At low temperatures the dynamical degrees of freedom in amorphous solids are tunneling two-level systems (TLSs). Concentrating on these degrees of freedom, and taking into account disorder and TLS-TLS interactions, we obtain a “TLS glass,” described by the random-field Ising model with random $1/r^3$ interactions. In this paper we perform a self-consistent mean-field calculation, previously used to study the electron-glass (EG) model [A. Amir et al., Phys. Rev. B 77, 165207 (2008)]. Similarly to the electron glass, we find a $\frac{1}{\lambda }$ distribution of relaxation rates $\lambda$, leading to logarithmic slow relaxation. However, with increased interactions the EG model shows slower dynamics whereas the TLS-glass model shows faster dynamics. This suggests that given system-specific properties, glass dynamics can be slowed down or sped up by the interactions.

The effects of confinement, disorder, and reduced dimensionality upon quantum fluids have been studied by the adsorption of liquid helium in porous media. The effects of extreme, nanoscale confinement upon its microscopic excitations are not presently understood. Several previous experiments have suggested that at sufficiently low temperature, the roton mean free path is set by the restricted geometry. Here we show that the lifetime of the roton excitation is unaffected when superfluid helium is confined within cylindrical pores only a few nanometers in diameter. The temperature dependence of its lifetime are found to be identical to the bulk fluid, implying that the lifetime is not set by the scale of the confinement. Our results demonstrate that the rotons in the pore center propagate without being modified by the confining media, unlike the collective excitations of classical fluids.

Scanning superconducting quantum interference device measurements reveal large-scale modulations of the superfluid density and the critical temperature in superconducting Nb, NbN, and underdoped $\mathrm{YB}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_{7-\delta }$ films deposited on $\mathrm{SrTi}{\mathrm{O}}_3$ (STO). We show that these modulations are a result of the STO domains and domain walls, forming below the 105 K structural phase transition of STO. We found that the flow of normal current, measured above the superconducting transition, is also modulated over the same domain structure, suggesting a modified carrier density. In clean STO, domain walls remain mobile down to low temperatures. Modulated superconductivity over mobile channels offers the opportunity to locally control superconducting properties and better understand the relations between superconductivity and the local structure.

We report the physical properties of ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ thin films on ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ single crystals. The manganite films are deposited using oxide molecular beam epitaxy on flux-grown (001)-oriented iridate crystals. Temperature-dependent magnetotransport and x-ray magnetic circular dichroism measurements reveal the presence of a ferromagnetic metallic ground state in the films, consistent with films grown on ${\mathrm{SrTiO}}_3$ and ${\mathrm{La}}_{0.3}{\mathrm{Sr}}_{0.7}{\mathrm{Al}}_{0.65}{\mathrm{Ta}}_{0.35}{\mathrm{O}}_3$. A parallel resistance model is used to separate conduction effects within the ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ substrate and the ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ thin films, revealing that the measured resistance maximum does not correspond to the manganite Curie temperature but results from a convolution of properties of the near-insulating substrate and metallic film. The ability to grow and characterize epitaxial perovskites on ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ crystals enables a new route for studying magnetism at oxide interfaces in the presence of strong spin-orbit interactions.

Grain boundaries are natural interfaces present in polycrystalline materials and have an important role in transport properties. In this work, the impact of grain boundary crystallographic mismatch, local impurity modulation, and spacing on lattice thermal conductivity is examined from the kinetic theory approach, with ZnO as a case study. We employ a dislocation-based model to describe the grain boundary scatterings of phonons, in which structural characteristics of grain boundaries are explicitly built-in and grain boundary scattering time depends on phonon frequency. This is in contrast to the gray model or the commonly used Casimir limit, which is blind to both grain boundary features and phonon frequency. We show that the lattice thermal conductivity generally decreases with grain boundary misorientation angle, and this dependence is significant for small grain boundary spacing while it tends to diminish for a large one. Intriguingly, our results show that local grain boundary chemistry can affect even more substantially than the crystallographic misfit on phonon relaxation time and interfacial thermal (Kapitza) resistance. Our results suggest new opportunities in tuning lattice thermal conductivity besides the nanostructure engineering approach, and demonstrates the synergetic effects of grain boundary characteristics on phonon conduction in polycrystals.

