We have investigated the effect of Ti doping on the transport properties coupled with the magnetic ones in ${\mathrm{Sm}}_{0.55}{\mathrm{Sr}}_{0.45}{\mathrm{Mn}}_{1-\eta }{\mathrm{Ti}}_{\eta }{\mathrm{O}}_3\left(0\le \eta \le 0.04\right)$. The parent compound, ${\mathrm{Sm}}_{0.55}{\mathrm{Sr}}_{0.45}{\mathrm{MnO}}_3$, exhibits a first-order paramagnetic-insulator to ferromagnetic-metal transition just below $T_{\mathrm{c}}=128$ K. With the substitution of Ti at Mn sites $(B$ site), $T_{\mathrm{c}}$ decreases approximately linearly at the rate of $22\mathrm{K}{\%}^{-1}$ while the width of thermal hysteresis in magnetization and resistivity increases almost in an exponential fashion. The most spectacular effect has been observed for the composition $\eta =0.03$, where a magnetic field of only 1 T yields a huge magnetoresistance, $1.2\times {10}^7\%$ at $T_c\approx 63$ K. With increasing magnetic field, the transition shifts towards higher temperature side, and the first-order nature gets weakened and eventually becomes crossover above a critical field $\left(H_{\text{cr}}\right)$ which increases with Ti doping. For $\eta \ge 0.04$, the system remains insulting without any ferromagnetic ordering down to 2 K. The Monte Carlo calculations based on a two-band double exchange model reveal that the rapid suppression of ferromagnetism with Ti doping is due to the increase of the lattice distortions around the doped Ti ions.

In a bilayer system of fermionic dipoles, full control over the strength of the attractive interactions between two layers leads to the BCS-BEC crossover. Here, using the BCS mean-field theory, we study such a crossover in symmetric bilayers of ultracold dipolar fermions with their dipole moments perpendicular to layers. In particular, we investigate how the pairing between two layers and the many-body screening of interlayer interaction affect each other. We compare results for pairings obtained with three different approximations for the interlayer interactions, namely, bare dipole-dipole interaction, the random-phase approximation for screening obtained in the normal phase, and the self-consistent superfluid phase screening within the random-phase approximation. We find that at weak couplings the screening further suppresses the pairing, while at strong couplings, the screening is suppressed due to the pairing gap in the quasiparticle spectrum. Therefore a self-consistent treatment of both screening and pairing on equal footings is necessary for obtaining the correct picture of the phase diagram and order parameter at both small and large layer spacings and densities. We also notice that the signals of highly speculated density-wave instability in dipolar bilayers only appear in the normal phase screened interlayer interaction.

Many-body localization (MBL) is a result of the balance between interference-based Anderson localization and many-body interactions in a high-dimensional Fock space. It is expected that dissipation is blurring interference and destroying that balance so that the asymptotic state of a system with an MBL Hamiltonian does not bear localization signatures. This is evidently true in the case of local dephasing which drives any system into an infinite-temperature state. We demonstrate, by using a set of dissipative operators, where each one is acting nontrivially on a pair of neighboring sites (or spins), that an MBL system can be brought into a Hamiltonian-specific steady state. The difference between ergodic and MBL Hamiltonians can be seen in statistics of imbalance, entanglement entropy, and level spacing of the steady-state density operator. By introducing pairwise dissipative operators into an MBL system already exposed to dephasing, these localization signatures can be restored.

Molecular dynamics simulations have been used to investigate the coupling process between plasticity and structural phase transformation in single-crystal iron under both shock and ramp compressions. In both cases, iron was found to yield via twinning. Then, the onset of the bcc-hcp phase transformation was shown to be tightly dependent on the plasticity history through a hardening-like effect, which in some conditions may inhibit the nucleation of the hcp phase.

