$P$-type doping in oxides is usually difficult due to their low valence-band energy. In order to make them $p$ type, the electronic structure of the oxides should be fundamentally changed; that is, the occupied valence band should be raised significantly. Here, using first-principles calculations, we propose that by adding a small amount of ${\mathrm{Bi}}_2{\mathrm{O}}_3$ into ${\mathrm{Ga}}_2{\mathrm{O}}_3$ to form dilute ${\left({\mathrm{Bi}}_x{\mathrm{Ga}}_{1–x}\right)}_2{\mathrm{O}}_3$ alloys and, more importantly, with properly chosen dopants, we can achieve efficient $p$-type doping in a transparent oxide. We show that adding a few percent of Bi to ${\mathrm{Ga}}_2{\mathrm{O}}_3$ leads to an intermediate valence band that is sufficiently high in energy to facilitate $p$-type doping; however, the commonly expected shallow acceptors, Mg and Zn substitution on the Ga site (${\mathrm{Mg}}_{\mathrm{Ga}}$ and ${\mathrm{Zn}}_{\mathrm{Ga}}$), are still deep acceptors, whereas the expected deep acceptor, ${\mathrm{Cu}}_{\mathrm{Ga}}$, actually creates a relatively shallow level in ${\left({\mathrm{Bi}}_x{\mathrm{Ga}}_{1–x}\right)}_2{\mathrm{O}}_3$ alloys. This trend is opposite to what is found in pure ${\mathrm{Ga}}_2{\mathrm{O}}_3$. The puzzling behavior of the acceptor levels in the ${\left({\mathrm{Bi}}_x{\mathrm{Ga}}_{1–x}\right)}_2{\mathrm{O}}_3$ alloys is attributed to the polaronic character of the holes in the Zn- and Mg-doped cases, and the decoupling of the hole state and the valence-band edge in the Cu-doped case. This understanding provides insights into the realization of $p$-type doping in dilute ${\left({\mathrm{Bi}}_x{\mathrm{Ga}}_{1–x}\right)}_2{\mathrm{O}}_3$ alloys and paves a way to dope semiconductor materials with a strongly correlated band edge state, i.e., materials with a tendency to form polaronic acceptor or donor states.

Quantum corrections to electrical resistance can serve as sensitive probes of the magnetic landscape of a material. For example, interference between time-reversed electron paths gives rise to weak localization effects, which can provide information about the coupling between spins and orbital motion, while the Kondo effect is sensitive to the presence of spin impurities. Here, we use low-temperature magnetotransport measurements to reveal a gradual transition from weak antilocalization (WAL) to Kondo scattering in the quasi-two-dimensional electron gas formed at the interface between $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ and the Mott insulator $\mathrm{Nd}\mathrm{Ti}{\mathrm{O}}_3$. This transition occurs as the thickness of the $\mathrm{Nd}\mathrm{Ti}{\mathrm{O}}_3$ layer is increased. Analysis of the Kondo scattering and WAL points to the presence of atomic-scale magnetic impurities coexisting with nanoscale magnetic regions that affect transport via a strong magnetic exchange interaction. This leads to distinct magnetoresistance behaviors that can serve as a sensitive probe of magnetic properties in two dimensions.

We investigate special solutions to the Bethe Ansatz equations (BAE) for open integrable XXZ Heisenberg spin chains containing phantom (infinite) Bethe roots. The phantom Bethe roots do not contribute to the energy of the Bethe state, so the energy is determined exclusively by the remaining regular excitations. We derive the phantom Bethe roots criterion and focus on BAE solutions for mixtures of phantom roots and regular (finite) Bethe roots. We prove that in the presence of phantom Bethe roots, all eigenstates are split between two invariant subspaces, spanned by chiral shock states. Bethe eigenstates are described by two complementary sets of Bethe ansatz equations for regular roots, one for each invariant subspace. The respective “semiphantom” Bethe vectors are states of chiral nature, with chirality properties getting less pronounced when more regular Bethe roots are added. For the easy plane case, “semiphantom” Bethe states carry nonzero magnetic current and are characterized by quasiperiodic modulation of the magnetization profile, the most prominent example being the spin helix states (SHS). We illustrate our results investigating “semiphantom” Bethe states generated by one regular Bethe root (the other Bethe roots being phantom), with a simple structure of the invariant subspace, in all details. We obtain the explicit expressions for Bethe vectors and calculate the simplest correlation functions, including the spin-current for all the states in the single-particle multiplet.

