We compute the thermoelectric transport parameterized by the Seebeck coefficient and thermal/electric conductance of random-alloy FeCo/MgO/FeCo(001) magnetic tunnel junctions (MTJs) from first principles using a generalized Landauer-Büttiker formalism. The thermopower is found to be typically smaller than those of Fe/MgO/Fe(001) MTJs. The (magneto-)Seebeck effect is sensitive to the details of the FeCo/MgO interfaces. Interfacial can greatly enhance the thermoelectric effects in MTJs. We also compute angular-dependent Seebeck coefficients that provide additional information about the transport physics. We report large deviations from the Wiedemann-Franz law at room temperature.

A method for deterministic control of magnetism using an electrical stimulus is highly desired for the new generation of magnetoelectronic devices. Much effort has been focused on magnetic domain-wall (DW) motion manipulated by a successive injection of spin-polarized current into a magnetic nanostructure. However, an integrant high-threshold current density of ${10}^{11}\sim {10}^{12}\mathrm{A}/{\mathrm{m}}^2$ inhibits the integration with low-energy-cost technology. Here, we report an approach to manipulate a single magnetic domain wall with a perpendicular anisotropy in a manganite/dielectric/metal capacitor using a probe-induced spin displacement. A spin-transfer torque (STT) occurs in the strongly correlated manganite film during the spin injection into the capacitor from the nanoscale magnetized tip with an ultralow voltage of 0.1 V, where a lower bound of the estimated threshold spin-polarized current density is $\sim {10}^8\mathrm{A}/{\mathrm{m}}^2$ at the tip/manganite interface. The dynamic of DW motions are analyzed using the Landau-Lifshitz-Gilbert method. This probe-voltage-controlled DW motion, at an ambient condition, demonstrates a critical framework for the fundamental understanding of the manipulation of the nanomagnet systems with low-energy consumption.

The phenomenon of different time scales of ultrafast demagnetization has attracted much attention. This so-called diversity of ultrafast demagnetization has been explained by the microscopic three temperature model (M3TM) and by the Landau-Lifshitz-Bloch model (LLBM). Here, we revisit the basic three temperature model (3TM) and provide a general criterion for explaining the different time scales observed. We focus on the role of magnetic heat capacity, which we find mainly determines the slowing down of the demagnetization time with increasing ambient temperature and laser fluence. In this context, we clarify the role of magnetic heat capacity in the M3TM and compare the 3TM with the LLBM. To illustrate the role of magnetic heat capacity, we present a simulation of ultrafast demagnetization of Ni. Furthermore, we present time-resolved magneto-optic Kerr effect measurements of ultrafast demagnetization and specific heat of ${\mathrm{Fe}}_{46}{\mathrm{Cu}}_6{\mathrm{Pt}}_{48}$ from 300 K to close to its Curie temperature. While most of the prior experimental research used high-fluence laser pulses causing sizable temperature excursions of the sample, our experiments involve small temperature excursions, which are crucial for studying the role of magnetic heat capacity in ultrafast demagnetization. Our experimental results corroborate that the slowing down of ultrafast demagnetization is dominated by the increase of the magnetic heat capacity near the Curie temperature.

Structural, magnetic, transport, and thermal properties of ${\mathrm{YbCu}}_{5-x}{\mathrm{Au}}_x$ alloys with Au concentration between $x=0.4$ (at the stability limit of ${\mathrm{AuBe}}_5$-type structure) and $x=0.7$ are reported. The outstanding features of this system are (i) the constant and record high value of $C_m/T\approx 6.7$ J/mol ${\mathrm{K}}^2$ below a characteristic temperature $T^{*}$ (ranging between 150 mK and 350 mK), (ii) a power law of temperature dependence $C_m/T(T>T^{*})=g/T^q$, with $q$ ranging between 1.3 and 0.95 for decreasing $x$ from 0.7 to 0.5, and (iii) an observation of incoherent electronic scattering in the resistivity at $T<1$ K for $x\le 0.6$ despite the fact that Yb magnetic atoms are placed in an ordered lattice. Geometric frustration, originating from the tetrahedral distribution of Yb atoms, appears to be responsible for the lack of magnetic order and the exotic behavior of this system.

Coexisting fluctuations towards various ordered states are ubiquitous in strongly correlated electronic systems. In particular, measurements of underdoped cuprate high-temperature superconductors reveal evidence for short-range charge order in parallel to large superconducting fluctuations. Here we use a nonlinear sigma model to describe a system with $N$ competing orders, and calculate its transverse thermoelectric transport coefficient in the analytically tractable limit of large $N$. Our results, which determine the contribution of order-parameter fluctuations to the Nernst signal, are appropriate for high temperatures in the case of finite $N$. They are similar to previously obtained results within a model of Gaussian superconducting fluctuations, but also indicate that the Nernst signal increases when the charge order fluctuations are suppressed.

