Understanding heat generation and transport processes in a magnetic tunnel junction (MTJ) is a significant step towards improving its application in current memory devices. Recent work has experimentally demonstrated the magneto-Seebeck effect in MTJs, where the Seebeck coefficient of the junction varies as the magnetic configuration changes from a parallel (P) to an antiparallel (AP) configuration. Here we report a study on its reciprocal effect, the magneto-Peltier effect, where the heat flow carried by the tunneling electrons is altered by changing the magnetic configuration of the MTJ. The magneto-Peltier signal that reflects the change in the temperature difference across the junction between the P and AP configurations scales linearly with the applied current in the small bias but is greatly enhanced in the large-bias regime, due to higher-order Joule heating mechanisms. By carefully extracting the linear response which reflects the magneto-Peltier effect, and comparing it with the magneto-Seebeck measurements performed on the same device, we observe results consistent with Onsager reciprocity. We estimate a magneto-Peltier coefficient of 13.4 mV in the linear regime using a three-dimensional thermoelectric model. Our result opens up the possibility of programmable thermoelectric devices based on the Peltier effect in MTJs.

We use angle-resolved photoemission spectroscopy to study the doping evolution of infinite-layer ${\mathrm{Sr}}_{1-x}{\mathrm{La}}_x{\mathrm{CuO}}_2$ thin films grown by molecular-beam epitaxy. At low doping, the material exhibits a dispersive lower Hubbard band typical of the superconducting cuprate parent compounds. As carriers are added to the system, a continuous evolution from charge-transfer insulator to superconductor is observed, with the initial lower Hubbard band pinned well below the Fermi level and the development of a coherent low-energy band with electron doping. This two-component spectral function emphasizes the important role that strong local correlations play even at relatively high doping levels. Electron diffraction probes reveal a $p(2\times 2)$ surface reconstruction of the material at low doping levels. Using a number of simple assumptions, we develop a model of this reconstruction based on the polar nature of the infinite-layer structure. Finally, we provide evidence for a thickness-controlled transition in ultrathin films of ${\mathrm{SrCuO}}_2$ grown on nonpolar ${\mathrm{SrTiO}}_3$, highlighting the diverse structural changes that can occur in polar complex oxide thin films.

The formation of a two-dimensional electron gas at oxide interfaces as a consequence of polar discontinuities has generated an enormous amount of activity due to the variety of interesting effects it gives rise to. Here, we study under what circumstances similar processes can also take place underneath ferroelectric thin films. We use a simple Landau model to demonstrate that in the absence of extrinsic screening mechanisms, a monodomain phase can be stabilized in ferroelectric films by means of an electronic reconstruction. Unlike in the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ heterostructure, the emergence with thickness of the free charge at the interface is discontinuous. This prediction is confirmed by performing first-principles simulations of free-standing slabs of ${\mathrm{PbTiO}}_3$. The model is also used to predict the response of the system to an applied electric field, demonstrating that the two-dimensional electron gas can be switched on and off discontinuously and in a nonvolatile fashion. Furthermore, the reversal of the polarization can be used to switch between a two-dimensional electron gas and a two-dimensional hole gas, which should, in principle, have very different transport properties. We discuss the possible formation of polarization domains and how such configuration competes with the spontaneous accumulation of free charge at the interfaces.

We report the discovery of ${\text{CaMn}}_2{\text{Al}}_{10}$, a metal with strong magnetic anisotropy and moderate electronic correlations. Magnetization measurements find a Curie-Weiss moment of $0.83{\mu }_{\mathrm{B}}/\mathrm{Mn}$, significantly reduced from the Hund's rule value, and the magnetic entropy obtained from specific heat measurements is correspondingly small, only $\approx 9\%$ of $R\ln 2$. These results imply that the Mn magnetism is highly itinerant, a conclusion supported by density functional theory calculations that find strong Mn-Al hybridization. Consistent with the layered nature of the crystal structure, the magnetic susceptibility $\chi$ is anisotropic below 20 K, with a maximum ratio of ${\chi }_{\left[010\right]}/{\chi }_{\left[001\right]}\approx 3.5$. A strong power-law divergence $\chi \left(T\right)\sim T^{-1.2}$ below 20 K implies incipient ferromagnetic order with a low Curie temperature $T_C<2\mathrm{K}$. Our experiments indicate that ${\text{CaMn}}_2{\text{Al}}_{10}$ is a rare example of a system where the weak and itinerant Mn-based magnetism is poised on the verge of order.

