Using electron energy loss spectroscopy, we have probed the spin waves of fcc(100) cobalt after capping with 1–3 atom layers of pseudomorphic nickel or up to 12 atom layers of copper. The intensity decay of the spin wave signal is quantitatively described by the mean-free path of the incident and the scattered electron within the capping layer. The observed spin waves are therefore localized at the cobalt side of the Co/Ni and Co/Cu interfaces. Compared to the free cobalt surface, the interface spin waves are downshifted in frequency. The effect is attributed to a reduced exchange interaction between cobalt atoms at the interface.

We show that the probability of electric-field-induced interband tunneling in solid-state systems is generically a nonmonotonic (oscillatory) function of the applied field. This unexpected behavior can be understood as arising due to a common path interference between two distinct tunneling solutions. The phenomenon is insensitive to magnetic field, and arises whenever the low-energy dispersion relation contains higher order terms in addition to the usual $p^2$ term. Such higher order terms are generically present, albeit with small coefficient, so that the oscillatory Zener tunneling is a universal phenomenon. However, the first “Zener oscillation” occurs at a transmission probability which is exponentially small when the coefficient of the higher order terms is small. This explains why this oscillatory aspect of Zener tunneling has been hitherto overlooked, despite its universality. The common path interference is also destroyed by the presence of odd powers of $p$ in the low-energy dispersion relation. Since odd powers of $p$ are strictly absent only when the tunneling barrier lies along an axis of mirror symmetry, it follows that the robustness of the oscillatory behavior depends on the orientation of the tunneling barrier. Bilayer graphene is identified as a particularly good material for observation of common path interference, due to its unusual nearly isotropic dispersion relation, where the $p^4$ term makes the leading contribution.

We consider the reduced density matrix (RDM) ${\rho }_A\left(t\right)$ for a finite subsystem $A$ after a global quantum quench in the infinite transverse-field Ising chain. It has been recently shown that the infinite time limit of ${\rho }_A\left(t\right)$ is described by the RDM ${\rho }_{\mathrm{GGE},A}$ of a generalized Gibbs ensemble. Here, we present some details on how to construct this ensemble in terms of local integrals of motion, and show its equivalence to the expression in terms of mode occupation numbers widely used in the literature. We then address the question of how ${\rho }_A\left(t\right)$ approaches ${\rho }_{\mathrm{GGE},A}$ as a function of time. To that end, we introduce a distance on the space of density matrices and show that it approaches zero as a universal power law $t^{-3/2}$ in time. As the RDM completely determines all local observables within $A$, this provides information on the relaxation of correlation functions of local operators. We then address the issue of how well a truncated generalized Gibbs ensemble with a finite number of local higher conservation laws describes a given subsystem at late times. We find that taking into account only local conservation laws with a range at most comparable to the subsystem size provides a good description. However, excluding even a single one of the most local conservation laws in general completely spoils this agreement.

Recent terahertz conductivity measurements observed low-power-law frequency dependence of optical conduction within the Mott gap of the kagome lattice spin-liquid candidate Herbertsmithite. We investigate mechanisms for this observed sub-gap conductivity for two possible scenarios in which the ground state is described by (1) a U(1) Dirac spin liquid with emergent fermionic spinons or (2) a nearly critical $Z_2$ spin liquid in the vicinity of a continuous quantum phase transition to magnetic order. We identify new mechanisms for optical absorption via magnetoelastic effects and spin-orbit coupling. In addition, for the Dirac spin liquid scenario, we establish an explicit microscopic origin for previously proposed absorption mechanisms based on slave-particle effective field theory descriptions.

The emergence of a ferromagnetic component in ${\text{LaMnO}}_3$ with low Cr-for-Mn substitution has been studied by x-ray absorption spectroscopy and x-ray magnetic circular dichroism at the Mn and Cr $K$ edges. The local magnetic moment strengths for Mn and Cr are proportional to each other and follow the macroscopic magnetization. The net ferromagnetic components of ${\text{Cr}}^{3+}$ and ${\text{Mn}}^{3+}$ are found antiferromagnetically coupled. Unlike hole doping by La site substitution, the inclusion of ${\text{Cr}}^{3+}$ ions up to $x=0.15$ does not decrease the Jahn-Teller (JT) distortion and consequently does not significantly affect the orbital ordering. This demonstrates that the emergence of the ferromagnetism is not related to JT weakening and likely arises from a complex orbital mixing.