We present a method for extrapolation of real-time dynamical correlation functions which can improve the capability of matrix product state methods to compute spectral functions. Unlike the widely used linear prediction method, which ignores the origin of the data being extrapolated, our recursion methods utilize a representation of the wavefunction in terms of an expansion of the same wavefunction and its translations at earlier times. This recursion method is exact for a noninteracting Fermi system. Surprisingly, the recursion method is also more robust than linear prediction at large interaction strength. We test this method on the Hubbard two-leg ladder, and present more accurate results for the spectral function than previous studies.

We study the transport of a hybrid superconductor–quantum-dot–superconductor junction, dominated by the interplay between the Kondo effect and the proximity-induced $p$-wave pairing states. Each superconductor lead is fabricated with a semiconductor with Rashba spin-orbit coupling (RSOC) and the combination of an $s$-wave superconductor and a ferromagnet. The RSOC breaks the SU(2) spin-rotation symmetry and creates the spin-triplet pairing components under the proximity-induced superconducting pairing interaction. Different from the s-wave pairing case, the Kondo screening of the dot spin involves the orbital angular momentum conserved transitions between the $p$-wave pairing states. The Zeeman field inherent from the ferromagnet removes the spin degeneracy of the quasiparticles excited in the triplet states. As a result, the spin-induced Yu-Shiba-Rusinov (YSR) state exhibits Zeeman-dependent splitting behaviors, and the splitting of the YSR state leads to the 0-$\pi$ phase transition when the ground state is a Kondo singlet. The temperature-dependent magnetic susceptibility indicates that the dot spin should be partially screened due to the breaking of time-reversal symmetry.

A circularly polarized ac pump field illuminated near resonance on two-dimensional transition metal dichalcogenides (TMDs) produces an anomalous Hall effect in response to a dc bias field. In this work, we develop a theory for this photoinduced anomalous Hall effect in undoped TMDs irradiated by a strong coherent laser field. The strong field renormalizes the equilibrium bands and opens up a dynamical energy gap where single-photon resonance occurs. The resulting photon dressed states, or Floquet states, are treated within the rotating-wave approximation. A quantum kinetic equation approach is developed to study the nonequilibrium density matrix and time-averaged transport currents under the simultaneous influence of the strong ac pump field and the weak dc probe field. Dissipative effects are taken into account in the kinetic equation that captures relaxation and dephasing. The photoinduced longitudinal and Hall conductivities display notable resonant signatures when the pump field frequency reaches the spin-split interband transition energies. Rather than valley polarization, we find that the anomalous Hall current is mainly driven by the intraband response of photon-dressed electron populations near the dynamical gap at both valleys, accompanied by a smaller contribution due to the interband response. These findings highlight the importance of photon-dressed bands and nonequilibrium distribution functions in achieving a proper understanding of the photoinduced anomalous Hall effect in a strong pump field.

We perform a detailed analysis of electronic polarizability of graphene with different theoretical approaches. From Kubo's linear response formalism, we give a general expression of frequency and wave-vector dependent polarizability within the random phase approximation. Four theoretical approaches have been applied to the single-layer graphene and their differences are on the band overlap of wave functions. By comparing with the ab initio calculation, we discuss the validity of methods used in literature. Our results show that the tight-binding method is as good as the time-demanding ab initio approach in calculating the polarizability of graphene. Moreover, due to the special Dirac-cone band structure of graphene, the Dirac model reproduces results of the tight-binding method for energy smaller than 3 eV. For doped graphene, the intraband transitions dominate at low energies and can be described by the Lindhard formula for two-dimensional electron gases. At zero temperature and long-wavelength limit, with the relaxation time approximation, all theoretical methods reduce to a long-wave analytical formula and the intraband contributions agree to the Drude polarizability of graphene. Effects of electrical doping and temperature are also discussed. This work may provide a solid reference for researches and applications of the screening effect of graphene.

The nonlinear sigma model is a well-established theoretical tool for studies of transport and thermodynamics in disordered electronic systems. The conventional sigma model approach for interacting systems does not account for particle-hole asymmetry. It is therefore not suited for studying quantities that are sensitive to this effect such as the thermoelectric transport coefficient. Here, we derive a minimal extension of the Keldysh nonlinear sigma model tailored for two-dimensional interacting systems. We argue that this model can be used to systematically study the combined effect of interactions and disorder on thermoelectric transport. As a first step in this direction, we use the model to analyze the structure of the heat density-density correlation function and calculate interaction corrections to its static part. The calculation of interaction corrections to the dynamical part of the correlation function and the thermodynamic transport coefficient is left for future work.

