When time-reversal symmetry is weakly broken and interactions are neglected, the surface of a ${\mathbb{Z}}_2$ topological insulator supports a half-quantized Hall conductivity ${\sigma }_S=e^2/\left(2h\right)$. A surface Hall conductivity in an insulator is equivalent to a bulk magnetoelectric polarizability, i.e., to a magnetic-field-dependent charge polarization. By performing an explicit calculation for the case in which the surface is approximated by a two-dimensional massive Dirac model and time-reversal symmetry is broken by weak ferromagnetism in the bulk, we demonstrate that there is a nonuniversal interaction correction to ${\sigma }_S$. Our prediction can be tested by measuring the capacitance of magnetized thin films in which the anomalous quantum Hall effect is absent.

We report on the spin-Hall magnetoresistance (SMR) and spin Seebeck effect (SSE) in multiferroic ${\mathrm{CoCr}}_2{\mathrm{O}}_4$ (CCO) spinel thin films with Pt contacts. We observe a large enhancement of both signals below the spin-spiral ($T_s=28\mathrm{K}$) and the spin lock-in ($T_{\mathrm{lock}\text{−}\mathrm{in}}=14\mathrm{K}$) transitions. The SMR and SSE responses in the spin lock-in phase are one order of magnitude larger than those observed at the ferrimagnetic transition temperature ($T_c=94\mathrm{K}$), which indicates that the interaction between spins at the $\mathrm{Pt}|\mathrm{CCO}$ interface is more efficient in the noncollinear magnetic state. At $T>T_c$, magnetic-field-induced SMR and SSE signals are observed, which can be explained by a high interface susceptibility. Our results show that the spin transport at the $\mathrm{Pt}|\mathrm{CCO}$ interface is sensitive to the magnetic phases but cannot be explained solely by the bulk magnetization.

Potassium niobate ${\mathrm{KNbO}}_3$ is a lead-free perovskite and a promising candidate to replace lead-containing ferroelectrics related to ${\mathrm{PbTiO}}_3$. In this study, we use atomistic computer simulation and transmission electron microscopy to investigate dislocations in ${\mathrm{KNbO}}_3$, first to establish the relationship between their atomic-scale properties and the macroscopic mechanical behavior, and second to study their influence on the ferroelectric properties of the material. The easiest dislocation glide system is found to be $\langle 110\rangle \left\{1\overline{1}0\right\}$ at all temperatures, independent from structural phase transformations. The mobility of dislocations and the evolution of the microstructure are measured from room temperature up to 1173 K. A sharp transition in the yield stress is found around 800 K, attributed to the additional activation of the $\langle 100\rangle \left\{010\right\}$ glide system at high temperature. Atomistic simulations quantify the effect of dislocations on the ferroelectric polarization, and TEM observations give indication of the nucleation of domain walls at dislocation cores.

We develop a theory for the vortex-unbinding transition in homogeneously disordered superconducting films. This theory incorporates the effects of quantum, mesoscopic, and thermal fluctuations stemming from length scales ranging from the superconducting coherence length down to the Fermi wavelength. In particular, we extend the renormalization group treatment of the diffusive nonlinear sigma model to the superconducting side of the transition. Furthermore, we explore the mesoscopic fluctuations of parameters in the Ginzburg-Landau functional. Using the developed theory, we determine the dependence of essential observables (including the vortex-unbinding temperature, the superconducting density, as well as the temperature-dependent resistivity and thermal conductivity) on microscopic characteristics such as the disorder-induced scattering rate and bare interaction couplings.

The magnetic structure of the quasi-two-dimensional antiferromagnet ${\mathrm{NiPS}}_3$ has been determined by magnetometry and a variety of neutron diffraction techniques. The experiments show that the samples must be carefully handled, as gluing influences the magnetometry measurements while preferred orientation complicates the interpretation of powder diffraction measurements. Our global set of consistent measurements show numerous departures from previously published results. We show that the compound adopts a k = $\left[010\right]$ antiferromagnetic structure with the moment directions mostly along the $a$ axis, and that the paramagnetic susceptibility is isotropic. The critical behavior was also investigated through the temperature dependence of the magnetic Bragg peaks below the Néel temperature.

