Manipulating thermal transport by designing materials with control of their properties over the entire spectral range of vibrational frequencies would provide a unique path to create solids with designer thermal conductivities. Traditional routes of nanostructuring to reduce the vibrational thermal conductivity of solids typically target narrow bands of the vibrational energy spectrum, which is often based on the characteristic dimensions of the nanostructure. In this work, we demonstrate the ability to simultaneously impact the phonon transport of both high- and low-frequency modes by creating defects that act as both high-frequency phonon scattering sites while coherently manipulating low-frequency waves via resonance with the long wavelength phonons. We use atomistic simulations to identify fullerenes functionalized on the sidewalls of carbon nanotubes (CNT) as efficient phonon blocks realized through localized resonances that appear due to hybridization between the modes of the fullerene and the underlying CNT. We show that with a large surface coverage and high periodicity in the inclusion of the covalently bonded fullerenes, the thermal conductivity of individual CNTs can be lowered by more than an order of magnitude, thus providing a large tunability in the thermal transport across these materials. We prescribe the large reduction in thermal conductivity to a combination of resonant phonon localization effects leading to phonon band anticrossings and vibrational scattering effects due to the inclusion of the strongly bonded fullerene molecules.

We propose a type of photoresponse induced in asymmetric Weyl semimetals in an external magnetic field. In usual symmetric Weyl semimetals in a magnetic field, the particles and holes produced by an incident light in different Weyl cones have opposite helicities and hence move in opposite directions, canceling each other's contributions to the photocurrent. However this cancellation does not occur if the Weyl semimetal possesses both a broken particle-hole symmetry and a broken spatial inversion symmetry. We call the resulting generation of photocurrent the helical magnetic effect because it is induced by the helicity imbalance in a magnetic field. We find that due to the large density of states in a magnetic field, the helical magnetic effect induces a remarkable large photocurrent for incident THz frequency light. This suggests a potential application of asymmetric Weyl semimetals for creating THz photosensors.

The single-ion physics of ${\mathrm{Ho}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ is well understood to produce strong Ising anisotropy, which is an essential ingredient to its low-temperature spin-ice state. We present inelastic neutron scattering measurements on ${\mathrm{Ho}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ that reveal a clear inconsistency with its established single-ion Hamiltonian. Specifically, we show that a crystal-field doublet near 60 meV is split by approximately 3 meV. Furthermore, this crystal-field splitting is not isolated to ${\mathrm{Ho}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ but can also be found in its chemical pressure analogs ${\mathrm{Ho}}_2{\mathrm{Ge}}_2{\mathrm{O}}_7$ and ${\mathrm{Ho}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$. We demonstrate that the origin of this effect is a vibronic bound state, resulting from the entanglement of a phonon and crystal-field excitation. We derive the microscopic Hamiltonian that describes the magnetoelastic coupling and provides a quantitative description of the inelastic neutron spectra.

Reentrant spin glasses are frustrated disordered ferromagnets developing vortexlike textures under an applied magnetic field. Our study of a ${\mathrm{Ni}}_{0.81}{\mathrm{Mn}}_{0.19}$ single crystal by small angle neutron scattering clarifies their internal structure and shows that these textures are randomly distributed. Spin components transverse to the magnetic field rotate over length scales of 3–15 nm in the explored field range, decreasing as field increases according to a scaling law. Monte Carlo simulations reveal that the internal structure of the vortices is strongly distorted and differs from that assumed for “frustrated” skyrmions, built upon a competition between symmetric exchange interactions. Isolated vortices have small noninteger topological charge. The vortices keep an anisotropic shape on a three-dimensional lattice, recalling “croutons” in a “ferromagnetic soup.” Their size and number can be tuned independently by the magnetic field and concentration $x$ (or heat treatment), respectively. This opens an original route to understand and control the influence of quenched disorder in systems hosting nontrivial spin textures.

We inspect the unusual scattering properties reported recently for structures alternating dielectric layers of subwavelength thicknesses near the critical angle for total reflection. In TE polarization, the unusual scattering properties are captured by an effective model with an accuracy less than 1% up to $kd\sim 0.1$. It is shown that the propagation is simply dispersive with local dispersion while the boundary layer effects are captured through a nonintuitive transmission condition. The resulting model involves two parameters depending only on the characteristics of the multilayer and which are given in closed forms. Besides, we show that a discrete description of the spectrum using the layer thickness $d$ as unit of measure misses the complexity of the continuous spectrum exhibiting strong variations within the scale $d$. This ultrasensitivity to variations below $d$ is attributable to strong boundary layer effects and, for large structures, to a cooperation between the boundaries and the phase accumulation within the structure.

