Plumbene, similar to silicene, has a buckled honeycomb structure with a large band gap ($\sim 400$ meV). All previous studies have shown that it is a normal insulator. Here, we perform first-principles calculations and employ a sixteen-band tight-binding model with nearest-neighbor and next-nearest-neighbor hopping terms to investigate electronic structures and topological properties of the plumbene monolayer. We find that it can become a topological insulator with a large bulk gap ($\sim 200$ meV) through electron doping, and the nontrivial state is very robust with respect to external strain. Plumbene can be an ideal candidate for realizing the quantum spin Hall effect at room temperature. By investigating effects of external electric and magnetic fields on electronic structures and transport properties of plumbene, we present two rich phase diagrams with and without electron doping and propose a theoretical design for a four-state spin-valley filter.

We present an experimental and computational study of the nonlinear optical response of conduction electrons to intense terahertz (THz) electric field. Our observations (saturable absorption and an amplitude-dependent group refractive index) can be understood on the qualitative level as the breakdown of the effective mass approximation. However, a predictive theoretical description of the nonlinear THz propagation has been missing. We propose a model based on the semiclassical electron dynamics, a realistic band structure, and the free electron Drude parameters to accurately calculate the experimental observables in InSb. Our results open a path to modeling of the conduction-electron optical nonlinearity that governs the THz propagation in semiconductors.

We numerically investigate the spectral control of near-field thermal radiation transfer using interband absorption in semiconductors and the band-folding effect in photonic crystals (PCs) for highly efficient thermophotovoltaics. We reveal that the near-field coupling between two semiconductors (Si and GaSb) realizes frequency-selective thermal radiation transfer concentrated above their bandgap energy when their thicknesses are optimized considering their absorption coefficient spectra. Moreover, we elucidate the role of PC structures in the near-field thermal radiation transfer and demonstrate that the band-folding effect in PCs can further increase both the radiation power and frequency selectivity of the near-field thermal radiation transfer.

We construct a nonperturbative nonequilibrium theory for graphene electrons interacting via the instantaneous Coulomb interaction by combining the functional renormalization group method with the nonequilibrium Keldysh formalism. The Coulomb interaction is partially bosonized in the forward scattering channel resulting in a coupled Fermi-Bose theory. Quantum kinetic equations for the Dirac fermions and the Hubbard-Stratonovich boson are derived in Keldysh basis, together with the exact flow equation for the effective action and the hierarchy of one-particle irreducible vertex functions, taking into account a possible nonzero expectation value of the bosonic field. Eventually, the system of equations is solved approximately under thermal equilibrium conditions at finite temperature, providing results for the renormalized Fermi velocity and the static dielectric function, which extends the zero-temperature results of Bauer et al., Phys. Rev. B 92, 121409 (2015).

In this paper we investigated the interaction between a graphene nanoflake anchored to the 2D graphene monolayer. This interaction is attractive but weak and is capable of setting a well defined registry in equilibrium. Rotational and linear displacements from equilibrium registry generate restoring forces, which can be controlled by external agents. Similar flakes can be self-assembled and can also execute simple harmonic motion as if a physical pendulum. Oscillation of a nanoflake about their equilibrium registries resulting in a characteristic libration frequency is predicted. This frequency depends on the size and geometry of the flake. Moreover, the libration frequency, as well as the electronic and magnetic properties of the flake+monolayer systems, can be tuned by a foreign molecule anchored to the flake, by electric charging and applied parallel and perpendicular electric and magnetic fields. When the sliding of the flake is combined with rotation, the friction force can be reduced dramatically. It is surprising that weak interaction can offer such features at nanoscale, which may offer potential applications. Our predictions are obtained by first-principles calculations based on density functional theory.

Ferroelectrics usually adopt a multidomain state with domain walls separating domains with polarization axes oriented differently. It has long been recognized that domain walls can dramatically impact the properties of ferroelectric materials. The enhancement of low-field susceptibility/permittivity under subswitching conditions is usually attributed to reversible domain wall vibration. Recent experiments highlight the stationary domain wall contribution to the dielectric susceptibility irrespective of any lateral displacements or deformations of the wall. We study the effects of domain walls on the low-field permittivity of ${\mathrm{PbTiO}}_3$ with density functional theory and molecular dynamics simulations. The static dielectric constant is calculated as a function of increasing domain wall density and temperature. We find an increase of dielectric permittivity with increasing domain wall density, which is expected to occur at a low driving field where the lateral motion of domain walls is forbidden. Real-space decomposition of the dielectric response reveals that frustrated dipoles within the finite width of the domain walls are responsible for the enhanced low-field permittivity. We explain the $100\%$ enhancement of the dielectric susceptibility form domain walls, which arises from the softer potential wells within them.