We take a two-step theoretical approach to study magnetism of rare-earth quasicrystals by considering Ising spins on quasiperiodic tilings, coupled via Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions. First, we compute RKKY interactions from a tight-binding Hamiltonian defined on the two-dimensional quasiperiodic tilings. We find that the magnetic interactions are frustrated and strongly dependent on the local environment. This results in the formation of clusters with strong bonds at certain patterns of the tilings that repeat quasiperiodically. Second, we examine the statistical mechanics of Ising spins with these RKKY interactions, using extensive Monte Carlo simulations. Although models that have frustrated interactions and lack translational invariance might be expected to display spin-glass behavior, we show that the spin system has a phase transition to low-temperature states with long-range quasiperiodic magnetic order. Additionally, we find that in some of the systems spin clusters can fluctuate much below the ordering temperature.

We formulate an ab initio downfolding scheme for electron-phonon-coupled systems. In this scheme, we calculate partially renormalized phonon frequencies and electron-phonon coupling, which include the screening effects of high-energy electrons, to construct a realistic Hamiltonian consisting of low-energy electron and phonon degrees of freedom. We show that our scheme can be implemented by slightly modifying the density functional-perturbation theory (DFPT), which is one of the standard methods for calculating phonon properties from first principles. Our scheme, which we call the constrained DFPT, can be applied to various phonon-related problems, such as superconductivity, electron and thermal transport, thermoelectricity, piezoelectricity, dielectricity, and multiferroicity. We believe that the constrained DFPT provides a firm basis for the understanding of the role of phonons in strongly correlated materials. Here, we apply the scheme to fullerene superconductors and discuss how the realistic low-energy Hamiltonian is constructed.

The electronic correlation strength is a basic quantity that characterizes the physical properties of materials such as transition metal oxides. Determining correlation strengths requires both precise definitions and a careful comparison between experiment and theory. In this paper, we define the correlation strength via the magnitude of the electron self-energy near the Fermi level. For the case of ${\mathrm{LaNiO}}_3$, we obtain both the experimental and theoretical mass enhancements $m^{☆}/m$ by considering high resolution angle-resolved photoemission spectroscopy (ARPES) measurements and density functional + dynamical mean field theory (DFT + DMFT) calculations. We use valence-band photoemission data to constrain the free parameters in the theory and demonstrate a quantitative agreement between the experiment and theory when both the realistic crystal structure and strong electronic correlations are taken into account. In addition, by considering DFT + DMFT calculations on epitaxially strained ${\mathrm{LaNiO}}_3$, we find a strain-induced evolution of $m^{☆}/m$ in qualitative agreement with trends derived from optics experiments. These results provide a benchmark for the accuracy of the DFT + DMFT theoretical approach, and can serve as a test case when considering other complex materials. By establishing the level of accuracy of the theory, this work also will enable better quantitative predictions when engineering new emergent properties in nickelate heterostructures.

Indirect absorption plays an important role in the transparency of transparent conducting oxides. We provide a detailed first-principles study of phonon- and charged-impurity-assisted free-carrier absorption, with the example of ${\mathrm{SnO}}_2$. The dominant phonon modes contributing to the electron-phonon coupling are identified and used to assess the validity of the Fröhlich model. We find that phonon-assisted indirect absorption dominates if the ionized dopant concentration is below ${10}^{20}{\mathrm{cm}}^{-3}$ and that phonon emission dominates this process. We also discuss subtle but important issues relating to the calculation of optical transition matrix elements.

The unusual magnetic and electronic ground states of $5d$ iridates have been shown to be driven by intrinsically enhanced spin-orbit coupling (SOC). The influence of appreciable but reduced SOC in creating the manifested magnetic insulating states in $4d$ oxides is less clear, with one hurdle being the existence of such compounds. Here, we present experimental and theoretical results on ${\mathrm{Sr}}_4{\mathrm{RhO}}_6$ that reveal SOC dominated behavior. Neutron measurements show the octahedra are both spatially separated and locally ideal, making the electronic ground state susceptible to alterations by SOC. Magnetic ordering is observed with a similar structure to an analogous $J_{\mathrm{eff}}=\frac{1}{2}$ Mott iridate. We consider the underlying role of SOC in this rhodate with density functional theory and x-ray absorption spectroscopy, and find a magnetic insulating ground state with $J_{\mathrm{eff}}=\frac{1}{2}$ character.

