 (2016).] forming a $[e_{tr}^{-}+\mathrm{O}–\mathrm{H}]$ center, which is lower in energy on average by about 0.2 eV. These electron trapping sites can affect both the dynamics and thermodynamics of the interaction of hydrogen with $\mathrm{a}\text{−}{\mathrm{HfO}}_2$ and the electrical behavior of amorphous hafnia films in CMOS devices.

We investigated the photoconductivity effect in $p$-type $\mathrm{P}{\mathrm{b}}_{1-x}\mathrm{E}{\mathrm{u}}_x\mathrm{Te}$ films for $x=0.01$, 0.02, 0.03, 0.05, and 0.06 at $T=300\mathrm{K}$. The measurements revealed a clear transition from negative to positive photoconductivity as the Eu content $x$ is increased at room temperature. This transition is related to the metal-insulator transition that occurs due to the disorder originated from the introduction of Eu atoms and it is an Anderson transition. Our investigation found that, from the potential application point of view, the sample $x=0.06$ is more suitable, i.e., it presents an almost noise-free signal and the higher photoconductivity amplitude response. The photoconductive amplitude response for the sample with $x=0.06$ was investigated further in the temperature range of 77–300 K and, surprisingly, multiple additional transitions were observed with amplitudes that reached around 200 times the original value before illumination. We show that this anomalous behavior is a consequence of the generation and recombination rates between the bands and the $4f$ level and a defect level located inside the band gap.

${\mathrm{Ge}}_{1-x}{\mathrm{Fe}}_x$ (Ge:Fe) shows ferromagnetic behavior up to a relatively high temperature of 210 K and hence is a promising material for spintronic applications compatible with Si technology. Unlike the prototypical system (Ga,Mn)As where itinerant holes induce long-range ferromagnetic order of the Mn spins, however, its ferromagnetism evolves from robust nanoscale ferromagnetic domains formed in Fe-rich regions. We have studied its underlying electronic structure by soft x-ray angle-resolved photoemission spectroscopy measurements and first-principles supercell calculation. We observed finite Fe $3d$ components in the states at the Fermi level $\left(E_{\mathrm{F}}\right)$ in a wide region of momentum space, and the $E_{\mathrm{F}}$ was located $\sim 0.35$ eV above the valence-band maximum of the host Ge. Our calculation indicates that the $E_{\mathrm{F}}$ is also within the deep acceptor-level impurity band induced by the strong $p\text{−}d\left(t_2\right)$ hybridization. We conclude that the additional minority-spin $d\left(e\right)$ electron characteristic of the ${\mathrm{Fe}}^{2+}$ state is responsible for the short-range ferromagnetic coupling between Fe atoms, making the magnetism markedly different from that of (Ga,Mn)As.

We report on the fabrication, transport measurements, and density functional theory (DFT) calculations of atomic-size contacts made of gadolinium (Gd). Gd is known to have local moments mainly associated with $f$ electrons. These coexist with itinerant $s$ and $d$ bands that account for its metallic character. Here we explore whether and how the local moments influence electronic transport properties at the atomic scale. Using both scanning tunneling microscope and lithographic mechanically controllable break junction techniques under cryogenic conditions, we study the conductance of Gd when only few atoms form the junction between bulk electrodes made of the very same material. Thousands of measurements show that Gd has an average lowest conductance, attributed to single-atom contact, below $\frac{2e^2}{h}$. Our DFT calculations for monostrand chains anticipate that the $f$ bands are fully spin polarized and insulating and that the conduction may be dominated by $s$, $p$, and $d$ bands. We also analyze the electronic transport for model nanocontacts using the nonequilibrium Green's function formalism in combination with DFT. We obtain an overall good agreement with the experimental results for zero bias and show that the contribution to the electronic transport from the $f$ channels is negligible and that from the $d$ channels is marginal.

Application of a strong magnetic field perpendicular to a two-dimensional electron system leads to a variety of quantum phases ranging from incompressible quantum Hall liquid to Wigner solid, charge density wave, and exotic non-Abelian states. A few quantum phases seen in past experiments on GaAs Hall samples of electrons show pronounced anisotropic magnetoresistance values at certain weak magnetic fields. We argue that this might be due to the piezoelectric effect that is inherent in a semiconductor host such as GaAs. Such an effect has the potential to create a sufficient in-plane internal strain that will be felt by electrons and will determine the direction of high and low resistance. When Wigner solid, charge density wave, and isotropic liquid phases are very close in energy, the overall stability of the system is very sensitive to local order and, thus, can be strongly influenced even by a weak perturbation such as the piezoelectric-induced effective electron-electron interaction, which is anisotropic. In this work, we argue that an anisotropic interaction potential may stabilize anisotropic liquid phases of electrons even in a strong magnetic field regime where normally one expects to see only isotropic quantum Hall or isotropic Fermi liquid states. We use this approach to support a theoretical framework that envisions the possibility of an anisotropic liquid crystalline state of electrons in the lowest Landau level. In particular, we argue that an anisotropic liquid state of electrons may stabilize in the lowest Landau level close to the liquid-solid transition region at filling factor $\nu =1/6$ for a given anisotropic Coulomb interaction potential. Quantum Monte Carlo simulations for a liquid crystalline state with broken rotational symmetry indicate stability of liquid crystalline order consistent with the existence of an anisotropic liquid state of electrons stabilized by anisotropy at filling factor $\nu =1/6$ of the lowest Landau level.