The anharmonic effect in thermoelectrics has been a central topic for decades in both condensed matter physics and material science. However, despite the long-believed strong and complex anharmonicity in the ${\mathrm{Bi}}_2{\mathrm{Te}}_{3-x}{\mathrm{Se}}_x$ series, experimental verification of anharmonicity and its evolution with doping remains elusive. We fill this important gap with high-resolution, temperature-dependent Raman spectroscopy in high-quality single crystals of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$, ${\mathrm{Bi}}_2{\mathrm{Te}}_2\mathrm{Se}$, and ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ over the temperature range from 4 to 293 K. Klemens's model was employed to explain the renormalization of their phonon linewidths. The phonon energies of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ are analyzed in detail from three aspects: lattice expansion, cubic anharmonicity, and quartic anharmonicity. For the first time, we explain the evolution of anharmonicity in various phonon modes and across the series. In particular, we find that the interplay between cubic and quartic anharmonicity is governed by their distinct dependence on the phonon density of states, providing insights into anomalous anharmonicity designing of new thermoelectrics.

A series of ${\mathrm{Yb}}_{2+x}{\mathrm{Ti}}_{2-x}{\mathrm{O}}_{7-\delta }$ doped samples demonstrates the effects of off-stoichiometry on ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$'s structure, properties, and magnetic ground state via x-ray diffraction, specific heat, and magnetization measurements. A stoichiometric single crystal of ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ grown by the traveling solvent floating zone technique (solvent = 30 wt % rutile ${\mathrm{TiO}}_2$ and 70 wt % ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$) is characterized and evaluated in light of this series. Our data shows that upon positive $x$ doping, the cubic lattice parameter $a$ increases and the Curie-Weiss temperature ${\theta }_{\mathrm{CW}}$ decreases. Heat capacity measurements of stoichiometric ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ samples exhibit a sharp, first-order peak at $T=268$(4) mK that is suppressed in magnitude and temperature in samples doped off ideal stoichiometry. The full entropy recovered per Yb ion is 5.7 J ${\mathrm{K}}^{-1}\approx Rln2$. Our work establishes the effects of doping on ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$'s physical properties, which provides further evidence indicating that previous crystals grown by the traditional floating zone method are doped off ideal stoichiometry. Additionally, we present how to grow high-quality colorless single crystals of ${\mathrm{Yb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ by the traveling solvent floating zone growth method.

Using Monte Carlo simulations, we study thermal and critical properties of two systems, in which domain walls and so-called $Z_2$ vortices as topological defects are presented. The main model is a lattice version of the $O\left(3\right)$ principal chiral model. We find a first-order transition and give qualitative arguments that the first order is induced by topological defects. We also consider the model of frustrated antiferromagnet on a square lattice with the additional exchange interaction between spins of the third range order. This model belongs to the same symmetry class. In this model, a transition is of first order too.

Unlike the widely studied $s$-type two-gap superconductor ${\mathrm{MgB}}_2$, the chemically similar compounds ${\mathrm{ZrB}}_2$ and ${\mathrm{HfB}}_2$ do not superconduct above 1 K. Yet it has been shown that small amounts of self or extrinsic doping (in particular with vanadium), can induce superconductivity in these materials. Based on results of different macroscopic and microscopic measurements, including magnetometry, nuclear magnetic resonance (NMR), resistivity, and muon-spin rotation (${\mu }^{+}\mathrm{SR}$), we present a comparative study of ${\mathrm{Zr}}_{0.96}{\mathrm{V}}_{0.04}{\mathrm{B}}_2$ and ${\mathrm{Hf}}_{0.97}{\mathrm{V}}_{0.03}{\mathrm{B}}_2$. Their key magnetic and superconducting features are determined and the results are considered within the theoretical framework of multiband superconductivity proposed for ${\mathrm{MgB}}_2$. Detailed Fermi surface (FS) and electronic structure calculations reveal the difference between ${\mathrm{MgB}}_2$ and transition-metal diborides.