Anisotropic magnetoresistance (AMR) was observed by Lord Kelvin one-and-half centuries ago in iron and nickel. The resistance of these ferromagnetic conductors showed a few percent change when a magnetic field was applied along or across the current. Subsequently, a 20% AMR was demonstrated in alloys of nickel and iron (permalloys). Efforts have then been devoted to extend this effect in multifunctional materials. The oxide heterostructure exhibiting two-dimensional electron liquid is one of the potential candidates as it has shown to exhibit emergent magnetic ordering, strong spin-orbit interactions, and anisotropic magnetoresistance. Here we show a giant crystalline AMR as large as 57% to 104% in anisotropic quantum wells based on nonmagnetic perovskite oxides $\mathrm{LaAl}{\mathrm{O}}_3$ and $\mathrm{SrTi}{\mathrm{O}}_3$, providing an alternative way in tailoring AMR with an extremely large effect. The AMR maximum appears when the magnetic field points along the in-plane $\left[1\overline{1}0\right]$ direction, irrespective of the direction of current flow, which is consistent with the idea of crystalline AMR. Data analysis and density functional theory calculation show that the observed giant crystalline AMR mainly originates from the strong anisotropic spin-orbit field at the interface due to its unique elliptical Fermi surface related to its orbital configuration and reconstruction. This work demonstrates that perovskite oxide interface is a unique platform for orbital physics.

Pillar-based phononic crystals belong to a class of acoustic metamaterials that can control heat conduction based on the design of the structure. In this work, we systematically investigate how various parameters of pillar-based phononic crystals affect thermal conductance at low temperatures. We find that the lowest thermal conductance is achieved when the pillars are short, have a large radius, a long period, and are made of materials with few local resonances, whereas pillars with many local resonances can, on the contrary, increase thermal conductance. We argue that properly designed pillar-based phononic crystals can serve as an alternative to conventional hole-based phononic crystals because local resonances in pillars introduce additional degrees of freedom, which allows not only suppressing but also enhancing heat conduction. Moreover, we propose hybrid hole/pillar-based phononic crystals that can further reduce thermal conductance.

We predict a predominance diagram for electron defects in the temperature-hydrostatic stress space for $\mathrm{SrTi}{\mathrm{O}}_3$ by combining density functional theory and the quasiharmonic approximation. We discovered two regimes where small polarons dominate: under tensile stress at lower temperature due to a larger relaxation volume of the defect $\mathrm{\Omega }$, and under compressive stress at higher temperature due to a smaller $\mathrm{\Omega }$ and larger formation entropy. This provides a means to modulate the electronic conductivity via controlling the underlying charge carrier. Furthermore, the results challenge the common association between larger $\mathrm{\Omega }$ and charge localization by demonstrating that at high temperature the free electron can induce larger $\mathrm{\Omega }$ compared to the small polaron. This finding is attributed to the ability of the free electron to generate greater vibrational entropy upon finite isothermal expansion.

We propose a computational approach to combining the plane-wave method and the real-space treatment to describe the periodic variation in the material plane and the decay of wave functions from the material surfaces. The proposed approach is natural for two-dimensional material systems and thus may circumvent some intrinsic limitations involving the artificial replication of material layers in traditional supercell methods. In particular, we show that the proposed method is easy to implement and, especially, computationally effective since low-cost computational algorithms, such as iterative and recursive techniques, can be used to treat matrices with block tridiagonal structure. Using this approach we show first-principles features that supplement the current knowledge of some fundamental issues in bilayer graphene systems, including the coupling between the two graphene layers, the preservation of the $\sigma$ band of monolayer graphene in the electronic structure of the bilayer system, and the differences in low-energy band structure between the AA- and AB-stacked configurations.

We determine the frequency dependence of four independent Cartesian tensor elements of the dielectric function for ${\mathrm{CdWO}}_4$ using generalized spectroscopic ellipsometry within mid-infrared and far-infrared spectral regions. Different single crystal cuts, (010) and (001), are investigated. From the spectral dependencies of the dielectric function tensor and its inverse we determine all long-wavelength active transverse and longitudinal optic phonon modes with $A_u$ and $B_u$ symmetry as well as their eigenvectors within the monoclinic lattice. We thereby demonstrate that such information can be obtained completely without physical model line-shape analysis in materials with monoclinic symmetry. We then augment the effect of lattice anharmonicity onto our recently described dielectric function tensor model approach for materials with monoclinic and triclinic crystal symmetries [M. Schubert et al., Phys. Rev. B 93, 125209 (2016)], and we obtain an excellent match between all measured and modeled dielectric function tensor elements. All phonon mode frequency and broadening parameters are determined in our model approach. We also perform density functional theory phonon mode calculations, and we compare our results obtained from theory, from direct dielectric function tensor analysis, and from model line-shape analysis, and we find excellent agreement between all approaches. We also discuss and present static and above reststrahlen spectral range dielectric constants. Our data for ${\mathrm{CdWO}}_4$ are in excellent agreement with a recently proposed generalization of the Lyddane-Sachs-Teller relation for materials with low crystal symmetry [M. Schubert, Phys. Rev. Lett. 117, 215502 (2016)].