Isotope-selective ${}^{57}\mathrm{Fe}$ nuclear resonant inelastic x-ray scattering (NRIXS) measurements and atomic-layer resolved density functional theory (DFT) calculations were used to investigate the effect of interfaces on the vibrational (phonon) density of states (VDOS) of (001)-oriented nanoscale Fe/Ag and Fe/Cr multilayers. The multilayers in the experiment contained isotopically enriched ${}^{57}\mathrm{Fe}$ monolayers as probe layers located either at the Fe/Ag or Fe/Cr interfaces or in the center of the Fe films. This allows probing of the vibrational dynamics of Fe sites either at the buried interfaces or in the center of the Fe films. For Fe/Ag multilayers, distinct differences were observed experimentally between the Fe-partial VDOS at the interface and in the center. At the Fe/Ag interface, the high-energy longitudinal-acoustic (LA) phonon peak of Fe near $\sim 35$ meV is suppressed and slightly shifted to lower energy, and the low-energy part of the VDOS below $\sim 20$ meV is drastically enhanced, as compared to the Fe-specific VDOS in the center Fe layers or in bulk Fe. Similar phenomena are found to a less degree in the Fe/Cr multilayers. The measured Fe-partial VDOS was used to determine the Fe site-selective vibrational thermodynamic properties of the multilayers. Our theoretical findings for the layer-dependent VDOS of the multilayers are in qualitative agreement with the experimental results obtained by NRIXS. For Fe/Ag multilayers, which are characterized by a large atomic mass ratio, the experimental and theoretical results demonstrate phonon confinement in the Fe layers and phonon localization at the Fe/Ag interfaces due to the energy mismatch between Ag and Fe LA phonons. These phenomena are reduced or suppressed in the Fe/Cr multilayers with their about equal atomic masses. Moreover, direction-projected Fe VDOS along the (nearly in-plane) incident x-ray beam was computed in order to address the intrinsic vibrational anisotropy of the Fe/Ag multilayer. We have also performed spin-resolved electronic band structure (DFT) calculations, predicting an enhanced magnetic moment $\left({\mu }_{\mathrm{Fe}}=2.8{\mu }_{\mathrm{B}}\right)$ of the interfacial Fe atoms and a high electron spin polarization (79%) at the Fermi energy for the Fe/Ag interface, as compared to the case of Fe center layers. This is a result of charge transfer from Fe to Ag at the interface. On the contrary, Cr tends to donate electrons to Fe, thus reducing the interfacial Fe moment $\left({\mu }_{\mathrm{Fe}}=1.9{\mu }_{\mathrm{B}}\right)$. This implies strong chemical bonding at the Fe/Ag and Fe/Cr interfaces, affecting the interfacial VDOS.

An unexplained discrepancy exists between the experimentally measured and theoretically calculated magnetic moments of Mn in α-Fe. In this study, we use density functional theory to suggest that this discrepancy is likely due to the local strain environment of a Mn atom in the Fe structure. The ferromagnetic coupling, found by experiment, was shown to be metastable and could be stabilized by a 2% hydrostatic compressive strain. The effects of Mn concentration, vacancies, and interstitial defects on the magnetic moment of Mn are also discussed. It was found that the ground-state, antiferromagnetic (AFM) coupling of Mn to Fe requires long-range tensile relaxations of the neighboring atoms along $\langle 111\rangle$ which is hindered in the presence of other Mn atoms. Vacancies and Fe interstitial defects stabilize the AFM coupling but are not expected to have a large effect on the average measured magnetic moment.

The superfluid drag coefficient of a weakly interacting three-component Bose-Einstein condensate is computed on a square optical lattice deep in the superfluid phase, starting from a Bose-Hubbard model with component-conserving, on-site interactions and nearest-neighbor hopping. At the mean-field level, Rayleigh-Schrödinger perturbation theory is employed to provide an analytic expression for the drag density. In addition, the Hamiltonian is diagonalized numerically to compute the drag within mean-field theory at both zero and finite temperatures to all orders in intercomponent interactions. Moreover, path integral Monte Carlo simulations, providing results beyond mean-field theory, have been performed to support the mean-field results. In the two-component case the drag increases monotonically with the magnitude of the intercomponent interaction ${\gamma }_{AB}$ between the two components $A$ and $B$. The increase is independent of the sign of the intercomponent interaction. This no longer holds when an additional third component $C$ is included. Instead of increasing monotonically, the drag can either be strengthened or weakened depending on the details of the interaction strengths, for weak and moderately strong interactions. The general picture is that the drag coefficient between component $A$ and $B$ is a nonmonotonic function of the intercomponent interaction strength ${\gamma }_{AC}$ between $A$ and a third component $C$. For weak ${\gamma }_{AC}$ compared to the direct interaction ${\gamma }_{AB}$ between $A$ and $B$, the drag coefficient between $A$ and $B$ can decrease, contrary to what one naively would expect. When ${\gamma }_{AC}$ is strong compared to ${\gamma }_{AB}$, the drag between $A$ and $B$ increases with increasing ${\gamma }_{AC}$, as one would naively expect. We attribute the subtle reduction of ${\rho }_{d,AB}$ with increasing ${\gamma }_{AC}$, which has no counterpart in the two-component case, to a renormalization of the intercomponent scattering vertex ${\gamma }_{AB}$ via intermediate excited states of the third condensate $C$. We briefly comment on how this generalizes to systems with more than three components.