Fast-heating calorimetry is used to study in operando the vitrification mechanism of off-eutectic ${\mathrm{Yb}}_{47}{\mathrm{Mg}}_{{}_{31}}{\mathrm{Zn}}_{19}{\mathrm{Cu}}_3$ liquid at a cooling rate of $10000\mathrm{K}{\mathrm{s}}^{–1}$. The glass forms only when it is quenched from between the melting and liquidus temperature, contradicting the need for full remelting. Zinc-rich precipitates, forming on quenching from above the liquidus, deteriorate vitrification—which also offers an alternative explanation to what has often been considered as liquid-liquid transition. Application of an up-quenching thermal protocol facilitates the glass formation by removing the precipitates in situ; it is generally applicable and may extend the glass-forming range of other off-eutectic glasses. This research demonstrates a strategy to develop bulk metallic glasses.

Using ab initio density functional theory and single-orbital Hubbard model calculations via the density matrix renormalization group method, we systematically studied the monolayer ${\mathrm{VOI}}_2$ with a $3d^1$ electronic configuration. Our phonon calculations indicate that the orthorhombic $Pmm2$ FE-II phase is the most likely ground state, involving a ferroelectric (FE) distortion along the $a$ axis and V-V dimerization along the $b$ axis. Specifically, the “pseudo Jahn-Teller” effect caused by the coupling between empty V ($d_{xz/yz}$ and $d_{3z^2-r^2}$) and O $2p$ states is proposed as the mechanism that stabilizes the FE distortion from the paraelectric phase. Moreover, the half-filled metallic $d_{xy}$ band displays a Peierls instability along the $b$ axis, inducing a V-V dimerization. We also found very short-range antiferromagnetic coupling along the V-V chain due to the formation of nearly decoupled spin singlets in the ground state.

The energy of the activation gaps in the quantum Hall effect has been thought to be reduced by the broadening of the Landau levels due to disorder. The isotopic mass can affect the electron-phonon interactions and the 0 K renormalization of the conduction and valence bands, leading to appreciable energy offsets between lattice sites with different nuclear masses. The isotopic disorder originating from the natural occurrence of nuclear masses could be the dominant broadening mechanism remaining, leading to the experimentally observed quantum Hall effect gaps in high-mobility devices. The Landau level broadening due to the isotopic disorder has been calculated microscopically without fitting parameters.

We report experimental studies on ${\mathrm{Ru}}_{2-x}{\mathrm{Fe}}_x\mathrm{CrSi}$ focusing on properties related to antiferromagnetism in ${\mathrm{Ru}}_2\mathrm{CrSi}$ and spin glass (SG) in ${\mathrm{Ru}}_{1.9}{\mathrm{Fe}}_{0.1}\mathrm{CrSi}$. By measuring the temperature dependence of magnetization $M\left(T\right)$, specific heat $C\left(T\right)$, electrical resistivity $\rho \left(T\right)$, and muon spin relaxation ($\mu \mathrm{SR}$), we observed that ${\mathrm{Ru}}_2\mathrm{CrSi}$ exhibited an antiferromagnetic (AF) transition at a temperature $T_N$ of $\sim 13$ K, and ${\mathrm{Ru}}_{1.9}{\mathrm{Fe}}_{0.1}\mathrm{CrSi}$ showed SG properties that could be interpreted as successive SG transitions, where the spin-freezing occurred at temperature $T_g$, and below that strong irreversibility in $M\left(T\right)$ and a gradual peak of $M\left(T\right)$ at $T^{*}(>T_g)$ appeared. When the data for $0\le x\le 0.1$ were compared, by substituting Fe the AF order was rapidly destroyed and it appeared to change to two anomalies at $T^{*}$ and $T_g$. With increasing $x$, there was a slight change in $T_g$ from $T_N$ for $x=0$; however, $T^{*}$ increased, suggesting that the AF and the SG states are closely related. Furthermore, the results of specific heat, resistivity, and magnetization in high fields were presented and compared. For ${\mathrm{Ru}}_2\mathrm{CrSi}$ in specific heat under high magnetic fields up to 14 T, the peak shape around $T_N$ and the $T_N$ value were constant. The resistivity and magnetization in pulsed fields suggested that $T_N$ of ${\mathrm{Ru}}_2\mathrm{CrSi}$ was constant up to over 50 T. These results demonstrated the unusual robustness of the AF transition to magnetic fields. In ${\mathrm{Ru}}_{1.9}{\mathrm{Fe}}_{0.1}\mathrm{CrSi}$, unusual hysteresis in magnetoresistance was observed in static and pulsed magnetic fields, although their appearances differed. These hystereses were considered a manifestation of the curious properties of the SG state with strong irreversibility.