We study under what conditions the quantum adiabaticity is maintained in a closed many-body system consisting of a one-dimensional fluid and an impurity particle dragged through the latter by an external force. We employ an effective theory describing the low-energy sector of the system to derive the time dependence of the adiabaticity figure of merit: the adiabatic fidelity. We find that in order to maintain adiabaticity in a large system the external force $F_N$ should vanish with the system size $N$ as $1/N$ or faster. This improves the necessary adiabatic condition $F_N=O(1/lnN)$ obtained for this system previously [O. Lychkovskiy et al., in Fourth International Conference on Quantum Technologies (ICQT-2017), AIP Conf. Proc. No. 1936 (AIP, New York, 2018), p. 020024]. The experimental implications of this result and its relation to the quasi-Bloch oscillations of the impurity are discussed.

We performed impedance spectroscopy and optical investigations below the reported mobility edge of two models systems ($X_n$/$X_{1.95-\delta }{\mathrm{Al}}_{1.95}{\mathrm{Si}}_{2.05}{\mathrm{O}}_{8.00}$; $X=\mathrm{K}$ or Rb, $n$ is the doping density of guest atoms) used to explore the metal-insulator transition in a deformable lattice. The bulk ac conductivity and imaginary dielectric permittivity of insulating samples near the mobility edge in both systems show a power-law behavior and variable-range hopping (VRH) conduction that is indeed related to the proposed polaron relaxation and conduction. We show that polaron relaxation and VRH occur simultaneously above a particular density of an interacting gas of small (bi)polarons, which is commensurate with the electron-lattice coupling strength and the intersite electron transfer energy. Based on our experimental results, we propose a rich phase diagram for the charge dynamics in a deformable lattice from the perspective of a Wigner lattice of small (bi)polarons.

We present a comprehensive study of a one-dimensional two-orbital model at and below quarter filling that realizes a number of unconventional phases. In particular, we find an excitonic density wave in which excitons quasicondense with finite center-of-mass momentum and an order parameter that changes phase with wave vector $Q$. In this phase, excitons behave as hard-core bosons without charge order. In addition, excitons can pair to form biexcitons in a state that is close to a charge- density-wave instability. When pairing dominates over the interorbital repulsion, we encounter a regime in which one orbital is metallic, while the other forms a spin-gapped superconductor, a genuine orbital-selective paired state. All these results are supported by both analytical and numerical calculations. By assuming a quasiclassical approximation, we solve the three-body hole-electron-spinon problem and show that excitons are held together by forming a bound state with spinons. In order to preserve the antiferromagnetic background, excitons acquire a dispersion that has a minimum away from $k=0$. The full characterization of the different phases is obtained by means of extensive density-matrix renormalization-group calculations.

A central issue of Mott physics, with symmetries being fully retained in the spin background, concerns the charge excitation. In a two-leg spin ladder with a spin gap, an injected hole can exhibit either a Bloch wave or a density wave by tuning the ladder anisotropy through a “quantum critical point” (QCP). The nature of such a QCP has been a subject of recent studies by density matrix renormalization group. In this paper, we reexamine the ground state of the one doped hole, and show that a two-component structure is present in the density wave regime, in contrast to the single component in the Bloch-wave regime. In the former, the density wave itself is still contributed by a standing-wave-like component characterized by a quasiparticle spectral weight $Z$ in a finite-size system. But there is an additional charge incoherent component emerging, which intrinsically breaks the translational symmetry associated with the density wave. The partial momentum is carried away by neutral spin excitations. Such an incoherent part does not manifest in the single-particle spectral function, directly probed by the angle-resolved photoemission spectroscopy measurement, however, it is demonstrated in the momentum distribution function. Landau's one-to-one correspondence hypothesis for a Fermi liquid breaks down here. The microscopic origin of this density wave state as an intrinsic manifestation of the doped Mott physics will be also discussed.

We investigate the electronic structure and several properties, and topological character, of the cubic time-reversal invariant intermetallic compounds ${\mathrm{PbPd}}_3$ and ${\mathrm{SnPd}}_3$ using density functional theory based methods. These compounds have a dispersionless band along the $\mathrm{\Gamma }\text{–}X$ line, forming the top of the Pd $4d$ bands and lying within a few meV of the Fermi level $E_F$. Effects of the flat band on transport and optical properties have been inspected by varying the doping concentration treated with the virtual crystal approximation for substitution on the Pb site. In the absence of spin-orbit coupling (SOC), we find triple nodal points and three-dimensional nodal loops, which are known to lead to surface bands and drumhead states, respectively, which we discuss for ${\mathrm{PbPd}}_3$. SOC removes degeneracy in most of the zone, providing a topological index $Z_2=1$ on the $k_z=0$ plane that indicates a topological character on that plane. The isovalent and isostructural compound ${\mathrm{SnPd}}_3$ shows only minor differences in its electronic structures, so it is expected to display similar electronic, transport, and topological properties.