We analyze the spin dynamics of an individual magnetic atom (Mn) inserted in a II-VI semiconductor quantum dot under resonant optical excitation. In addition to standard optical pumping expected for a resonant excitation, we show that for particular conditions of laser detuning and excitation intensity, the Mn spin population can be trapped in the state which is resonantly excited. This effect is modeled considering the coherent spin dynamics of the coupled electronic and nuclear spin of the Mn atom optically dressed by a resonant laser field. This “spin population trapping” mechanism is controlled by the combined effect of the coupling with the laser field and the coherent interaction between the different Mn spin states induced by an anisotropy of the strain in the plane of the quantum dot.

We demonstrate rapid, first-order sideband transitions between a superconducting resonator and a frequency-modulated transmon qubit. The qubit contains a substantial asymmetry between its Josephson junctions leading to a linear portion of the energy band near the resonator frequency. The sideband transitions are driven with a magnetic flux signal of a few hundred MHz coupled to the qubit. This modulates the qubit splitting at a frequency near the detuning between the dressed qubit and resonator frequencies, leading to rates up to 85 MHz for exchanging quanta between the qubit and resonator.

Using first-principles calculation, we propose an interface structure for single triple-layer FeSe on the SrTiO${}_3$(001) surface, a high-$T_{\mathrm{c}}$ superconductor found recently. The key component of this structure is the oxygen deficiency on the top layer of the SrTiO${}_3$ substrate, as a result of Se etching used in preparing the high-$T_{\mathrm{c}}$ samples. The O vacancies strongly bind the FeSe triple layer to the substrate giving rise to a (2×1) reconstruction, as observed by scanning tunneling microscopy. The enhanced binding correlates to the significant increase of $T_{\mathrm{c}}$ observed in experiment. The O vacancies also serve as the source of electron doping, which modifies the Fermi surface of the first FeSe layer by filling the hole pocket near the center of the surface Brillouin zone, as suggested from angle-resolved photoemission spectroscopy measurement.

We present transport measurements in superconductor-nanowire devices with a gated constriction forming a quantum point contact. Zero-bias features in tunneling spectroscopy appear at finite magnetic fields and oscillate in amplitude and split away from zero bias as a function of magnetic field and gate voltage. A crossover in magnetoconductance is observed: Magnetic fields above $\sim$0.5 T enhance conductance in the low-conductance (tunneling) regime but suppress conductance in the high-conductance (multichannel) regime. We consider these results in the context of Majorana zero modes as well as alternatives, including the Kondo effect and analogs of 0.7 structure in a disordered nanowire.

The generation, relaxation, and diffusion dynamics of optically induced lattice distortion in the relaxed excited state of SrTiO${}_3$ are studied by using polarization spectroscopy with the pump-probe technique. The relaxed excited state is generated with a rise time on the order of 100 ps. Three kinds of thermal activation processes of the localized lattice distortion are found, and these processes are considered to be caused by photogenerated carriers in trapped states, which play important roles in photoluminescence or photoconductivity. We observed the lattice distortion induced by a separated pump beam from the probe beam to investigate its itineracy. The lattice-distortion signal appears later as the separation along the [100] axis becomes larger. The temperature dependence of the mobility suggests that the observed diffusive lattice distortion is caused by photogenerated electrons accompanied by lattice distortion, or electron polarons. Thus, the spatial and temporal dynamics of polaron diffusion were observed directly in our experiment.

We experimentally investigate and quantitatively analyze the spin Hall magnetoresistance effect in ferromagnetic insulator/platinum and ferromagnetic insulator/nonferromagnetic metal/platinum hybrid structures. For the ferromagnetic insulator, we use either yttrium iron garnet, nickel ferrite, or magnetite and for the nonferromagnet, copper or gold. The spin Hall magnetoresistance effect is theoretically ascribed to the combined action of spin Hall and inverse spin Hall effect in the platinum metal top layer. It therefore should characteristically depend upon the orientation of the magnetization in the adjacent ferromagnet and prevail even if an additional, nonferromagnetic metal layer is inserted between Pt and the ferromagnet. Our experimental data corroborate these theoretical conjectures. Using the spin Hall magnetoresistance theory to analyze our data, we extract the spin Hall angle and the spin diffusion length in platinum. For a spin-mixing conductance of $4\times {10}^{14}{\Omega }^{-1}{\mathrm{m}}^{-2}$, we obtain a spin Hall angle of $0.11\pm 0.08$ and a spin diffusion length of $(1.5\pm 0.5)\mathrm{nm}$ for Pt in our thin-film samples.