We study three proposals for broken symmetry in the cuprate pseudogap—oxygen antiferromagnetism, ${\Theta }_{II}$ orbital loop currents, and circulating currents involving apex oxygens—through numerical exploration of multiorbital Hubbard models. Our numerically exact results show no evidence for the existence of oxygen antiferromagnetic order or the ${\Theta }_{II}$ phase in the three-orbital Hubbard model. The model also fails to sustain an ordered current pattern even with the presence of additional apex oxygen orbitals. We thereby conclude that it is difficult to stabilize the aforementioned phases in the multiorbital Hubbard models for parameters relevant to cuprate superconductors. However, the ${\Theta }_{II}$ phase might be stabilized through explicit flux terms. We find an enhanced propensity for circulating currents with such terms in calculations simulating applied stress or strain, which skew the copper-oxygen plane to resemble a kagome lattice. We propose an experimental viewpoint to shed additional light on this problem.

We report on the doping evolution of magnetic susceptibility $\chi \left(T\right)$ and Hall coefficient $R_H$ in high-quality ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ $\left(0.13\le x\le 1\right)$ single crystals. It is found that the normal-state magnetic susceptibility of ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ compounds undergoes a crossover from linear-$T$ dependence in the undoped and underdoped samples into ${\mathrm{KFe}}_2{\mathrm{As}}_2$-type magnetic response in the overdoped samples with increasing K content. Although magnetic susceptibility $\chi \left(T\right)$ of optimally doped samples $\left(0.34\le x\le 0.47\right)$ still follows a monotonic increase with increasing temperature, a big hump around 300 K emerges. As $x$ exceeds 0.53, a broad peak forms in overdoped samples $\left(0.53\le x\le 1\right)$, which shifts toward 120 K for the end member ${\mathrm{KFe}}_2{\mathrm{As}}_2$. Above the peak temperature $T^{*}=120$ K, a Curie-Weiss-like behavior is observed in ${\mathrm{KFe}}_2{\mathrm{As}}_2$. The Hall coefficient $R_H$ of underdoped sample $x=0.22$ shows a rapid increase above spin-density-wave transition temperature $T_{SDW}$. Below $T_{SDW}$, it increases slowly. $R_H$ of optimally doped and slightly overdoped samples $\left(0.34\le x\le 0.65\right)$ shows relatively weak temperature dependence and a saturation tendency below 150 K. However, $R_H$ of K heavily overdoped samples $\left(0.80\le x\le 1\right)$ increases rapidly below 150 K. Meanwhile, the Hall angle $cot{\theta }_H$ displays a concave temperature dependence within the doping range $0.22\le x\le 0.55$, whereas it changes to a convex temperature dependence within the doping range $0.65\le x\le 1$. The dramatic change coincides with the Lifshitz transition occurring around the critical doping $x=0.80$, where angle photoemission spectroscopy measurements had confirmed that the electron pocket disappears with excess hole doping in the ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ system. It is suggested that the characteristic temperature $T^{*}$ at around $120\sim 150$ K observed in susceptibility and the Hall coefficient, as well as previously reported resistivity data, may indicate an incoherence-coherence crossover in the ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ system.

Symmetry-protected topological states cannot be deformed to a trivial state so long as the symmetry is preserved, yet there is no local order parameter that can distinguish them from a trivial state. We demonstrate how to detect whether a two-dimensional ground state has symmetry-protected topological order; the measurements play a similar role as the topological entanglement entropy does for detecting anyons. For any finite Abelian onsite symmetry, the measurement completely determines the third cohomology class that characterizes the order. The proposed measurement is validated numerically using the infinite density matrix renormalization group for a model with ${\mathbb{Z}}_2$ symmetry-protected order.

The strain-dependent phonon properties and thermal conductivities of a soft system [Lennard-Jones (LJ) argon] and a stiff system (silicon modeled using first-principles calculations) are predicted using lattice dynamics calculations and the Boltzmann transport equation. As is commonly assumed for materials under isotropic strain, the thermal conductivity of LJ argon decreases monotonically as the system moves from compression into tension. The reduction in thermal conductivity is attributed to decreases in both the phonon lifetimes and group velocities. The thermal conductivity of silicon, however, is constant in compression and only begins to decrease once the system is put in tension. The silicon lifetimes show an anomalous behavior, whereby they increase as the system moves from compression into tension, which is explained by examining the potential energy surface felt by an atom. The results emphasize the need to separately consider the harmonic and anharmonic effects of strain on material stiffness, phonon properties, and thermal conductivity.