We theoretically investigate a pure spin Hall current driven by a longitudinal temperature gradient, i.e., the spin Nernst effect (SNE), in a paramagnetic state of a collinear antiferromagnetic insulator with the Dzyaloshinskii-Moriya interaction. The SNE in a magnetic ordered state in such an insulator was proposed by Cheng et al. [R. Cheng, S. Okamoto, and D. Xiao, Phys. Rev. Lett. 117, 217202 (2016)]. Here we show that the Dzyaloshinskii-Moriya interaction can generate a pure spin Hall current even without magnetic ordering. By using a Schwinger boson mean-field theory, we calculate the temperature dependence of SNE in a disordered phase. We also discuss the implication of our results to experimental realizations.

Here we propose and study a novel type of plasmonic resonators based on a metal-insulator-metal waveguide and a side-coupled magneto-optical disk controlled by an external magnetic field. The wave-number change and the transmission of surface-plasmon-polaritons (SPPs) can be tuned by altering the magnetic field and reversible on/off switching of the running SPP modes by a reversal of the direction of the external magnetic field is demonstrated. Resonant enhancement of the magnetoplasmonic modulation by more than 200 times leads to a modulation contrast ratio more than tenfold ratio (90-% modulation) keeping a moderate insertion loss within an optical bandwidth of hundreds of GHz. Numerical simulations confirm the predictions by the derived analytical formulas of a high-contrast magnetoplasmonic modulation by the submicrometer ultrasmall disk resonator.

In two dimensions, interacting Floquet topological phases may arise even in the absence of any protecting symmetry, exhibiting chiral edge transport that is robust to local perturbations. We explore a similar class of Floquet topological phases in three dimensions, with translational invariance but no other symmetry, which also exhibit anomalous transport at a boundary surface. By studying the space of local 2D unitary operators, we show that the boundary behavior of such phases falls into equivalence classes, each characterized by an infinite set of reciprocal lattice vectors. In turn, this provides a classification of the 3D bulk, which we conjecture is complete. We demonstrate that such phases may be generated by exactly solvable “exchange drives” in the bulk. In the process, we show that the edge behavior of a general exchange drive in two or three dimensions can be deduced from the geometric properties of its action in the bulk, a form of bulk-boundary correspondence.

We present direct experimental evidence of Anderson localization induced by the intrinsic alloy compositional disorder of InGaN/GaN quantum wells. Our approach relies on the measurement of the luminescence spectrum under local injection of electrons from a scanning tunneling microscope tip into a near-surface single quantum well. Fluctuations in the emission line shape are observed on a few-nanometer scale. Narrow emission peaks characteristic of single localized states are resolved. Calculations in the framework of the localization landscape theory provide the effective confining potential map stemming from composition fluctuations. This theory explains well the observed nanometer scale carrier localization and the energies of these Anderson-type localized states. The energy spreading of the emission from localized states is consistent with the usually observed very broad photo- or electroluminescence spectra of InGaN/GaN quantum well structures.

We discuss transport through an interferometer formed by helical edge states tunnel-coupled to metallic leads. We focus on the experimentally relevant case of relatively high temperature as compared to the level spacing and discuss a response of the setup to the external magnetic flux $\varphi$ (measured in units of flux quantum) piercing the area encompassed by the edge states. We demonstrate that tunneling conductance of the interferometer is structureless in the ballistic case but shows sharp antiresonances, as a function of magnetic flux $\varphi$—with the period $1/2$—in the presence of a magnetic impurity. We interpret the resonance behavior as a coherent enhancement of backward scattering off the magnetic impurity at integer and half-integer values of flux, which is accompanied by suppression of the effective scattering at other values of flux. Both enhancement and suppression are due to the interference of processes with multiple returns to the magnetic impurity after a number of clockwise and counterclockwise revolutions around the setup. This phenomenon is similar to the well-known weak-localization-induced enhancement of backscattering in disordered systems. The quantum correction to the tunneling conductance is shown to be proportional to the flux-dependent “ballistic Cooperon.” The obtained results can be used for flux-tunable control of the magnetic disorder in Aharonov-Bohm interferometers built on helical edge states.