Specific heat, magnetization, and electrical resistivity have been studied on single crystalline samples of ${\mathrm{Ce}}_2{\mathrm{Pt}}_2\mathrm{Pb}$. These results clearly indicate that ${\mathrm{Ce}}_2{\mathrm{Pt}}_2\mathrm{Pb}$ undergoes an antiferromagnetic transition at approximately 3.4 K under a zero magnetic field. The magnetic field induces another ordered phase when the field is applied along the $\langle 100\rangle$ or $\langle 110\rangle$ directions of the crystallography, whereas we have not found another ordered phase along the $\langle 001\rangle$ direction of the field. The specific heat and the magnetization along three directions of the field reveal an easy-plane type magnetic anisotropy even in the antiferromagnetic phase. The magnetic structures of the ordered phases are discussed from the standpoint of macroscopic measurement, which results in a helical magnetic structure as a reasonable scenario. A large Sommerfeld coefficient of the specific heat and squared-temperature dependence of the electrical resistivity strongly suggest construction of the heavy-fermion state in ${\mathrm{Ce}}_2{\mathrm{Pt}}_2\mathrm{Pb}$, while the influence of the frustration is hardly found.

We investigate quantum transport and thermoelectrical properties of a finite-size Su-Schrieffer-Heeger model, a paradigmatic model for a one-dimensional topological insulator, which displays topologically protected edge states. By coupling the model to two fermionic reservoirs at its ends, we can explore the nonequilibrium dynamics of the system. Investigating the energy-resolved transmission, the current, and the noise, we find that these observables can be used to detect the topologically nontrivial phase. With specific parameters and asymmetric reservoir coupling strengths, we show that we can dissipatively prepare the edge states as stationary states of a nonequilibrium configuration. In addition, we point out that the edge states can be exploited to design a refrigerator driven by chemical work or a heat engine driven by a thermal gradient, respectively. These thermal devices do not require asymmetric couplings and are topologically protected against symmetry-preserving perturbations. Their maximum efficiencies significantly exceed that of a single quantum dot device at comparable coupling strengths.

Layered transition-metal dichalcogenides that host coexisting charge-density wave (CDW) and superconducting orders provide ideal systems for exploring the effects of dimensionality on correlated electronic phases. Dimensionality has a profound effect on both superconductivity and CDW instabilities. Here we report a substantial enhancement of the superconducting $T_c$ to 3.4 K for $2H\text{−}{\mathrm{TaS}}_2$ in the monolayer limit, compared to 0.8 K in the bulk. In addition, the transport signature of a CDW phase transition vanishes in the two-dimensional limit. In our analysis of electronic and vibrational properties of this material, we show that a reduction of the CDW amplitude results in a substantial increase of the density of states at the Fermi energy, which can boost $T_c$ by an amount similar to that seen in experiment. Our results indicate competition between CDW order and superconductivity in ultrathin $2H\text{−}{\mathrm{TaS}}_2$ down to the monolayer limit, providing insight toward understanding correlated electronic phases in reduced dimensions.

We examined the influence of microwave radiation on both the amplitude of Shubnikov–de Haas (SdH) oscillations and the null field longitudinal magnetoresistance at liquid helium temperatures, in GaAs/AlGaAs Hall bar devices. Microwave radiation over the frequency range $30\le f\le 50\text{GHz}$ with source power $0\le P\le 4\text{mW}$ served to photoexcite the high mobility ($\simeq {10}^7{\text{cm}}^2/\text{V}\text{s}$) two-dimensional electron system (2DES) as magnetoresistance traces were obtained as a function of the microwave power $P$ and temperature $T$. Line-shape study of SdH oscillations has been carried out over the span $2.3<{\omega }_c/\omega \le 5.2$, where ${\omega }_c=eB/m^{*},\omega =2\pi f,B$ is the magnetic field, $m^{*}$ is the effective mass, and $f$ is the microwave frequency. Here, fits of the SdH line shape served to determine the electron temperature ($T_e$) as a function of $P$ and $T$. Theory has proposed that, in the ${\omega }_c/\omega \ge 1$ regime, both the electron temperature and radiation energy absorption rate ($S_p$) exhibit relatively small response, while in the ${\omega }_c/\omega \le 1$ regime, both $T_e$ and $S_p$ are enhanced and exhibit oscillatory behavior. We compare the experimental results with these theoretical predictions, and comment upon the relative role of electron heating in the microwave photoexcited high-mobility 2DES.

