We investigate the impact of various phonon-scattering mechanisms on the in-plane thermal conductivity of suspended silicon thin films with two-dimensional periodic arrays of holes, i.e., phononic crystal (PnC) nanostructures. A large amount of data on the PnC structures with square, hexagonal, and honeycomb lattices reveals that the thermal conductivity is mostly determined by the surface-to-volume ratio. However, as the characteristic size of the structure is reduced down to several tens of nanometers, thermal conductivity becomes independent of the surface-to-volume ratio, lattice type, and other geometrical parameters, being controlled solely by the distance between adjacent holes (neck size).

Nonlocal spin valves (NLSVs) generate pure spin currents, providing unique insight into spin injection and relaxation at the nanoscale. Recently it was shown that the puzzling low temperature nonmonotonicity of the spin accumulation in all-metal NLSVs occurs due to a manifestation of the Kondo effect arising from dilute local-moment-forming impurities in the nonmagnetic material. Here it is demonstrated that precise control over interdiffusion in Fe/Cu NLSVs via thermal annealing can induce dramatic increases in this Kondo suppression of injection efficiency, observation of injector/detector separation-dependent Kondo effects in both charge and spin channels simultaneously, and, in the limit of large interdiffusion, complete breakdown of standard Valet-Fert-based models. The Kondo effect in the charge channel enables extraction of the exact interdiffusion profile, quantifying the influence of local moment density on the injection efficiency and presenting a well-posed challenge to theory.

Ta/CoFeB/MgO trilayers with perpendicular magnetic anisotropy are expected to play a key role in the next generation of current and electric field switched memory and logic devices. In this study, we combine scanning Kerr microscopy with electrical transport measurements to gain insight into the underlying mechanisms of current-induced switching within such devices. We find switching to be a stochastic, domain-wall-driven process, the speed of which is strongly dependent on the switching current. Kerr imaging shows domain nucleation at one edge of the device, which modeling reveals is likely assisted by the out-of-plane component of the Oersted field. Further domain growth, leading to magnetization reversal, may still be dominated by spin torques, but the Oersted field provides an additional mechanism with which to control the switching process.

We use sub-Kelvin scanning tunneling spectroscopy to investigate the suppression of superconductivity in homogeneously disordered ultrathin MoC films. We observe that the superconducting state remains spatially homogeneous even on the films of 3-nm thickness. The vortex imaging suggests the global phase coherence in our films. Upon decreasing thickness, when the superconducting transition drops from 8.5 to 1.2 K, the superconducting energy gap Δ follows $T_c$ perfectly. All this is pointing to a two-stage fermionic scenario of the superconductor-insulator transition (SIT) via a metallic state as an alternative to the direct bosonic SIT scenario with a Cooper-pair insulating state evidenced by the past decade STM experiments.

The complex nature of the structural disorder in the lead-free ferroelectric $\mathrm{N}{\mathrm{a}}_{1/2}\mathrm{B}{\mathrm{i}}_{1/2}\mathrm{Ti}{\mathrm{O}}_3$ has a profound impact on the perceived global structure and polar properties. In this paper, we have investigated the effect of electric field and temperature on the local structure around the Bi and Ti atoms using extended x-ray absorption fine structure. Detailed analysis revealed that poling brings about a noticeable change in the bond distances associated with the Bi-coordination sphere, whereas the Ti coordination remains unaffected. We also observed discontinuity in the Bi-O bond lengths across the depolarization temperature of the poled specimen. These results establish that the disappearance of the monoclinic-like ($Cc$) global distortion, along with the drastic suppression of the short-ranged in-phase octahedral tilt after poling [B. N. Rao et al., Phys. Rev. B 88, 224103 (2013)] is a result of the readjustment of the A-O bonds by the electric field, so as to be in conformity with the rhombohedral $R3c$ structure.

We study the influence of molecular monolayers on the thermal conductivities and heat capacities of hybrid inorganic/organic superlattice thin films fabricated via atomic/molecular layer deposition. We measure the cross plane thermal conductivities and volumetric heat capacities of ${\mathrm{TiO}}_2$- and ZnO-based superlattices with periodic inclusion of hydroquinone layers via time domain thermoreflectance. In comparison to their homogeneous counterparts, the thermal conductivities in these superlattice films are considerably reduced. We attribute this reduction in the thermal conductivity mainly due to incoherent phonon boundary scattering at the inorganic/organic interface. Increasing the inorganic/organic interface density reduces the thermal conductivity and heat capacity of these films. High-temperature annealing treatment of the superlattices results in a change in the orientation of the hydroquinone molecules to a 2D graphitic layer along with a change in the overall density of the hybrid superlattice. The thermal conductivity of the hybrid superlattice increases after annealing, which we attribute to an increase in crystallinity.