Measurements of the London penetration depth $\mathrm{\Delta }\lambda \left(T\right)$ and tunneling conductance in single crystals of the recently discovered stoichiometric iron-based superconductor ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$ (CaK1144) show nodeless, two-effective-gap superconductivity with a larger gap of about 6–10 meV and a smaller gap of about 1–4 meV. Having a critical temperature $T_{c,\text{onset}}\approx 35.8$ K, this material behaves similar to slightly overdoped $\left({\mathrm{Ba}}_{1-x}{\mathrm{K}}_x\right){\mathrm{Fe}}_2{\mathrm{As}}_2$ (e.g., $x=0.54,T_c\approx 34$ K), a known multigap $s_{\pm }$ superconductor. We conclude that the superconducting behavior of stoichiometric CaK1144 demonstrates that two-gap $s_{\pm }$ superconductivity is an essential property of high-temperature superconductivity in iron-based superconductors, independent of the degree of substitutional disorder.

Using first principles density functional theory calculations combined with quasi-harmonic approximation, we demonstrate that the recently reported nonsymmorphic “hourglass fermion” materials $\text{KHg}X$ $(X=\mathrm{As}$, Sb) belong to a type of negative thermal expansion (NTE) material with an abnormal volumetric thermal expansion. It is revealed that the NTE is caused by the peculiar layered structures of KHgAs and KHgSb, composed of alternately arranged alkali metal and Hg-$X$ atomic layers with residual tensions. Specifically, the coefficients of negative thermal expansion (CNTE) of KHgAs and KHgSb can reach up to $-2.7\times {10}^{-6}{\mathrm{K}}^{-1}$ and $-4.9\times {10}^{-6}{\mathrm{K}}^{-1}$ along the $a$ axis, respectively, as well as a larger volumetric CNTE of $-4.98\times {10}^{-6}{\mathrm{K}}^{-1}$ for KHgSb. The analyses of Grüneisen parameters and vibrational modes show that the NTE of $\text{KHg}X$ is driven by the cooperation of membrane and tension effects. It is most likely that the weaker bonds in the Hg-$X$ layer and the smaller mass of alkali metal facilitate the membrane and tension effects, therefore producing a large NTE. Our findings offer insights for understanding the underlying mechanism of NTE behavior in the hourglass fermion materials $\text{KHg}X$.

We propose a dynamic Jahn-Teller approach to elucidate the generation mechanism of asymmetric modes of coherent phonons induced in crystals by irradiation with a short optical pulse in the opaque energy region. This is a natural extension of the impulsive excitation model of symmetric modes to multi dimensions in the configuration coordinate space. We show that the two generation mechanisms of coherent phonons coexist in this case, namely the impulsive absorption (IA) mechanism and impulsive stimulated Raman scattering (ISRS) mechanism. The dependence of the phonon amplitude on the polarization of the pump pulse is exactly the same in IA and ISRS processes and is in agreement with the prediction of the argument based on Raman tensors. The dependence of the excitation efficiency of the coherent phonons on the frequency of the pump pulse is calculated using a simplified model of the optical response function of the crystal. Generally, the IA mechanism predominates in the opaque region, although ISRS makes a comparable contribution to phonon generation in the near-edge opaque region. The initial phase of the coherent phonon is always cosine-like in IA but depends on the excitation frequency in ISRS.

To date, endeavors in nanoscale spintronics are dominated by the use of single-electron or single-spin transistors having at their heart a semiconductor, metallic, or molecular quantum dot whose localized states are non-spin-degenerate and can be controlled by an external bias applied via a gate electrode. Adjusting the bias of the gate one can realign those states with respect to the chemical potentials of the leads and thus tailor the spin-polarized transmission properties of the device. Here we show that similar functionality can be achieved in a purely metallic junction comprised of a metallic magnetic chain attached to metallic paramagnetic leads and biased by a gate electrode. Our ab initio calculations of electron transport through mixed Pt-Fe (Fe-Pd and Fe-Rh) atomic chains suspended between Pt (Pd and Rh) electrodes show that spin-polarized confined states of the chain can be shifted by the gate bias causing a change in the relative contributions of majority and minority channels to the nanocontact's conductance. As a result, we observe strong dependence of conductance spin polarization on the applied gate potential. In some cases the spin polarization of conductance can even be reversed in sign upon gate potential application, which is a remarkable and promising trait for spintronic applications.