We investigate electronic transport in twisted bilayer graphene (tBLG) under variable temperatures ($T$), carrier densities ($n$), and transverse magnetic fields, focusing on samples with small twist angles (θ). These samples show prominent signatures associated with the van Hove singularities (VHSs) and superlattice-induced minigaps (SMGs). Temperature-dependent field-effect measurement shows that the difference between temperature-dependent resistivity and residual resistivity, ${\rho }_{xx}\left(T,n\right)-{\rho }_0\left(n\right)$, follows $\sim T^{\beta }$ for $n$ between the main Dirac point (DP) and SMG. The evolution of the temperature exponent β with $n$ exhibits a W-shaped dependence, with minima of $\beta \sim 0.9$ near the VHSs and maxima of $\beta \sim 1.7$ toward the SMGs. This W-shaped behavior can be qualitatively understood with a theoretical picture that considers both the Fermi surface smearing near the VHSs and flexural-acoustic phonon scattering. In the quantum Hall regime, we observe only Landau level crossings in the massless Dirac spectrum originating from the main DP but not in the parabolic band near the SMG. Such crossings enable the measurement of an enhanced interlayer dielectric constant, attributed to a reduced Fermi velocity. Moreover, we measure the Fermi velocity, interlayer coupling strength, VHS energy relative to the DP, and gap size of SMG, four important parameters used to describe the peculiar band structure of the small-θ tBLG.

We consider the excitation of single-electron wave packets by means of a time-dependent voltage applied to the ballistic edge channels of the integer quantum Hall effect at filling factor $\nu =2$. Due to electron-electron interactions, fractional excitations emerge along the edge. Their detailed structure is analyzed by evaluating the nonequilibrium momentum distributions associated with the different edge channels. We provide results for a generic time-dependent drive both in the stationary regime and for intermediate times, where the overlap between fractionalized wave packets carries relevant information on interaction strength. As a particular example we focus on a Lorentzian drive, which provides a clear signature of the minimal excitations known as Levitons. Here we argue that inner-channel fractionalized excitations can be exploited to extract information about interchannel interactions. We further confirm this idea by calculating the zero frequency noise due to the partitioning of these excitations at a quantum point contact and we propose a measurable quantity as a tool to directly probe electron-electron interactions and determine the so-called mixing angle of copropagating quantum Hall channels.

Using density-functional studies, we show that a spin-orbital entangled two-dimensional electron gas (2DEG) forms at the (001) ${\mathrm{LaAlO}}_3/{\mathrm{Sr}}_2{\mathrm{IrO}}_4$ polar interface between an ordinary band insulator and a spin-orbit coupled Mott insulator, aided by the combination of the spin-orbit coupling, Coulomb interaction, and polar catastrophe. Quite remarkably, the 2DEG is found to be localized on a single ${\mathrm{IrO}}_2$ monolayer, unlike other polar interfaces such as ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$, where the 2DEG is several monolayers thick. The electron gas occupies the upper $J_{\mathrm{eff}}=1/2$ Hubbard band in the interface layer, which becomes half filled with a simple squarelike Fermi surface. If successfully grown, this would be a candidate material to host the spin-orbital entangled 2DEG.

We study one-dimensional systems of two-orbital SU(4) fermionic cold atoms. In particular, we focus on an SU(4) spin model [named SU(4) $e\text{−}g$ spin model] that is realized in a low-energy state in the Mott insulator phase at the filling $n_g=3,n_e=1$ ($n_g,n_e$: numbers of atoms in ground and excited states, respectively). Our numerical study with the infinite-size density matrix renormalization group shows that the ground state of SU(4) $e\text{−}g$ spin model is a nontrivial symmetry protected topological (SPT) phase protected by $Z_4\times Z_4$ symmetry. Specifically, we find that the ground state belongs to an SPT phase with the topological index $2\in {\mathbb{Z}}_4$ and show sixfold degenerate edge states. This is topologically distinct from SPT phases with the index $1\in {\mathbb{Z}}_4$ that are realized in the SU(4) bilinear model and the SU(4) Affleck-Kennedy-Lieb-Tasaki (AKLT) model. We explore the phase diagram of SU(4) spin models including $e\text{−}g$ spin model, bilinear-biquadratic model, and AKLT model, and identify that antisymmetrization effect in neighboring spins (that we quantify with Casimir operators) is the driving force of the phase transitions. Furthermore, we demonstrate by using the matrix product state how the ${\mathbb{Z}}_4$ SPT state with six edge states appears in the SU(4) $e\text{−}g$ spin model.