We propose a new approach leading to giant gain enhancement. It is based on unconventional slow-wave resonance associated with a degenerate band edge (DBE) in the dispersion diagram for a special class of photonic crystals supporting four modes at each frequency. We show that the gain enhancement in a Fabry-Pérot cavity (FPC) when operating at the DBE is several orders of magnitude stronger when compared to a cavity of the same length made of a standard photonic crystal with a regular band edge (RBE). The giant gain condition is explained by a significant increase in the photon lifetime and in the local density of states. We have demonstrated the existence of DBE operated special cavities that provide for superior gain conditions for solid-state lasers, quantum cascade lasers, traveling wave tubes, and distributed solid-state amplifiers. We also report the possibility to achieve low-threshold lasing in FPC with DBE compared to RBE-based lasers.

We report measurements of the in-plane magnetoelastic coupling in both as-deposited and annealed ultrathin Ta/CoFeB/MgO layers as a function of uniaxial strain, conducted using a four-point bending apparatus. While as-deposited samples show only a weak dependence of the magnetoelastic coupling on the CoFeB layer thickness in the ultrathin regime $(<2\mathrm{nm})$, we observe the onset of a strong thickness dependence upon annealing. This dependence can be modeled as arising from a combination of effective surface and volume contributions to the magnetoelastic coupling. We point out that if similar thickness dependence exists for magnetoelastic coupling in response to biaxial strain, then the standard Néel model for the magnetic anisotropy energy acquires a term inversely proportional to the magnetic layer thickness. This contribution can significantly change the overall magnetic anisotropy, and provides a natural explanation for the strongly nonlinear dependence of magnetic anisotropy energy on magnetic layer thickness that is commonly observed for ultrathin annealed CoFeB/MgO films with perpendicular magnetic anisotropy.

A major focus of experimental interest in ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ has been to clarify how the magnetic excitations of this strongly spin-orbit coupled system differ from the predictions of an isotropic 2D spin-1/2 Heisenberg model and to explore the extent to which strong spin-orbit coupling affects the magnetic properties of iridates. Here, we present a high-resolution inelastic light (Raman) scattering study of the low-energy magnetic excitation spectrum of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ and Eu-doped ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ as functions of both temperature and applied magnetic field. We show that the high-field ($H>1.5$ T) in-plane spin dynamics of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ are isotropic and governed by the interplay between the applied field and the small in-plane ferromagnetic spin components induced by the Dzyaloshinskii-Moriya interaction. However, the spin dynamics of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ at lower fields ($H<1.5$ T) exhibit important effects associated with interlayer coupling and in-plane anisotropy, including a spin-flop transition at $H_c$ in ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ that occurs either discontinuously or via a continuous rotation of the spins, depending on the in-plane orientation of the applied field. These results show that in-plane anisotropy and interlayer coupling effects play important roles in the low-field magnetic and dynamical properties of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$.

We investigate the optical control of magnetism of $\mathrm{L}{\mathrm{a}}_{1/2}\mathrm{S}{\mathrm{r}}_{1/2}\mathrm{Mn}{\mathrm{O}}_{3-\delta }$ via the manipulation of anisotropic magnetoresistance under the irradiation of a common red-light light-emitting diode. The window of the anisotropic magnetoresistance curve under a sweeping magnetic field is enlarged under the irradiation of red light, indicating an increase of saturation magnetization. The optical control of magnetization is reversible and strongly dependent on the light intensity and the $\mathrm{M}{\mathrm{n}}^{4+}$ content of $\mathrm{L}{\mathrm{a}}_{1/2}\mathrm{S}{\mathrm{r}}_{1/2}\mathrm{Mn}{\mathrm{O}}_{3-\delta }$. A model of the photon-induced transition of spin electrons is proposed to explain the optical manipulation, according to the half-metallic electronic structure given by first-principles calculations. In addition to the fundamental significance, our findings advance the application of optical manipulation of magnetism in spintronics.

We report an angle-resolved photoemission spectroscopy study of a ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$-type superconductor ${\mathrm{YFe}}_2{\mathrm{Ge}}_2$ ($T_c\approx 1.8$ K) in the normal state. The resolved low-energy band structure mainly consists of several flat Fe bands, while the renormalization factors of different bands range from 1.7 to 2.8. Around the Fermi level, these bands contribute to a high density of states, which may account for the unusually high Sommerfeld coefficient $\gamma$. Besides, a 30 meV kink is observed, which is probably induced by electron-phonon interactions. Our results indicate that ${\mathrm{YFe}}_2{\mathrm{Ge}}_2$ is a moderately correlated compound, while electron-phonon coupling should be taken into account to better understand its properties.