We report a detailed low-temperature study of the two-dimensional (2D) electron gas in a 6.1-nm-wide HgTe quantum well with $\mathrm{H}{\mathrm{g}}_{0.3}\mathrm{C}{\mathrm{d}}_{0.7}\mathrm{Te}$ barriers by terahertz magnetophotoconductivity and magnetotransmission combined with magnetotransport measurements ($R_{xx}$ and $R_{xy}$) in magnetic fields up to 10 T. This well width, close to that at the topological phase transition, corresponds to conventional band ordering, and we probe the “bulk” quasi-2D Landau-level (LL) spectrum of the conduction band at high energies $(\approx 135-160\mathrm{meV})$ above the Dirac point. The calculated separations between adjacent LLs of the same spin based on published parameters for this structure are in fair agreement with the measured cyclotron resonance energies. However, the very large spin splittings observed $(E_{\mathrm{spin}}>E_{\mathrm{cyclotron}})$ require a significantly larger $g$-parameter $g_e$ for electrons. Tilted field coincidence experiments are consistent with the large spin splitting showing coincidences at 3/2 and twice the cyclotron energy. This large value of $g_e$ also leads to interesting crossings of the calculated LLs, and we find direct evidence of these crossings in the $R_{xx}$ measurements at lower electron densities (Fermi energies) produced by negative gate bias.

We consider electronic transport through semiconducting nanowires (W) with spin-orbit interaction (SOI) in a hybrid N-W-N setup where the wire is contacted by normal-metal leads (N). We investigate the conductance behavior of the system as a function of gate and bias voltage, magnetic field, wire length, temperature, and disorder. The transport calculations are performed numerically and are based on standard recursive Green's function techniques. In particular, we are interested in understanding if and how it is possible to deduce the strength of the SOI from the transport behavior. This is a very relevant question since so far no clear experimental observation in that direction has been produced. We find that the smoothness of the electrostatic potential profile between the contacts and the wire plays a crucial role, and we show that in realistic regimes the N-W-N setup may mask the effects of SOI, and a trivial behavior with apparent vanishing SOI is observed. We identify an optimal parameter regime, with neither too smooth nor too abrupt potentials, where the signature of SOI is best visible, with and without Fabry-Perot oscillations, and is most resilient to disorder and temperature effects.

We investigate theoretically a Mach-Zehnder interferometer driven by a time-dependent voltage. Motivated by recent experiments, we focus on a train of Lorentzian voltage pulses which we compare to a sinusoidal and a constant voltage. We discuss the visibilities of Aharonov-Bohm oscillations in the current and in the noise. For the current, we find a strikingly different behavior in the driven as compared to the static case for voltage pulses containing multiple charges. For pulses containing fractional charges, we find a universality at path-length differences equal to multiples of the spacing between the voltage pulses. These observations can be explained by the electronic energy distribution of the driven contact. In the noise oscillations, we find additional features which are characteristic to time-dependent transport. Finite electronic temperatures are found to have a qualitatively different influence on the current and the noise.

We report a quantum phase transition between orbital-selective Mott states, with different localized orbitals, in a Hund's metals model. Using the density matrix renormalization group, the phase diagram is constructed varying the electronic density and Hubbard $U$, at robust Hund's coupling. We demonstrate that this transition is preempted by charge fluctuations and the emergence of free spinless fermions, as opposed to the magnetically driven Mott transition. The Luttinger correlation exponent is shown to have a universal value in the strong-coupling phase, whereas it is interaction dependent at intermediate couplings. At weak coupling we find a second transition from a normal metal to the intermediate-coupling phase.

In realizing practical nontrivial topological electronic phases stable structures need to be determined first. Tin and lead do stabilize an optimal two-dimensional high-buckled phase—a hexagonal close-packed bilayer structure with ninefold atomic coordination—and they do not stabilize topological fullerenes, as demonstrated by energetics, phonon dispersion curves, and structural optimization of finite-size samples. The high-buckled phases are metallic due to their high atomic coordination. The optimal structure of fluorinated tin lacks threefold symmetry and it stabilizes small samples too. It develops two oblate conical valleys in the first Brillouin zone coupling valley, sublattice, and spin degrees of freedom with a novel ${\tau }_z{\sigma }_x{s}_x$ term, thus making it a new two-dimensional platform for valleytronics.

We study the interplay of correlation and thermal fluctuation in a system consisting of two species of classical particles with up and down spin on a geometrically frustrated anisotropic triangular lattice, described by an extended four-state Potts model. The model corresponds to the strong coupling limit of the extended Hubbard model at quarter-filling, which is known to host several competing charge ordered phases as well as an exotic quantum state called pinball liquid. The frustrated intersite Coulomb interactions together with the on-site Coulomb interaction generate macroscopically degenerate manifolds of low-energy states. They compete entropically at finite temperature and two characteristic states emerge; a threefold periodic charge ordered state and a quasi-one-dimensionally disordered state called “good defect state” characterized by the systematic generation of ferroelectric bonds. The two states show good correspondence with the threefold charge order and the pinball liquid in the extended Hubbard model, and are separated by the partial Mott transition taking place on one of the three sublattices of the triangular lattice.