The intermetallic compound Y${}_3$Pt${}_4$Ge${}_{13}$ has been synthesized by high-pressure high-temperature technique. It crystalizes with its own noncentrosymmetric type of crystal structure in the monoclinic space group $Cc$ and is superconducting below $T_c=4.5$ K. Thermodynamic properties as well as transport measurements were performed down to 0.4 K. The upper critical field is ${\mu }_0{H}_{c2}=3.8$ T, the lower one only ${\mu }_0{H}_{c1}=5$ mT. All observations are consistent with an $s$-wave symmetry single-energy gap for the superconducting phase.

Double resonant sum-frequency generation (SFG) is demonstrated in Cu${}_2$O. The $1S$ yellow orthoexciton-polariton is resonantly excited by a single frequency dye laser (quadrupole transition) and coupled to the blue $1S$ exciton-polariton by a single frequency infrared laser (dipole transition). Sum-frequency (SF) spectra as a function of the energy of the infrared laser are measured for the two pump laser beams parallel and antiparallel. In the antiparallel configuration, pronounced oscillations are observed with a period scaling inversely with the crystal length. The origin of these oscillations is discussed in detail and attributed to a phase-matching effect. The appearance of phase matching can be understood if the blue polaritons—generated by SFG—move coherently through the crystal. This observation is very surprising, since blue exciton-polaritons excited in a one-photon transmission experiment exhibit an absorption length of only $100\text{nm}$ due to strong scattering.

We identify an angle-dependent momentum scale as the fundamental property of a bilayer composed of mutually rotated graphene layers. The interlayer scattering processes at these characteristic momentum values define an effective Brillouin zone (a Jones zone) which, in general, differs from the Brillouin zone generated by the real-space lattice, which is physically irrelevant. From this we develop a numerical method that increases, for the twist bilayer, the efficiency of the standard tight-binding method by a factor of $\approx {10}^3$ at no loss of accuracy. The efficiency of the method is based on (i) the fact that the twist Hamiltonian is exceptionally sparse in a basis of single-layer graphene (SLG) states, (ii) a solution of trivial Diophantine problem (Bézout's identity) allows one to know in advance which matrix elements take nonzero values, and (iii) to access the electronic structure in a few electron volts about the Dirac point a truncated SLG basis consisting only of states in a somewhat larger energy window are required, leading to a much reduced size of the Hamiltonian. This allows a complete survey of the system which reveals (i) an angle-dependent series of van Hove singularities, (ii) an increasing mixing of SLG states as the twist angle is reduced leading to the appearance of localization of the twist bilayer wave functions at all energies in the small-angle limit, and (iii) a zero-energy peak in the density of states in an approximately self-similar small-angle regime.

Charge carrier transport in single-layer graphene with one-dimensional charged defects is studied theoretically. Extended charged defects, considered an important factor for mobility degradation in chemically vapor-deposited graphene, are described by a self-consistent Thomas-Fermi potential. A numerical study of electronic transport is performed by means of a time-dependent real-space Kubo approach in honeycomb lattices containing millions of carbon atoms, capturing the linear response of realistic size systems in the highly disordered regime. Our numerical calculations are complemented with a kinetic transport theory describing charge transport in the weak scattering limit. The semiclassical transport lifetimes are obtained by computing scattered amplitudes within the second Born approximation. The transport electron-hole asymmetry found in the semiclassical approach is consistent with the Kubo calculations. In the strong scattering regime, the conductivity is found to be a sublinear function of electronic density and weakly dependent on the Thomas-Fermi screening wavelength. We attribute this atypical behavior to the extended nature of one-dimensional charged defects. Our results are consistent with recent experimental reports.