Chains of magnetic atoms placed on the surface of an $s$-wave superconductor with large spin-orbit coupling provide a promising platform for the realization of topological superconducting states characterized by the presence of Majorana zero-energy modes. In this work, we study the properties of one-dimensional chains of Yu-Shiba-Rusinov states induced by magnetic impurities using a realistic model for the magnetic atoms that includes the presence of multiple scattering channels. These channels are mixed by spin-orbit coupling and, via the hybridization of the Yu-Shiba-Rusinov states at different sites of the chain, result in a multiband structure for the chain. We obtain the topological phase diagram for such a band structure and show that the inclusion of higher bands can greatly enlarge the phase space for the realization of topological states.

${\mathrm{Ba}}_{0.95}{\mathrm{Na}}_{0.05}{\mathrm{Ti}}_2{\mathrm{Sb}}_2\mathrm{O}$ is a titanium-based oxypnictide superconductor with possible density wave order. We have performed high-resolution angle-resolved photoemission spectroscopy and scanning tunneling microscopy (STM) studies on ${\mathrm{Ba}}_{0.95}{\mathrm{Na}}_{0.05}{\mathrm{Ti}}_2{\mathrm{Sb}}_2\mathrm{O}$. Our STM studies find a charge density wave (CDW) order with the wave vector of ($\pi , \pi$). The electronic structure shows both multiorbital and three-dimensional nature, consistent with the theoretical calculations. The observed Fermi surfaces are well nested along the ($\pi , \pi$) direction, which might drive the CDW transition. Likely due to the weak strength of the CDW order, we do not observe a leading-edge gap. Instead, we observe an overall spectral weight enhancement with decreasing temperature, but the rate of this enhancement is abruptly reduced or diminished in the CDW state, suggesting the suppression of spectral weight or opening of a partial CDW gap. Our results give a comprehensive picture of the electronic structure and direct observation of the CDW order in ${\mathrm{Ba}}_{0.95}{\mathrm{Na}}_{0.05}{\mathrm{Ti}}_2{\mathrm{Sb}}_2\mathrm{O}$.

Quantum transport of strongly correlated fermions is of central interest in condensed matter physics. While the stationary expansion dynamics have recently been measured with cold atoms in 2D optical lattices, ab initio simulations have been limited to 1D setups so far. Here, we present the first precise fermionic quantum dynamics simulations for 2D and 3D. The simulations are based on nonequilibrium Green functions and incorporate strong correlations via $T$-matrix self-energies. The simulations predict the short-time dynamics, and we discover a universal scaling of the expansion velocity with the particle number. Our predictions can be verified experimentally using the recently developed fermionic atom microscopes.

The one-dimensional extended bosonic Hubbard model has been shown to exhibit a variety of phases ranging from Mott insulator and superfluid to exotic supersolids and Haldane insulators depending on the filling and the relative value of the contact ($U$) and near neighbor ($V$) interaction strengths. In this paper we use the density matrix renormalization group and the time evolving block decimation numerical methods to study in detail the dynamics and excitation spectra of this model in its various phases. In particular, we study in detail the behavior of the charge and neutral gaps which characterize the Mott, charge density, and Haldane insulating phases. We also show that in addition to the gapless modes at $k=0$, the supersolid phase exhibits gapless modes at a finite $k$ which depends on the filling.

We investigate the effect of on-site Coulomb repulsion $U$ on the band gap of the electrically gated bilayer graphene by employing coherent potential approximation in the paramagnetic state, based on an ionic two-layer Hubbard model. We find that, while either the on-site Coulomb repulsion $U$ or the external perpendicular electric field $E$ alone will favor a gapped state in the bilayer graphene, competition between them will surprisingly lead to a suppression of the gap amplitude. Our results can be applied to understand the discrepancies of gap size reported from optical and transport measurements, as well as the puzzling features observed in angular resolved photoemission spectroscopic study.

A solid understanding of the implantation process of N and Si ions into diamond is needed for the controlled creation of shallow color centers for quantum computing, simulation, and sensing applications. Here, molecular dynamics simulations of the shallow implantation of N and Si ions into diamond is simulated at 100–5000 eV kinetic energies and different angles of incidence. We find that ion channeling is an important effect with an onset energy depending on the crystal orientation. Consequently, the molecular dynamics simulations produce improved predictions as compared to standard Monte Carlo simulations. When implanting in a channeling direction, the spatial distribution of the channeled ions becomes markedly narrow, allowing a higher degree of control over the location of the nitrogen vacancy (${\mathrm{NV}}^{-}$) centers. A contamination layer on the ion entry surface reduces the fraction of channeled ions. A comparison to an experimentally determined depth profile based on a NMR signal from protons yields a quantitative agreement, validating the simulation approach.