We present results of single-crystal neutron-diffraction experiments on the rare-earth, half-Heusler antiferromagnet (AFM) NdBiPt. This compound exhibits an AFM phase transition at $T_{\mathrm{N}}=2.18$ K with an ordered moment of $1.78\left(9\right){\mu }_B$ per Nd atom. The magnetic moments are aligned along the $\left[001\right]$ direction, arranged in a type-I AFM structure with ferromagnetic planes, alternating antiferromagnetically along a propagation vector $\tau$ of $\left(100\right)$. The $R\mathrm{BiPt}$ ($R$ = Ce–Lu) family of materials has been proposed as candidates for a new family of antiferromagnetic topological insulators (AFTIs) with a magnetic space group that corresponds to a type-II AFM structure where ferromagnetic sheets are stacked along the space diagonal. The resolved structure makes it unlikely that NdBiPt qualifies as an AFTI.

Phase-sensitive measurements of the superconducting gap in Fe-based superconductors have proven more difficult than originally anticipated. While quasiparticle interference (QPI) measurements based on scanning tunneling spectroscopy are often proposed as definitive tests of gap structure, the analysis typically relies on details of the model employed. Here we point out that the temperature dependence of momentum-integrated QPI data can be used to identify gap sign changes in a qualitative way, and present an illustration for $s_{\pm }$ and $s_{++}$ states in a system with typical Fe-pnictide Fermi surface.

We show that the $\nu =8$ integer quantum Hall state can support Majorana zero modes at domain walls between its two different stable chiral edge phases without superconductivity. This is due to the existence of an edge phase that does not support gapless fermionic excitations; all gapless excitations are bosonic in this edge phase. Majorana fermion zero modes occur at a domain wall between this edge phase and the more conventional one that does support gapless fermions. Remarkably, due to the chirality of the system, the topological degeneracy of these zero modes has exponential protection, as a function of the relevant length scales, in spite of the presence of gapless excitations, including gapless fermions. These results are compatible with charge conservation, but do not require it. We discuss generalizations to other integer and fractional quantum Hall states, and classify possible mechanisms for appearance of Majorana zero modes at domain walls.

We present a theory of out-of-equilibrium ultrafast spin dynamics in magnetic heterostructures based on the $s\text{−}d$ model of ferromagnetism. Both in the bulk and across interfaces, the exchange processes between the itinerant $s$ and the localized $d$ electrons are described by kinetic rate equations for electron-magnon spin-flop scattering. In our treatment, the magnon distribution function remains nonthermalized on the relevant time scales of the demagnetization process, and the relaxation of the out-of-equilibrium spin accumulation among itinerant electrons provides the principal channel for dissipation of spin angular momentum from the combined electronic system.

Torque magnetization measurements on ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_y$ (YBCO) at doping $y=6.67$ $\left(p=0.12\right)$, in dc fields ($B$) up to 33 T and temperatures down to 4.5 K, show that weak diamagnetism persists above the extrapolated irreversibility field $H_{\mathrm{irr}}(T=0)\approx 24$ T. The differential susceptibility $dM/dB$, however, is more rapidly suppressed for $B\gtrsim 16$ T than expected from the properties of the low field superconducting state, and saturates at a low value for fields $B\gtrsim 24$ T. In addition, torque measurements on a $p=0.11$ YBCO crystal in pulsed field up to 65 T and temperatures down to 8 K show similar behavior, with no additional features at higher fields. We offer two candidate scenarios to explain these observations: (a) superconductivity survives but is heavily suppressed at high field by competition with charge-density-wave (CDW) order; (b) static superconductivity disappears near 24 T and is followed by a region of fluctuating superconductivity, which causes $dM/dB$ to saturate at high field. The diamagnetic signal observed above 50 T for the $p=0.11$ crystal at 40 K and below may be caused by changes in the normal state susceptibility rather than bulk or fluctuating superconductivity. There will be orbital (Landau) diamagnetism from electron pockets and possibly a reduction in spin susceptibility caused by the stronger three-dimensional ordered CDW.

We study the interplay between the dynamic nuclear spin polarization and resonant spin-flip Raman scattering of the resident electron in an ensemble of singly charged (In,Ga)As/GaAs quantum dots by using a two-color laser excitation scheme. The shift of the electron spin-flip Raman line gives a direct measure of the optically induced Overhauser shift, while the linewidth indicates nuclear spin fluctuations. The dynamic nuclear spin polarization leads only to a reduction in the electron spin splitting induced by wetting-layer excitation that is copolarized with the resonant quantum dot excitation. The respective mechanism of the two-color spin-flip Raman scattering is discussed together with the electron-nuclear hyperfine interaction and Pauli exclusion principle. The temporal evolution of the Overhauser shift further demonstrates a nuclear spin depolarization within several seconds depending strongly on the temperature.