We explore the role of the spin diffusion of conducting electrons in two-dimensional magnetic textures (domain walls and skyrmions) with spatial variation of the order of the spin precession length ${\lambda }_{\text{ex}}$. The effect of diffusion reflects in four additional torques that are third order in spatial derivatives of magnetization and bilinear in ${\lambda }_{\text{ex}}$ and in the nonadiabatic parameter ${\beta }^{\prime }$. In order to study the dynamics of the solitons when these diffusive torques are present, we derive the Thiele equation in the limit of steady motion and we compare the results with the nondiffusive limit. When considering a homogenous current these torques increase the longitudinal velocity of transverse domain walls of width $\mathrm{\Delta }$ by a factor ${({\lambda }_{\text{ex}}/\mathrm{\Delta })}^2(\alpha /3), \alpha$ being the magnetic damping constant. In the case of single skyrmions with core radius $r_0$ these new contributions tend to increase the Magnus effect in an amount proportional to ${({\lambda }_{\text{ex}}/r_0)}^2(1+2\alpha {\beta }^{\prime })$.

${\mathrm{GdFe}}_2{\mathrm{Zn}}_{20}$ is a complex cagelike compound with an unusually high ferromagnetic ordering temperature ($T_C=86$ K) for a very diluted ${\mathrm{Gd}}^{3+}$ magnetic sublattice, embedded in a matrix that features strong electron-electron correlations. Here, we report on a magnetic and electronic study of the substitutional intermetallic system $\mathrm{Gd}{\left({\mathrm{Co}}_{1-y}{\mathrm{Fe}}_y\right)}_2{\mathrm{Zn}}_{20}$ combining magnetization measurements plus first-principles density functional theory (DFT) calculations with temperature-dependent electron spin resonance (ESR). After accounting for electron-electron correlations and itinerant molecular field effects, the ESR results indicate that the exchange interaction between the ${\mathrm{Gd}}^{3+}$ is processed via a single band of $d$-type electrons at the Fermi level and the exchange interaction is covalent in nature $[J{\left(0\right)}_{fd}<0]$ with a strong conduction electron ($ce$) momentum transfer dependence $\left[J_{fd}\left(q\right)\right]$. The DFT calculations support this scenario by indicating a major contribution of $d$-type $ce$ at the Fermi level and a spin polarization in $(\text{Y},\mathrm{Gd}){\mathrm{Fe}}_2{\mathrm{Zn}}_{20}$ wherein the most stable configuration is antiferromagnetic between ${\mathrm{Gd}}^{3+}$ and $ce$ spins. Our results demonstrate that the standard Ruderman-Kittel-Kasuya-Yosida mechanism cannot explain the ferromagnetic behavior of ${\mathrm{GdFe}}_2{\mathrm{Zn}}_{20}$ and a superexchangelike mechanism is proposed for this magnetic interaction. An “extended phase diagram” for the double substitution sequence ${\mathrm{YCo}}_2{\mathrm{Zn}}_{20}\to {\mathrm{GdCo}}_2{\mathrm{Zn}}_{20}\to {\mathrm{GdFe}}_2{\mathrm{Zn}}_{20}$ is presented and discussed.

The nature of Na ion distribution, diffusion path, and the spin structure of $P2$-type ${\mathrm{Na}}_2{\mathrm{Ni}}_2{\mathrm{TeO}}_6$ with a Ni honeycomb lattice has been explored. The nuclear density distribution of Na ions reveals a two-dimensional (2D) chiral pattern within Na layers without breaking the original 3D crystal symmetry, which has been achieved uniquely via an inverse Fourier transform (iFT)-assisted neutron-diffraction technique. The Na diffusion pathway described by the calculated isosurface of the Na ion bond valence sum (BVS) map is found consistent to a chiral diffusion mechanism. The Na site occupancy and ${\mathrm{Ni}}^{2+}$ spin ordering were examined in detail with the neutron diffraction, magnetic susceptibility, specific heat, thermal conductivity, and transport measurements. Signatures of both strong incommensurate (ICM) and weak commensurate (CM) antiferromagnetic (AFM) spin ordering were identified in the polycrystalline sample studied, and the CM-AFM spin ordering was confirmed by using a single-crystal sample through the $k$ scan in the momentum space corresponding to the AFM peak of ($\frac{1}{2},0,1$).