Heteroepitaxial growth of transition-metal oxide films on the open (111) surface of $\mathrm{SrTi}{\mathrm{O}}_3$ results in significant restructuring due to the polar mismatch. Monitoring the structure and composition on an atomic scale of $\mathrm{LaNi}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ (111) interface as a function of processing conditions has enabled the avoidance of the expected polar catastrophe. Using atomically resolved transmission electron microscopy and spectroscopy as well as low-energy electron diffraction, the structure of the thin film, from interface to the surface, has been studied. In this paper, we show that the proper processing can lead to a structure that is ordered, coherent with the substrate without intermediate structural phase. Angle-resolved x-ray photoemission spectroscopy shows that the oxygen content of thin films increases with the film thickness, indicating that the polar mismatch is avoided by the presence of oxygen vacancies.

We investigate the origin of ubiquitous low-energy kinks found in angle-resolved photoemission experiments in a variety of correlated matter. Such kinks are unexpected from weakly interacting electrons and hence identifying their origin should lead to fundamental insights in strongly correlated matter. We devise a protocol for extracting the kink momentum and energy from the experimental data which relies solely on the two asymptotic tangents of each dispersion curve, away from the feature itself. It is thereby insensitive to the different shapes of the kinks as seen in experiments. The body of available data are then analyzed using this method. We proceed to discuss two alternate theoretical explanations of the origin of the kinks. Some theoretical proposals invoke local bosonic excitations (Einstein phonons or other modes with spin or charge character), located exactly at the energy of observed kinks, leading to a momentum-independent self-energy of the electrons. A recent alternate is the theory of extremely correlated Fermi liquids (ECFL). This theory predicts kinks in the dispersion arising from a momentum-dependent self-energy of correlated electrons. We present the essential results from both classes of theories, and identify experimental features that can help distinguish between the two mechanisms. The ECFL theory is found to be consistent with currently available data on kinks in the nodal direction of cuprate superconductors, but conclusive tests require higher-resolution energy distribution curve data.

Hyperbolic (or indefinite) materials have attracted significant attention due to their unique capabilities for engineering electromagnetic space and controlling light propagation. A current challenge is to find a hyperbolic material with wide working frequency window, low energy loss, and easy controllability. Here, we propose that naturally existing electride materials could serve as high-performance hyperbolic medium. Taking the electride ${\mathrm{Ca}}_2\mathrm{N}$ as a concrete example and using first-principles calculations, we show that the material is hyperbolic over a wide frequency window from short-wavelength infrared to near infrared (from about $3.3\mu \mathrm{m}$ to 880 nm). More importantly, it is almost lossless in the window. We clarify the physical origin of these remarkable properties and show its all-angle negative refraction effect. Moreover, we find that the optical properties can be effectively tuned by strain. With moderate strain, the material can even be switched between elliptic and hyperbolic for a particular frequency. Our result points out a new route toward high-performance natural hyperbolic materials, and it offers realistic materials and novel methods to achieve controllable hyperbolic dispersion with great potential for applications.

The magnetic circular dichroism in threshold photoemission (TPMCD) for perpendicularly magnetized fcc Co films on Pt(111) has been revisited. A complete mapping of the spectral function $I(E_B,k_x,k_y)$ (binding energy $E_B$, momentum parallel to surface $k_x$, $k_y$) and the corresponding TPMCD asymmetry distribution $A_{\mathrm{MCD}}(E_B,k_x,k_y)$ has been performed for one-photon and two-photon photoemission using time-of-flight momentum microscopy. The experimental results allow distinguishing direct from indirect transitions. The measurements reveal clear band features of direct transitions from bulk bands that show a nontrivial asymmetry pattern. A significant homogeneous background with substantial asymmetry stemming from indirect transitions superposes direct transitions. Two-photon photoemission reveals enhanced emission intensity via an image potential state, acting as intermediate state. The image potential state enhances not only intensity but also asymmetry. The present results demonstrate that two-photon photoemission is a powerful method for mapping the spin-polarized unoccupied band structures and points out pathways for applying TPMCD as a contrast mechanism for various classes of magnetic materials.

We propose an extension of the Sachdev-Ye-Kitaev (SYK) model that exhibits a quantum phase transition from the previously identified non-Fermi-liquid fixed point to a Fermi-liquid-like state, while still allowing an exact solution in a suitable large-$N$ limit. The extended model involves coupling the interacting $N$-site SYK model to a new set of $pN$ peripheral sites with only quadratic hopping terms between them. The conformal fixed point of the SYK model remains a stable low-energy phase below a critical ratio of peripheral sites $p<p_c\left(n\right)$ that depends on the fermion filling $n$. The scrambling dynamics throughout the non-Fermi-liquid (NFL) phase is characterized by a universal Lyapunov exponent ${\lambda }_{\mathrm{L}}\to 2\pi T$ in the low-temperature limit; however, the temperature scale marking the crossover to the conformal regime vanishes continuously at the critical point $p_c$. The residual entropy at $T\to 0$, nonzero in the NFL, also vanishes continuously at the critical point. For $p>p_c$ the quadratic sites effectively screen the SYK dynamics, leading to a quadratic fixed point in the low-temperature and low-frequency limit. The interactions have a perturbative effect in this regime leading to scrambling with Lyapunov exponent ${\lambda }_{\mathrm{L}}\propto T^2$.