In Fe-based superconductors, electron doping is often realized by the substitution of transition-metal atoms for Fe. In order to investigate how the electronic structure of the parent compound is influenced by Zn substitution, which supplies nominally four extra electrons per substituted atom but is expected to induce the strongest impurity potential among the transition-metal atoms, we have performed an angle-resolved photoemission spectroscopy measurement on Ba(Fe${}_{1-x}$Zn${}_x$)${}_2$As${}_2$ (Zn-122). In Zn-122, the temperature dependence of the resistivity shows a kink around $T\sim$ 135 K, indicating antiferromagnetic order below the N$\acute{\mathrm{e}}$el temperature of $T_{\mathrm{N}}\sim$ 135 K. In fact, folded Fermi surfaces (FSs) similar to those of the parent compound have been observed below $T_{\mathrm{N}}$. The hole and electron FS volumes are, therefore, different from those expected from the rigid-band model. The results can be understood if all the extra electrons occupy the Zn 3$d$ state $\sim$10 eV below the Fermi level and do not participate in the formation of the FSs.

Spin-flip Raman scattering of electrons and heavy holes is studied for resonant excitation of neutral and charged excitons in a CdTe/Cd${}_{0.63}$Mg${}_{0.37}$Te quantum well. The spin-flip scattering is characterized by its dependence on the incident and scattered light polarization as well as on the magnetic field strength and orientation. Model schemes of electric-dipole-allowed spin-flip Raman processes in the exciton complexes are compared to the experimental observations, from which we find that lowering the exciton symmetry, time of carrier spin relaxation, and mixing between electron states and, respectively, light- and heavy-hole states play an essential role in the scattering. At the exciton resonance, anisotropic exchange interaction induces heavy-hole spin-flip scattering, while acoustic phonon interaction is mainly responsible for the electron spin-flip. In resonance with the positively and negatively charged excitons, anisotropic electron-hole exchange as well as mixed electron states allow spin-flip scattering. Variations in the resonant excitation energy and lattice temperature demonstrate that localization of resident electrons and holes controls the Raman process probability and is also responsible for symmetry reduction. We show that the intensity of the electron spin-flip scattering is strongly affected by the lifetime of the exciton complex, and in tilted magnetic fields it is affected by the angular dependence of the anisotropic electron-hole exchange interaction.

Selection rules and interference effects in angle-resolved photoemission spectra (ARPES) from twisted graphene bilayers are studied within a long-wavelength theory for the electronic structure. Using a generic model for the interlayer coupling, we identify features in the calculated ARPES momentum distributions that are controlled by the singularities and topological character of its long-wavelength spectrum. We distinguish spectral features that are controlled by single-layer singularities in the spectrum, their modification by gauge potentials in each layer generated by the interlayer coupling, and energy-dependent interference effects that directly probe the interlayer coherence. The results demonstrate how the energy- and polarization-dependence of ARPES spectra can be used to characterize the interlayer coupling in twisted bilayer graphenes.

The lifting of the fourfold degeneracy of the zeroth Landau level in graphene under high magnetic fields has been the subject of numerous experimental studies, and attributed to various mechanisms such as pure spin splitting, spin splitting combined with subsequent valley splitting, or the formation of a quantum Hall insulator. Unexplored, however, is the influence of an energy gap on the quantum Hall effect (QHE) states in graphene. Here we demonstrate, using measurements of the magnetoresistance of graphene antidot lattices (GALs) in magnetic fields up to 30 T and temperatures between 2 and 100 K, that gap opening in these samples is accompanied by valley polarization and a change from linear to parabolic band structure at low carrier energies. The emergence of a massive character of the carriers profoundly alters the transport characteristics of the zeroth Landau level, which manifests itself in a linear increase of the activated gap with magnetic field. Furthermore, samples of the highest quality display spin splitting on top of the valley splitting, albeit of significantly smaller magnitude.

We report crystal growth at high pressure, structure determination, and magnetic and transport studies of an oxypnictide superconductor Pr${}_4$Fe${}_2$As${}_2$Te${}_{1-x}$O${}_4$. Its structure resembles the known 1111 phase except for the considerably larger $c$ lattice constant and intercalated tellurium atoms and it crystallizes in a tetragonal lattice [$a$ $=$ 4.0165(2) Å, $c$ $=$ 29.8572(16) Å, and space group $I$4/mmm (no. 139)]. The electrical resistivity ρ(T) and magnetization measurement show a transition at $T_{\mathrm{c}}\approx 25$ K. The lower ($H_{\mathrm{c}1}$) and upper ($H_{\mathrm{c}2}$) critical fields are 2 mT and 6.5 T, respectively. The Ginzburg-Landau parameter of κ ≈ 80 places this compound in the family of strong type-II superconductors.