Using density-functional theory calculations, we investigate the dominant defects formed by boron (B) and carbon (C) impurities in a CoFe/MgO/CoFe magnetic tunnel junction (MTJ) and their influence on conductivity and tunneling magnetoresistance (TMR). We find that, in the O-poor conditions relevant to experiment, B forms the substitutional defect ${\mathrm{B}}_{\mathrm{Co}}$ and C forms the interstitial site ${\mathrm{C}}_{\mathrm{i}}$ at the CoFe/MgO interface. The C-doped MTJ is predicted to have a significantly higher TMR than the B-doped MTJ. This is due to interface state densities associated with the majority spin ${\mathrm{\Delta }}_1$-symmetry bands being more heavily suppressed by the ${\mathrm{B}}_{\mathrm{Co}}$ defects than by the ${\mathrm{C}}_{\mathrm{i}}$ defects. Our results indicate that carbon can serve as a viable alternative to boron as a dopant for MTJ fabrication.

Using density functional dynamical mean-field theory we investigate the site-selective electronic structure of ${\mathrm{Ca}}_2{\mathrm{O}}_3{\mathrm{Fe}}_3{\mathrm{S}}_2$. We confirm that the parent compound with two distinct iron sites is a multiorbital Mott insulator similar to ${\mathrm{La}}_2{\mathrm{O}}_3{\mathrm{Fe}}_2{\mathrm{S}}_2$. Upon electron/hole doping, carrier localization is found to persist in the two active iron channels because the chemical potential lies in a gap structure with anisotropic and almost vanishing states near the Fermi energy. This emergent behavior stems from large electronic reconstruction caused by dynamical spectral weight transfer involving states with distinct $d$-shell occupancies and orbital character at low energies. We detail the implications of our microscopic analysis and discuss the underlying physics which will emerge in future experiments on ${\mathrm{Ca}}_2{\mathrm{O}}_3{\mathrm{Fe}}_3{\mathrm{S}}_2$.

Charge trapping and the formation of polarons is a pervasive phenomenon in transition-metal oxide compounds, in particular at the surface, affecting fundamental physical properties and functionalities of the hosting materials. Here we investigate via first-principles techniques the formation and dynamics of small polarons on the reduced surface of titanium dioxide, an archetypal system for polarons, for a wide range of oxygen vacancy concentrations. We report how the essential features of polarons can be adequately accounted for in terms of a few quantities: the local structural and chemical environment, the attractive interaction between negatively charged polarons and positively charged oxygen vacancies, and the spin-dependent polaron-polaron Coulomb repulsion. We combined molecular-dynamics simulations on realistic samples derived from experimental observations with simplified static models, aiming to disentangle the various variables at play. We find that depending on the specific trapping site, different types of polarons can be formed, with distinct orbital symmetries and a different degree of localization and structural distortion. The energetically most stable polaron site is the subsurface Ti atom below the undercoordinated surface Ti atom, due to a small energy cost to distort the lattice and a suitable electrostatic potential. Polaron-polaron repulsion and polaron-vacancy attraction determine the spatial distribution of polarons as well as the energy of the polaronic in-gap state. In the range of experimentally reachable oxygen vacancy concentrations, the calculated data are in excellent agreement with observations, thus validating the overall interpretation.

We study narrow zigzag graphene nanoribbons (ZGNRs), employing density functional theory (DFT) simulations and the tight-binding (TB) method. The main result of these calculations is the braiding of the conduction and valence bands, generating Dirac cones for noncommensurate wave-vectors $\vec{k}$. Employing a TB Hamiltonian, we show that the braiding is generated by the third-neighbor hopping. We calculate the band structure, the density of states, and the conductance; new conductance channels are opened, and the conductance at the Fermi energy assumes integer multiples of the quantum conductance unit $G_o=2e^2/h$. We also investigate the satisfaction of the Stoner criterion by these ZGNRs. We calculate the magnetic properties of the fundamental state, employing the random-phase approximation and employing local spin-density approximation (LSDA) (spin-unrestricted DFT) we confirm that ZGNRs with $N=(2,3)$ do not satisfy the Stoner criterion and as such, the magnetic order could not be developed at their edges. These results are confirmed by both tight-binding and LSDA calculations.

Transport and magnetoresistance measurements are performed on metallic, high-carrier density ${\mathrm{YTiO}}_3\text{−}{\mathrm{CaTiO}}_3$ superlattices acting as a unique probe of an emergent magnetic order in ${\mathrm{YTiO}}_3$. On varying the thickness of ${\mathrm{YTiO}}_3$ while keeping the ${\mathrm{CaTiO}}_3$ layer thickness constant, a non-Fermi-liquid charge transport and positive magnetoresistance are observed. Analyses of the origin of such effects reveal that an unusual antiferromagnetic order is realized in the ultrathin, epitaxially strained ${\mathrm{YTiO}}_3$ layers, which is in excellent agreement with recent theoretical predictions.