We studied the effect of hydrostatic pressure on the magnetic properties of the highly anisotropic van der Waals ferromagnetic metal ${\mathrm{Fe}}_3\mathrm{Ge}{\mathrm{Te}}_2$ (FGT) with the field applied along the easy axis. The paramagnetic-to-ferromagnetic transition occurs at the Curie temperature $T_c=180\mathrm{K}$ at ambient pressure, and $T_c$ decreases monotonically by up to 15 K as the pressure increases up to 1.44 GPa, while the magnetization is suppressed by the pressure. By using high-pressure x-ray diffraction techniques, we found that the Fe-Fe bond lengths tend to decrease, and the Fe-Ge(Te)-Fe bond angles deviate away from ${90}^{\circ }$ under hydrostatic pressures, indicating the modification of the exchange interactions. First-principles calculations further confirm the pressure effects. These results suggest that the competition between direct-, super-, and double-exchange interactions plays a crucial role in the pronounced magnetic response under the hydrostatic pressure, i.e., the direct-exchange becomes stronger at a higher pressure and, hence, leading to increased antiferromagnetic components and thus deceased $T_c$. The highly tunable magnetic properties under hydrostatic pressure in this system provide robust routes for spin manipulation in low-dimensional material systems.

Semiconductor/ferromagnet hybrid systems are attractive platforms for investigation of spin conversion physics, such as the (inverse) spin Hall effect. However, the superimposed rectification currents originating from anisotropic magnetoresistance have been a serious problem preventing unambiguous detection of dc spin Hall electric signals in semiconductors. In this study, we applied a microwave frequency inductive technique immune to such rectification effects to investigate the spin to charge conversion in heterostructures based on Si, one of the primitive semiconductors. The Si doping dependence of the spin-orbit torque conductivity was obtained for the Si/Cu/NiFe trilayer system. A monotonous modulation of the spin-orbit torque conductivity by doping and relative sign change of spin to charge conversion between the degenerate $n$- and $p$-type Si samples were observed. These results unveil spin to charge conversion mechanisms in semiconductor/metal heterostructures and show a pathway for further exploration of spin-conversion physics in metal/semiconductor heterostructures.

We present a temperature-dependent investigation of the local structure and magnetic dynamics of the FeAs binary. The magnetic susceptibility $\chi \left(T\right)$ result shows an anomalous broad feature up to 550 K, with χ continuing to increase above the Néel antiferromagnetic ordering temperature ($T_{\mathrm{N}}=70\mathrm{K}$), peaking at ∼250 K, then decreasing gently above. It is remarkable that this peak susceptibility temperature corresponds to the onset of anisotropic negative thermal expansion in both the $a$ and $c$ axes, suggesting that magnetic interactions are affecting the structure even well above the Néel point. A systematic investigation into local bonding correlations, from time-of-flight neutron pair distribution function analyses, shows the octahedral volume around each Fe site growing monotonically while adjacent octahedra tilt toward one another before relaxing away past this peak in the anomalous magnetic susceptibility. We use inelastic-neutron scattering to map spin-wave excitations in FeAs at temperatures above and below the $T_{\mathrm{N}}$. We find magnetic excitations near $T_{\mathrm{N}}$ to be very different from the excitations in the ground state at 1.5 K. Spin waves measured at 1.5 K are three dimensional (3D), however, in the vicinity of the magnetic transition, the magnetic fluctuations clearly indicate two-dimensional (2D) character in this intrinsically 3D crystal structure. Unlike the undoped 2D parents of iron-arsenide superconductors, where the magnetic correlations are considerably weaker along the $c$ axis than in the $\mathit{ab}$ plane, inelastic neutron scattering here shows that the spin fluctuations in the 3D FeAs binary are nearly 2D in the $\mathit{bc}$ plane at 90 K. These results demonstrate the importance of short-range correlations in understanding the magnetic properties of transition-metal binaries, and suggest how 2D excitations, even in a 3D structure, can potentially become a breeding ground for unconventional superconductivity.