The effective spin exchange RKKY coupling between impurities (adatoms) on graphene mediated by conduction electrons is studied as a function of the strength of the potential part of the on-site energy $U$ of the electron-adatom interaction. With increasing $U$, the exchange coupling becomes long range, determined largely by the impurity levels with energies close to the Dirac points. When adatoms reside on opposite sublattices, their exchange coupling, normally antiferromagnetic, becomes ferromagnetic and resonantly enhanced at a specific distance where an impurity level crosses the Dirac point.

Quantum oscillations are conventionally understood to arise from the Fermi level; hence, they are considered to be a proof of the existence of an underlying Fermi surface. In this article we show that in certain situations quantum oscillations can also arise from inside the Fermi sea. We establish this analytically, supporting it with numerical calculations. Possible scenarios where such unusual behavior can occur are pointed out. In particular, in strongly particle-hole asymmetric insulators, models of which have been recently used in the context of the topological Kondo insulator ${\text{SmB}}_6$, we show that the oscillations arise from inside the filled band, and are not related to the gap.

We give evidence for intrinsic defect-induced bulk paramagnetism in SiC by means of ${}^{13}\mathrm{C}$ and ${}^{29}\mathrm{Si}$ nuclear magnetic resonance (NMR) spectroscopy. The temperature dependence of the internal dipole-field distribution, probed by the spin part of the NMR Knight shift and the spectral linewidth, follows the Curie law and scales very well with the macroscopic dc susceptibility. In order to quantitatively analyze the NMR spectra, a microscopic model based on dipole-dipole interactions was developed. The very good agreement between these simulations and the NMR data establishes a direct relation between the frequency distribution of the spectral intensity and the corresponding real-space volumes of nuclear spins. The presented approach by NMR can be applied to a variety of similar materials and, thus, opens a new avenue for the microscopic exploration and exploitation of diluted bulk magnetism in semiconductors.

Gapless boundary modes at the interface between topologically distinct regions are one of the most salient manifestations of topology in physics. Metallic boundary states of time-reversal-invariant topological insulators (TIs), a realization of topological order in condensed matter, have been of much interest not only due to such a fundamental nature, but also due to their practical significance. These boundary states are immune to backscattering and localization owing to their topological origin, thereby opening up the possibility to tailor them for potential uses in spintronics and quantum computing. The heterojunction between a TI and a normal insulator (NI) is a representative playground for exploring such a topologically protected metallic boundary state and expected to constitute a building block for future electronic and spintronic solid-state devices based on TIs. Here, we report a first-principles study of two experimentally realized lattice-matched heterojunctions between TIs and NIs, ${\mathrm{Bi}}_2{\mathrm{Se}}_3$(0001)/InP(111) and ${\mathrm{Bi}}_2{\mathrm{Te}}_3$(0001)/${\mathrm{BaF}}_2$(111). We evaluate the band offsets at these interfaces from many-body perturbation theory within the $GW$ approximation as well as density-functional theory. Furthermore, we investigate the topological interface states, demonstrating that at these lattice-matched heterointerfaces, they are strictly localized and their helical spin textures are as well preserved as those at the vacuum-facing surfaces. These results taken together may help in designing devices relying on spin-helical metallic boundary states of TIs.

We study lasing in distributed feedback lasers made from square lattices of silver particles in a dye-doped waveguide. We present a systematic analysis and experimental study of the band structure underlying the lasing process as a function of the detuning between the particle plasmon resonance and the lattice Bragg diffraction condition. To this end, as gain medium we use either a polymer doped with Rh6G only, or polymer doped with a pair of dyes (Rh6G and Rh700) that act as a Förster energy transfer (FRET) pair. This allows for gain, respectively, at 590 or 700 nm when pumped at 532 nm, compatible with the achievable size tunability of silver particles embedded in the polymer. By polarization-resolved spectroscopic Fourier microscopy, we are able to observe the plasmonic/photonic band structure of the array, unraveling both the stop gap width, as well as the loss properties of the four involved bands at fixed lattice Bragg diffraction condition and as a function of detuning of the plasmon resonance. To explain the measurements we derive an analytical model that sheds insights on the lasing process in plasmonic lattices, highlighting the interaction between two competing resonant processes, one localized at the particle level around the plasmon resonance, and one distributed across the lattice. Both are shown to contribute to the lasing threshold and the overall emission properties of the array.