We explore the ferromagnetic quantum critical point in a three-dimensional semimetallic system with upward- and downward-dispersing bands touching at the Fermi level. Evaluating the static spin susceptibility to leading order in the coupling between the fermions and the fluctuating ferromagnetic order parameter, we find that the ferromagnetic quantum critical point is masked by an incommensurate, longitudinal spin density wave phase. We first analyze an idealized model that, despite having strong spin-orbit coupling, still possesses $O\left(3\right)$ rotational symmetry generated by the total angular momentum operator. In this case, the direction of the incommensurate spin density wave propagation can point anywhere, while the magnetic moment is aligned along the direction of propagation. Including symmetry-allowed anisotropies in the fermion dispersion and the coupling to the order parameter field, however, we find that the ordering wavevector instead breaks a discrete symmetry and aligns along either the [111] or [100] direction, depending on the signs and magnitudes of these two types of anisotropy.

Control over the helicity degree of freedom of Dirac fermions is identified in thin films of topological materials which act as a tunable “chiral-metamaterial-like” platform to tame left- and right-handed Dirac fermions in two dimensions. Using topological crystalline insulator SnTe(111) thin films as an example, we perform the first-principles calculations and show that giant helicity splitting in the band structures can be induced under moderate electric field. Based on this result, helicity-resolved functionalities, including pronounced electron dichroism, helicity switching, helical negative refraction, and birefraction, are demonstrated, where the intrahelical scattering always dominates over the interhelical one. Such intriguing control strategy for helical Dirac fermions may hold great promise for the applications of helicity-based electron optics and nanoelectronics.

Using electronic structure calculations, we conduct an extensive investigation into the Hf-Ta-C system, which includes the compounds that have the highest melting points known to date. We identify three major chemical factors that contribute to the high melting temperatures. Based on these factors, we propose a class of materials that may possess even higher melting temperatures and explore it via efficient ab initio molecular dynamics calculations in order to identify the composition maximizing the melting point. This study demonstrates the feasibility of automated and high-throughput materials screening and discovery via ab initio calculations for the optimization of “higher-level” properties, such as melting points, whose determination requires extensive sampling of atomic configuration space.

Direction-dependent anisotropic exchange is a common feature of magnetic systems with strong spin-orbit coupling. Here we study the effect of such exchange upon macroscopic magnetic anisotropy for a face-centered-cubic model. By several theoretical techniques, we show that, both in the paramagnetic and ordered phases, the magnetic anisotropy is induced by fluctuations. Moreover, the magnetic anisotropy differs in the paramagnetic and ordered phases: in the paramagnetic phase the susceptibility is maximum along the $\langle 111\rangle$ directions, while the magnetic moments orient along $\langle 110\rangle$ or $\langle 100\rangle$ in the ordered phase. We suggest that such “anisotropy switching” can be a common feature of strongly spin-orbit-coupled magnets.

Single crystals of $A_2{\mathrm{Cr}}_3{\mathrm{As}}_3(A=\text{K}, \mathrm{Rb})$ were successfully grown using a self-flux method and studied via structural, transport, and thermodynamic measurement techniques. The superconducting state properties between the two species are similar, with critical temperatures of 6.1 and 4.8 K in ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ and ${\mathrm{Rb}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$, respectively. However, the emergence of a strong normal state electronic anisotropy in ${\mathrm{Rb}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ suggests a unique electronic tuning parameter is coupled to the interchain spacing in the $A_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ structure, which increases with alkali-metal ionic size while the one-dimensional ${\left[{\left({\mathrm{Cr}}_3{\mathrm{As}}_3\right)}^{2-}\right]}_{\infty }$ chain structure itself remains essentially unchanged. Together with dramatic enhancements in both conductivity and magnetoresistance (MR), the appearance of a strong anisotropy in the MR of ${\mathrm{Rb}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ is consistent with the proposed quasi-one-dimensional character of band structure and its evolution with alkali-metal species in this new family of superconductors.