$\mathrm{Mo}{\mathrm{S}}_2$ sheets are decorated by silver nanoparticles of size 10–12 nm. Localized holes are generated in Ag $4d$ levels carrying a magnetic moment due to the charge transfer effect at the interface and formation of Ag-S bonds. Temperature-dependent charge transport shows a metal-insulator (M-I) transition; the metallic phase is supported by the transition from negative to positive magnetoresistance and enhanced polarizability near the M-I transition. Observed ferromagnetism arises due to dominant coupling among the spins of localized holes in Ag nanoparticles at lower temperature. Density functional theory calculation is carried out to establish the charge transfer effect. The amount of charge transferred from Ag to $\mathrm{Mo}{\mathrm{S}}_2$ is evaluated from Bader charge analysis and the temperature effect is verified by a Nosé thermostat with the NVT ensemble method.

We present a systematic study on the heavily Cr doped iron pnictides ${\mathrm{BaFe}}_{1.9-x}{\mathrm{Ni}}_{0.1}{\mathrm{Cr}}_x{\mathrm{As}}_2$ by using elastic neutron scattering, high-resolution synchrotron x-ray diffraction (XRD), resistivity, and Hall transport measurements. When the Cr concentration increases from $x=0$ to 0.8, neutron diffraction experiments suggest that the collinear antiferromagnetism persists in the whole doping range, where the Néel temperature $T_N$ coincides with the tetragonal-to-orthorhombic structural transition temperature $T_s$, and both of them keeps around 35 K. The magnetic ordered moment, on the other hand, increases within increasing $x$ until $x=0.5$, and then decreases with further increasing $x$. Detailed refinement of the powder XRD patterns reveals that the Cr substitutions actually stretch the ${\mathrm{FeAs}}_4$ tetrahedron along the $c$ axis and lift the arsenic height away from the Fe-Fe plane. Transport results indicate that the charge carriers become more localized upon Cr doping, then changes from electron type to hole type around $x=0.5$. Our results suggest that the ordered moment and the ordered temperature of static magnetism in iron pnictides can be decoupled and tuned separately by chemical doping.

Magnetocaloric ${\mathrm{LaFe}}_{13-x}{\mathrm{Si}}_x$-based compounds belong to the outstanding materials with potential for efficient solid-state refrigeration. We have performed temperature-dependent ${}^{57}\mathrm{Fe}$ nuclear resonant inelastic x-ray scattering measurements (in a field ${\mu }_{0}H$ of $\sim 0.7$ T) of the vibrational (phonon) density of states, VDOS, in ${\mathrm{LaFe}}_{11.6}{\mathrm{Si}}_{1.4}$ across the metamagnetic isostructural first-order phase transition at $T_C\sim 192$ K from the low-temperature ferromagnetic (FM) to the high-temperature paramagnetic (PM) phase, in order to determine the change in thermodynamic properties of the Fe lattice at $T_C$. The experimental results are compared with density-functional-theory-based first-principles calculations using the fixed-spin moment approach. Our combined experimental and theoretical results reveal distinct and abrupt changes in the VDOS of the Fe sublattice across $T_C$, occurring within a small temperature interval of $\mathrm{\Delta }T\le 12$ K around $T_C$. This indicates that strong magnetoelastic coupling (at the atomic scale) is present up to $T_C$, leading to a pronounced lattice softening (phonon redshift) in the PM phase. These changes originate from the itinerant electron magnetism associated with Fe and are correlated with distinct modifications in the Fe-partial electronic density of states $D(E_F$) at the Fermi energy $E_F$. From the experimental VDOS we can infer an abrupt increase (jump) in the Fe-partial vibrational entropy $\mathrm{\Delta }{S}_{\mathrm{vib}}$ of $+6.9\pm 2.6$ J/(kg K) and in the vibrational specific heat $\mathrm{\Delta }{C}_{\mathrm{vib}}$ of $+2.7\pm 1.6$ J/(kg K) upon heating. The increase in magnitude of the vibrational entropy $|\mathrm{\Delta }{S}_{\mathrm{vib}}|=6.9$ J/(kg K) of the Fe sublattice at $T_C$ upon heating is substantial, if compared with the magnitude of the isothermal entropy change $|\mathrm{\Delta }{S}_{\mathrm{iso}}|$ of 14.2 J/(kg K) in a field change $\mathrm{\Delta }B$ from 0 to 1 T, as obtained from isothermal magnetization measurements on our sample and using the Maxwell relation. We demonstrate that $\mathrm{\Delta }{S}_{\mathrm{vib}}$ obtained by nuclear resonant inelastic x-ray scattering is a sizable quantity and contributes directly and cooperatively to the total entropy change $\mathrm{\Delta }{S}_{\mathrm{iso}}$ at the phase transition of ${\mathrm{LaFe}}_{13-x}{\mathrm{Si}}_x$.