We show, using density functional calculations, that quasi-one-dimensional ${\mathrm{K}}_2{\mathrm{Mo}}_3{\mathrm{As}}_3$, which is closely related to ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ and has very similar superconducting properties, is not close to magnetism and has conventional $s$ wave electron-phonon superconductivity. This superconductivity is of multigap character due to different coupling on different Fermi surface sheets. This is discussed in relation to the properties of this family of quasi-one-dimensional pnictide superconductors. The results show that this family of superconductors provides a unique opportunity for studying the interplay of spin fluctuations and electron-phonon superconductivity in transition metal pnictides, and offer a path for sorting out the different proposed superconducting scenarios in this fascinating family of pnictide superconductors.

The low-temperature structure of ${\mathrm{Bi}}_{0.68}{\mathrm{Ca}}_{0.32}{\mathrm{MnO}}_3$ has been solved from electron and neutron diffraction data. The quantitative simultaneous refinement indicates an ordering of the Mn cations in a “stripe/chess”-like pattern. The ordering is accompanied by the formation of short Mn—Mn distances and the rearrangement of the Mn—O bonds indicating the development of complex extended “orbital molecules.” The primary order parameter breaks inversion symmetry and allows the generation of a spontaneous electrical polarization as the secondary order parameter. The neutron data at low temperature indicate the coexistence of a pseudo-CE long-range-ordered structure with a strongly reduced moment and short-range ferromagnetic correlations. These results indicate an intricate competition between the charge, orbital, and magnetic degrees of freedom and the ${\mathrm{Bi}}^{3+}$ stereoactivity in this manganite system.

We investigate the normal-mode spectrum of a trapped ion chain at the symmetry-breaking linear to zigzag transition and at finite temperatures. For this purpose, we modulate the amplitude of the Doppler cooling laser to excite and measure mode oscillations. The expected mode softening at the critical point, a signature of the second-order transition, is not observed. Numerical simulations show that this is mainly due to the finite temperature of the chain. Inspection of the trajectories suggest that the thermal shifts of the normal-mode spectrum can be understood by the ions collectively jumping between the two ground-state configurations of the symmetry-broken phase. We develop an effective analytical model, which allows us to reproduce the low-frequency spectrum as a function of the temperature and close to the transition point. In this model, the frequency shift of the soft mode is due to the anharmonic coupling with the high-frequency modes of the spectrum, acting as an averaged effective thermal environment. Our study could prove important for implementing ground-state laser cooling close to the critical point.

We present a random matrix approach to study the general vibrational properties of stable amorphous solids with translational invariance using the correlated Wishart ensemble. Within this approach, both analytical and numerical methods can be applied. Applying the random matrix theory to the correlated Wishart ensemble, we found the analytical form of the vibrational density of states and the dynamical structure factor. The ratio between the number of bonds and the number of degrees of freedom controls the Ioffe-Regel frequency, which determines the crossover between low-frequency propagating phonons and diffusons at higher frequencies. The reduced vibrational density of states shows the boson peak, whose frequency is close to the Ioffe-Regel crossover. In the isostatic case, we obtain the low-frequency cusplike singularity of the vibrational density of states, which was observed numerically. We also present a simple numerical random matrix model with finite interaction radius, whose properties rapidly converge to the analytical results with increasing the interaction radius. For a finite interaction radius, the numerical model demonstrates the presence of quasilocalized vibrations with a power-law low-frequency density of states.

A recent polarized neutron diffraction experiment on the $5d^2$ rhenium double perovskite ${\mathrm{Ba}}_2\mathrm{YRe}{\mathrm{O}}_6$ held at a low temperature uncovered weak magnetic diffraction peaks. Data analysis inferred a significantly reduced Re dipole moment, and long-range order compatible with an antiferromagnetic, noncollinear motif. To interpret the experimental findings, we present a model wave function for Re ions derived from the crystal field potential, Coulomb interaction, and spin-orbit coupling that fully respects the symmetry of the low-temperature ordered state. It is used to calculate in analytic form all multipole moments visible in neutron and resonance enhanced x-ray diffraction. A minimal model consistent with available neutron diffraction data predicts significant multipolar moments up to the hexadecapole and, in particular, a dominant chargelike quadrupole moment. Calculated diffraction patterns embrace single crystal x-ray diffraction at the Re $L$ edge, and renewed neutron diffraction, to probe the presumed underlying multipolar order.