A continuum model for low-energy physisorption on a membrane under tension is proposed and studied with variational mean-field theory. A discontinuous change in the energy-dependent sticking coefficient is predicted under certain conditions. This singularity is a result of the bosonic orthogonality catastrophe of the vibrational states of the membrane. The energy-dependent sticking coefficient is predicted to have exponential scaling in $1/E$ at energies above the singularity. The application of this model to the quantum sticking of cold hydrogen to suspended graphene is discussed. The model predicts that a beam of atomic hydrogen can be completely reflected by suspended graphene at ultralow energies.

Hyperfine interaction (HF) is of key importance for the functionality of solid-state quantum information processing, as it affects qubit coherence and enables nuclear-spin quantum memories. In this work, we complete the theory of the basic HF mechanisms (Fermi contact, dipolar, orbital) in carbon nanotube quantum dots by providing a theoretical description of the orbital HF. We find that orbital HF induces an interaction between the nuclear spins of the nanotube lattice and the valley degree of freedom of the electrons confined in the quantum dot. We show that the resulting nuclear-spin–electron-valley interaction (i) is approximately of Ising type; (ii) is essentially local, in the sense that a radius- and dot-length-independent atomic interaction strength can be defined; and (iii) has an atomic interaction strength that is comparable to the combined strength of the Fermi contact and dipolar interactions. We argue that orbital HF provides a new decoherence mechanism for single-electron valley qubits and spin-valley qubits in a range of multivalley materials. We explicitly evaluate the corresponding inhomogeneous dephasing time $T_2^{*}$ for a nanotube-based valley qubit.

The Oersted field about a nanocontact on a thin film can nucleate vortex formation in the film, and spin torque will drive the vortex in large-amplitude gyrotropic motion about the nanocontact. In a system where the nanocontact is a center of symmetry, the Oersted field provides the restoring force toward the nanocontact center, and spin torque balances the dissipation force from damping to give a linear dependence of the gyrotropic frequency on nanocontact current. For the lower symmetry case when the nanocontact is placed close to the edge of a film there is a significant magnetostatic force repelling the vortex from the edge as well as a deformation of the vortex structure. Both effects combine to produce an anisotropic restoring force, which in turn result in the existence of a lower critical current for gyrotropic motion and a nonlinear dependence of the gyrotropic frequency on the nanocontact current.

The anisotropic magnetoresistance (AMR) in epitaxial $\mathrm{Pt}/\mathrm{Fe}/\mathrm{MgO}\left(001\right)$ bilayers was systematically studied as a function of Pt thickness, and a strong enhancement of AMR by the $\mathrm{Pt}$ layer was observed. The enhancement of AMR persists with a 1 nm Au layer inserted between $\mathrm{Pt}$ and Fe but disappears with a 1 nm $\mathrm{MgO}$ inserted layer. Both the spin Hall magnetoresistance theory and induced $\mathrm{Pt}$ moments located at the $\mathrm{Fe}/\mathrm{Pt}$ interface cannot explain the observed AMR enhancement. The enhanced AMR in the $\mathrm{Pt}/\mathrm{Fe}$ bilayer can be attributed to the $\mathrm{Pt}$ magnetic moment in the conduction band induced by the spin-polarized conduction electron, which has a long decay length of $1.7\text{–}2.6\text{nm}$.

We report ${}^{11}\mathrm{B}$ nuclear magnetic resonance (NMR) and zero-field muon spin relaxation (ZF-$\mu \mathrm{SR})$ measurements of the anisotropic spin tetramer system ${\mathrm{CdCu}}_2\left({\mathrm{BO}}_3\right){}_2$, which consists of strongly interacting Cu(1) dimers and weakly coupled nonfrustrated Cu(2) spins. Long-range magnetic order is observed at $T_N=9.8$ K by a critical divergence of the nuclear spin-lattice relaxation rate $1/T_1$ and an appearance of well-defined muon-spin precessions. In addition, we find the spectroscopic signature for a magnetic anomaly at $T^{*}=6.5$ K, a few kelvins below $T_N$. For temperatures below $T^{*},1/T_1$ ceases to follow a $T^{4.9\left(1\right)}$ behavior. Instead, upon cooling below $T^{*}$, both $1/T_1$ and $1/T_2$ become temperature-independent with a subsequent small drop. The muon frequency and the relative fraction of the Cu magnetic sites show anomalies mostly for Cu(2) spins at $T^{*}$. This is ascribed to a Cu(2) spin reorientation. Site-specific change in magnetic structure is discussed in terms of the energy hierarchy of involved exchange interactions and an additional intertetramer interaction that controls magnetism at temperature $T^{*}$.