A promising route towards nanodevice applications relies on the association of graphene and transition metal dichalcogenides with hexagonal boron nitride ($h\text{−}\mathrm{BN}$). Due to its insulating nature, $h\text{−}\mathrm{BN}$ has emerged as a natural substrate and gate dielectric for graphene-based electronic devices. However, some fundamental properties of bulk $h\text{−}\mathrm{BN}$ remain obscure. For example, the band structure and the position of the Fermi level have not been experimentally resolved. Here, we report a direct observation of parabolic dispersions of $h\text{−}\mathrm{BN}$ crystals using high-resolution angle-resolved photoemission spectroscopy (ARPES). We find that $h\text{−}\mathrm{BN}$ exfoliation on epitaxial graphene enables overcoming the technical difficulties of using ARPES with insulating materials. We show trigonal warping of the intensity maps at constant energy. The valence-band maxima are located around the K points, 2.5 eV below the Fermi level, thus confirming the residual $p$-type character of typical $h\text{−}\mathrm{BN}$.

Unlike the two-terminal device, in which the time-reversal invariant spin-orbit interaction alone cannot polarize the spins, such a polarization can be generated when electrons from one source reservoir flow into two (or more) separate drain reservoirs. We present analytical solutions for two examples. First, we demonstrate that the electrons transmitted through a “diamond” interferometer into two drains can be simultaneously fully spin polarized along different tunable directions, even when the two arms of the interferometer are not identical. Second, we show that a single helical molecule attached to more than one drain can induce a significant spin polarization in electrons passing through it. The average polarization remains nonzero even when the electrons outgoing into separate leads are eventually mixed incoherently into one absorbing reservoir. This may explain recent experiments on spin selectivity of certain helical-chiral molecules.

Three Mo-based transition-metal dichalcogenides ${\mathrm{MoS}}_2,{\mathrm{MoSe}}_2$, and ${\mathrm{MoTe}}_2$ share at ambient conditions the same structure $2H_c$, consisting of layers where Mo atoms are surrounded by six chalcogen atoms in trigonal prism coordination. The knowledge of their high-pressure behavior is, however, limited, particularly in case of ${\mathrm{MoSe}}_2$ and ${\mathrm{MoTe}}_2$. The latter materials do not undergo a layer-sliding transition $2H_c\to$ $2H_a$ known in ${\mathrm{MoS}}_2$ and currently no other stable phase aside from $2H_c$ is known in these systems at room temperature. Employing evolutionary crystal structure prediction in combination with ab initio calculations, we study the zero-temperature phase diagram of both materials up to Mbar pressures. We find a tetragonal phase with space group P4/mmm, previously predicted in ${\mathrm{MoS}}_2$, to become stable in ${\mathrm{MoSe}}_2$ at 118 GPa. In ${\mathrm{MoTe}}_2$, we predict at 50 GPa a transition to a new layered tetragonal structure with space group I4/mmm, similar to ${\mathrm{CaC}}_2$, where Mo atoms are surrounded by eight Te atoms. The phase is metallic already at the transition pressure and becomes a good metal beyond 1 Mbar. We discuss chemical trends in the family of Mo-based transition-metal dichalcogenides and suggest that ${\mathrm{MoTe}}_2$ likely offers the easiest route towards the post-$2H$ phases.

We build a theoretical model for the electronic properties of the two-dimensional (2D) electron gas that forms at the interface between insulating ${\mathrm{SrTiO}}_3$ and a number of polar cap layers, including ${\mathrm{LaTiO}}_3$, ${\mathrm{LaAlO}}_3$, and ${\mathrm{GdTiO}}_3$. The model treats conduction electrons within a tight-binding approximation and the dielectric polarization via a Landau-Devonshire free energy that incorporates strontium titanate's strongly nonlinear, nonlocal, and temperature-dependent dielectric response. The self-consistent band structure comprises a mix of quantum 2D states that are tightly bound to the interface and quasi-three-dimensional (3D) states that extend hundreds of unit cells into the ${\mathrm{SrTiO}}_3$ substrate. We find that there is a substantial shift of electrons away from the interface into the 3D tails as temperature is lowered from 300 K to 10 K. This shift is least important at high electron densities $\left(\sim {10}^{14}{\mathrm{cm}}^{-2}\right)$ but becomes substantial at low densities; for example, the total electron density within 4 nm of the interface changes by a factor of two for 2D electron densities $\sim {10}^{13}{\mathrm{cm}}^{-2}$. We speculate that the quasi-3D tails form the low-density high-mobility component of the interfacial electron gas that is widely inferred from magnetoresistance measurements.