Quantum theory of spin relaxation in the elastic environment is revised with account of the concept of a phonon spin recently introduced by Zhang and Niu [L. Zhang and Q. Niu, Phys. Rev. Lett. 112, 085503 (2014)]. Similar to the case of the electromagnetic field, the division of the angular momentum associated with elastic deformations into the orbital part and the part due to phonon spins proves to be useful for the analysis of the balance of the angular momentum. Such analysis sheds important light on microscopic processes leading to the Einstein–de Haas effect.

We report measurements of in-plane electrical and thermal transport properties in the limit $T\to 0$ near the unconventional quantum critical point in the heavy-fermion metal $\beta -{\mathrm{YbAlB}}_4$. The high Kondo temperature $T_K\simeq 200$ K in this material allows us to probe transport extremely close to the critical point, at unusually small values of $T/T_K<5\times {10}^{-4}$. Here we find that the Wiedemann-Franz law is obeyed at the lowest temperatures, implying that the Landau quasiparticles remain intact in the critical region. At finite temperatures we observe a non-Fermi-liquid $T$-linear dependence of inelastic-scattering processes to energies lower than those previously accessed. These processes have a weaker temperature dependence than in comparable heavy fermion quantum critical systems, revealing a temperature scale of $T\sim 0.3\text{K}$ which signals a sudden change in the character of the inelastic scattering.

We report extremely large magnetoresistance (MR) in an extended temperature regime from 1.5 K to 300 K in nonmagnetic binary compounds TaP and NbP. TaP exhibits linear MR around $1.8\times {10}^4$ at 2 K in a magnetic field of 9 T, which further follows its linearity up to $1.4\times {10}^5$ in a magnetic field of 56 T at 1.5 K. At room temperature the MR for TaP and NbP follows a power law of the exponent about $1.5$ with the values larger than $300\%$ in a magnetic field of 9 T. Such large MR in a wide temperature regime is likely not only due to a resonance of the electron-hole balance, but also indicates a complicated mechanism underneath.

We realize resonant driving of the magnetization precession by monochromatic phonons in a thin ferromagnetic layer embedded into a phononic Fabry-Pérot resonator. A femtosecond laser pulse excites resonant phonon modes of the structure in the $10-40\mathrm{GHz}$ frequency range. By applying an external magnetic field, we tune the precession frequency relative to the frequency of the phonons localized in the cavity and observe an enormous increase in the amplitude of the magnetization precession when the frequencies of free magnetization precession and phonons localized in the cavity are equal.

The critical behavior of quantum Hall transitions in two-dimensional disordered electronic systems can be described by a class of complicated, nonunitary conformal field theories with logarithmic correlations. The nature and the physical origin of these logarithmic correlation functions remain, however, mysterious. Using the replica trick and the underlying symmetries of these quantum critical points, we show here how to construct nonperturbatively disorder-averaged observables in terms of Green's functions that scale logarithmically at criticality. In the case of the spin quantum Hall transition, which may occur in disordered superconductors with spin-rotation symmetry and broken time reversal invariance, we argue that our results are compatible with an alternative approach based on supersymmetry. The generalization to the integer quantum Hall plateau transition is also discussed.

We demonstrate the formation of large sheets of layered organic-inorganic perovskite (OIPC) crystals, as thin as a single unit cell, prepared by mechanical exfoliation. The resulting two-dimensional OIPC nanosheets of 2.4 nm thickness are direct semiconductors with an optical band gap of 2.4 eV. They exhibit unusually strong light-matter interaction with an optical absorption as high as 25% at the main excitonic resonance, as well as bright photoluminescence. We extract an exciton binding energy of 490 meV from measurement of the series of excited exciton states. The properties of the excitons are shown to be strongly influenced by the changes in the dielectric surroundings. The environmental sensitivity of these ultrathin OIPC sheets is further reflected in the strong suppression of a thermally driven phase transition present in the bulk crystals.