We present magnetic-field-induced metastability of the magnetic soliton lattice in a bilayer ruthenate $\mathrm{C}{\mathrm{a}}_3{\left(\mathrm{R}{\mathrm{u}}_{1-x}\mathrm{F}{\mathrm{e}}_x\right)}_2{\mathrm{O}}_7\left(x=0.05\right)$ through single-crystal neutron diffraction study. We show that the incommensurability of the modulation wave vector at zero field strongly depends on the history of magnetic field at low temperature, and that the equilibrium ground state can be achieved by warming above a characteristic temperature $T_{\mathrm{g}}\sim 37\mathrm{K}$. We suggest that such metastability might be associated with the domain wall pinning by the magnetic Fe dopants.

Magnetic frustration and low dimensionality can prevent long-range magnetic order and lead to exotic correlated ground states. ${\mathrm{SrDy}}_2{\mathrm{O}}_4$ consists of magnetic ${\mathrm{Dy}}^{3+}$ ions forming magnetically frustrated zigzag chains along the $c$ axis and shows no long-range order to temperatures as low as $T=60$ mK. We carried out neutron scattering and ac magnetic susceptibility measurements using powder and single crystals of ${\mathrm{SrDy}}_2{\mathrm{O}}_4$. Diffuse neutron scattering indicates strong one-dimensional (1D) magnetic correlations along the chain direction that can be qualitatively accounted for by the axial next-nearest-neighbor Ising model with nearest-neighbor and next-nearest-neighbor exchange $J_1=0.3$ meV and $J_2=0.2$ meV, respectively. Three-dimensional (3D) correlations become important below $T^{*}\approx 0.7$ K. At $T=60$ mK, the short-range correlations are characterized by a putative propagation vector ${\mathbf{k}}_{1/2}=(0,\frac{1}{2},\frac{1}{2})$. We argue that the absence of long-range order arises from the presence of slowly decaying 1D domain walls that are trapped due to 3D correlations. This stabilizes a low-temperature phase without long-range magnetic order, but with well-ordered chain segments separated by slowly moving domain walls.

We explore critical properties of two-dimensional lattices of spins interacting via an anisotropic Heisenberg Hamiltonian that are subject to incoherent spin flips. We determine the steady-state solution of the master equation for the density matrix via the corner-space renormalization method. We investigate the finite-size scaling and critical exponent of the magnetic linear susceptibility associated with a dissipative ferromagnetic transition. We show that the von Neumann entropy increases across the critical point, revealing a strongly mixed character of the ferromagnetic phase. Entanglement is witnessed by the quantum Fisher information, which exhibits a critical behavior at the transition point, showing that quantum correlations play a crucial role in the transition.

We report experiments on the generation of spin currents by simultaneous spin pumping effect and spin Seebeck effect (SSE) in bilayers of yttrium iron garnet (YIG)/Pt and YIG/IrMn. The experiments are performed with the ferromagnetic resonance (FMR) in YIG excited by microwave radiation in the range of 2–6 GHz and subject to a temperature gradient as in studies of the longitudinal SSE. Measurements of the FMR linewidth show that the magnetic damping in YIG can be controlled by spin currents generated by the thermal gradient across the thickness of the bilayer. The simultaneous measurements allow the determination of the spin current generated by SSE and a quantitative comparison of the models proposed for thermal control of the magnetic damping with experimental data. We show that the model based on the flow of angular momentum into or out of the YIG film carried by the coherent part of the spin current influenced by the temperature gradient is the one that best accounts for the observed change in the damping.

A nanoscale reconfigurable magnonic crystal is designed using voltage-controlled perpendicular magnetic anisotropy (PMA) in ferromagnetic-dielectric heterostructures. A periodic array of gate metallic strips is placed on top of a MgO/Co structure in order to apply a periodic electric field and to modify the PMA in Co. It is numerically demonstrated that the introduction of PMA, which can be realized experimentally via applying a voltage, modifies the spin-wave propagation and leads to the formation of band gaps in the spin-wave spectrum. The band gaps can be controlled, i.e., it is possible to switch band gaps on and off within a few tens of nanoseconds. The width and the center frequency of the band gaps are defined by the applied voltage. Finally, it is shown that the introduction of PMA to selected, rather than to all gate strips allows for a predefined modification of the band-gap spectra. The proposed voltage-controlled reconfigurable magnonic crystal opens a way to low power consumption magnonic applications.