Amongst the $X(\text{Sn},\text{Pb})Y_3$ perovskites currently under scrutiny for their photovoltaic applications, the cubic $\mathrm{B}-\alpha$ phase of ${\text{CsSnI}}_3$ is arguably the best characterized experimentally. Yet, according to the standard harmonic theory of phonons, this deceptively simple phase should not exist at all due to rotational instabilities of the ${\mathrm{SnI}}_6$ octahedra. Here, employing self-consistent phonon theory, we show that these soft modes are stabilized at experimental conditions through anharmonic phonon-phonon interactions between the Cs ions and their iodine cages. We further calculate the renormalization of the electronic energies due to vibrations and find an unusual opening of the band gap, estimated as 0.24 and 0.11 eV at 500 and 300 K, which we attribute to the stretching of Sn-I bonds. Our work demonstrates the important role of temperature in accurately describing these materials.

Hubbard ladders are an important stepping stone to the physics of the two-dimensional Hubbard model. While many of their properties are accessible to numerical and analytical techniques, the question of whether weakly hole-doped Hubbard ladders are dominated by superconducting or charge-density-wave correlations has so far eluded a definitive answer. In particular, previous numerical simulations of Hubbard ladders have seen a much faster decay of superconducting correlations than expected based on analytical arguments. We revisit this question using a state-of-the-art implementation of the density matrix renormalization group algorithm that allows us to simulate larger system sizes with higher accuracy than before. Performing careful extrapolations of the results, we obtain improved estimates for the Luttinger liquid parameter and the correlation functions at long distances. Our results confirm that, as suggested by analytical considerations, superconducting correlations become dominant in the limit of very small doping.

We discuss how to reveal the massive Goldstone mode, often referred to as the Higgs amplitude mode, near the superfluid-to-insulator quantum critical point (QCP) in a system of two-dimensional ultracold bosonic atoms in optical lattices. The spectral function of the amplitude response is obtained by analytic continuation of the kinetic energy correlation function calculated by Monte Carlo methods. Our results enable a direct comparison with the recent experiment [M. Endres, T. Fukuhara, D. Pekker, M. Cheneau, P. Schauß, C. Gross, E. Demler, S. Kuhr, and I. Bloch, Nature (London) 487, 454 (2012)] and demonstrate a good agreement for temperature shifts induced by lattice modulation. Based on our numerical analysis, we formulate the necessary conditions in terms of homogeneity, detuning from the QCP and temperature in order to reveal the massive Goldstone resonance peak in spectral functions experimentally. We also propose to apply a local modulation at the trap center to overcome the inhomogeneous broadening caused by the parabolic trap confinement.

Dynamical nuclear polarization holds the key for orders of magnitude enhancements of nuclear magnetic resonance signals which, in turn, would enable a wide range of novel applications in biomedical sciences. However, current implementations of DNP require cryogenic temperatures and long times for achieving high polarization. Here we propose and analyze in detail protocols that can achieve rapid hyperpolarization of ${}^{13}\mathrm{C}$ nuclear spins in randomly oriented ensembles of nanodiamonds at room temperature. Our protocols exploit a combination of optical polarization of electron spins in nitrogen-vacancy centers and the transfer of this polarization to ${}^{13}\mathrm{C}$ nuclei by means of microwave control to overcome the severe challenges that are posed by the random orientation of the nanodiamonds and their nitrogen-vacancy centers. Specifically, these random orientations result in exceedingly large energy variations of the electron spin levels that render the polarization and coherent control of the nitrogen-vacancy center electron spins as well as the control of their coherent interaction with the surrounding ${}^{13}\mathrm{C}$ nuclear spins highly inefficient. We address these challenges by a combination of an off-resonant microwave double resonance scheme in conjunction with a realization of the integrated solid effect which, together with adiabatic rotations of external magnetic fields or rotations of nanodiamonds, leads to a protocol that achieves high levels of hyperpolarization of the entire nuclear-spin bath in a randomly oriented ensemble of nanodiamonds even at room temperature. This hyperpolarization together with the long nuclear-spin polarization lifetimes in nanodiamonds and the relatively high density of ${}^{13}\mathrm{C}$ nuclei has the potential to result in a major signal enhancement in ${}^{13}\mathrm{C}$ nuclear magnetic resonance imaging and suggests functionalized and hyperpolarized nanodiamonds as a unique probe for molecular imaging both in vitro and in vivo.