The influence of the disorder-induced Anderson localization and electron-electron interaction on the superconductivity in two-dimensional systems is explored. We determine the superconducting transition temperature $T_c$, the temperature dependence of the resistivity, the phase diagram, and the magnetoresistance. The analysis is based on the renormalization group (RG) for a nonlinear sigma model. The derived RG equations are valid to the lowest order in disorder but for an arbitrary electron-electron interaction strength in a particle-hole and the Cooper channels. Systems with preserved and broken spin-rotational symmetry are considered, with both short-range and long-range (Coulomb) interactions. In the case of short-range interaction, we identify parameter regions where the superconductivity is enhanced by localization effects. Our RG analysis indicates that the superconductor-insulator transition is controlled by a fixed point with a resistivity $R_c$ of the order of the quantum resistance $R_q=h/4e^2$. When a transverse magnetic field is applied, we find a strong nonmonotonous magnetoresistance for temperatures below $T_c$.

We have explored the hidden symmetries of a generic four-parameter nearest-neighbor spin model, allowed in honeycomb-lattice compounds under trigonal compression. Our method utilizes a systematic algorithm to identify all dual transformations of the model that map the Hamiltonian on itself, changing the parameters and providing exact links between different points in its parameter space. We have found the complete set of points of hidden SU(2) symmetry at which a seemingly highly anisotropic model can be mapped back on the Heisenberg model and inherits therefore its properties such as the presence of gapless Goldstone modes. The procedure used to search for the hidden symmetries is quite general and may be extended to other bond-anisotropic spin models and other lattices, such as the triangular, kagome, hyperhoneycomb, or harmonic-honeycomb lattices. We apply our findings to the honeycomb-lattice iridates ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ and ${\mathrm{Li}}_2{\mathrm{IrO}}_3$, and illustrate how they help to identify plausible values of the model parameters that are compatible with the available experimental data.

We present a first-principles approach to describe magnetic and superconducting systems and the phenomena of competition between these electronic effects. We develop a density functional theory SpinSCDFT by extending the Hohenberg-Kohn theorem and constructing the noninteracting Kohn-Sham system. An exchange-correlation functional for SpinSCDFT is derived from the Sham-Schlüter connection between the SpinSCDFT Kohn-Sham and a self-energy in Eliashberg approximation. The reference Eliashberg equations for superconductors in the presence of magnetism are also derived and discussed.

We numerically investigate the spin density functional theory for superconductors (SpinSCDFT) and the approximated exchange-correlation functional, derived and presented in the preceding Paper I [A. Linscheid et al., Phys. Rev. B 92, 024505 (2015)]. As a test system, we employ a free-electron gas featuring an exchange splitting, a phononic pairing field, and a Coulomb repulsion. SpinSCDFT results are compared with Sarma, the Bardeen-Cooper-Schrieffer theory, and with an Eliashberg type of approach. We find that the spectrum of the superconducting Kohn-Sham SpinSCDFT system is not in agreement with the true quasiparticle structure. Therefore, starting from the Dyson equation, we derive a scheme that allows to compute the many-body excitations of the superconductor and represents the extension to superconductivity of the $G_0{W}_0$ method in band-structure theory. This superconducting $G_0{W}_0$ method vastly improves the predicted spectra.

We construct two exactly soluble lattice spin models that demonstrate the importance of three-loop braiding statistics for the classification of three-dimensional gapped quantum phases. The two models are superficially similar: both are gapped and both support particlelike and looplike excitations similar to those of charges and vortex lines in a ${\mathbb{Z}}_2\times {\mathbb{Z}}_2$ gauge theory. Furthermore, in both models the particle excitations are bosons, and in both models the particle and loop excitations have the same mutual braiding statistics. The difference between the two models is only apparent when one considers the recently proposed three-loop braiding process in which one loop is braided around another while both are linked to a third loop. We find that the statistical phase associated with this process is different in the two models, thus proving that they belong to two distinct phases. An important feature of this work is that we derive our results using a concrete approach: we construct string and membrane operators that create and move the particle and loop excitations and then we extract the braiding statistics from the commutation algebra of these operators.