The unfilled skutterudite ${\mathrm{CoSb}}_3$ is a small-gap semiconductor which was predicted to be close to a strain-induced transition to a topological-insulator phase passing through a topological quantum critical point. As a starting point to strain measurements, the aim of this work is to characterize the electronic structure of as-grown samples. Two types of samples, one grown in Sb flux and the other by an inclined rotary Bridgman technique, are compared based on structural properties, resistivity, Hall effect, and magnetization. All samples appear metallic with small growth-dependent hole doping. Measurements of quantum oscillations in magnetization and angle-dependent electronic transport confirm the calculated band structure at ambient pressure. The Fermi surface consists of a single spherical sheet at the $\mathrm{\Gamma }$ point, and the purest samples grown by the Bridgman technique have a Fermi energy lying 25 meV below the valence-band edge. Band-structure calculations provide an accurate description of the experimental results. Hence, this compound is suitable for an investigation of topological states under strain.

We comprehensively studied the magnetotransport properties of ${\mathrm{LaAgSb}}_2$ under high pressure up to 4 GPa, which showed unique successive charge density wave (CDW) transitions at $T_{\mathrm{CDW}1}\sim 210$ K and $T_{\mathrm{CDW}2}\sim 190$ K at ambient pressure. With the application of pressure, both $T_{\mathrm{CDW}1}$ and $T_{\mathrm{CDW}2}$ were suppressed and disappeared at the critical pressures of $P_{\mathrm{CDW}1}=3.0–3.4$ GPa and $P_{\mathrm{CDW}2}=1.5–1.9$ GPa, respectively. At $P_{\mathrm{CDW}1}$, the Hall conductivity showed a steplike increase, which is consistently understood by the emergence of a two-dimensional hollow Fermi surface at $P_{\mathrm{CDW}1}$. We also observed a significant negative magnetoresistance effect when the magnetic field and current were applied parallel to the $c$ axis. The negative contribution was observed in the whole pressure region from 0 to 4 GPa. Shubnikov–de Haas (SdH) oscillation measurements under pressure directly showed the changes in the Fermi surface across the CDW phase boundaries. In $P<P_{\mathrm{CDW}2}$, three major oscillation components, $\alpha , \beta$, and $\gamma$, were identified, whose frequencies were increased by application of pressure. The increment rate of these frequencies was considerably larger than that expected from the shrinkage of lattice constant, indicating the unignorable band modification under pressure. In the normal metallic phase above $P>P_{\mathrm{CDW}1}$, we observed a single frequency of $\sim 48$ T with a cyclotron effective mass of $0.066m_0$, whose cross section in the reciprocal space corresponded to only 0.22% of the first Brillouin zone. Besides, we observed another oscillation component with frequency of $\sim 9.2$ T, which is significantly enhanced in the limited pressure range of $P_{\mathrm{CDW}2}<P<P_{\mathrm{CDW}1}$. The amplitude of this oscillation was anomalously suppressed in the high-field and low-temperature region, which cannot be explained by the conventional Lifshitz-Kosevich formula.

Thermal expansion behavior of two-dimensional (2D) nitrides and graphene were studied by ab initio molecular dynamics (MD) simulations as well as quasiharmonic approximation (QHA). Anharmonicity of the acoustic phonon modes are related to the unusual negative thermal expansion (NTE) behavior of the nitrides. Our results also hint that direct ab initio MD simulations are a more elaborate method to investigate thermal expansion behavior of 2D materials than the QHA. Nevertheless, giant NTE coefficients are found for $h$-GaN and $h$-AlN within the covered temperature range 100–600 K regardless of the chosen computational method. This unusual NTE of 2D nitrides is reasoned with the out-of-plane oscillations related to the rippling behavior of the monolayers.

We study the response of ballistic electron waves in graphene to a square array composed of gate-defined quantum dots when the incident energy is at the neutral point of the Dirac conical dispersion. The effective medium theory shows that the array will behave as a zero-refractive-index medium. Our simulations based on the rigorous multiple scattering theory corroborate that it can indeed exhibit various typical applications of zero-refractive-index media, such as the focusing effect of electron waves, the control over the propagation direction, and directional emission. The array is usually about one-wavelength thick so that the incident wave can penetrate it and the wave front can be tailored by engineering the geometrical shape of the emergent surface. The wave-manipulating behaviors based on zero-index metamaterials could open unprecedented opportunities to steer the flow of graphene electrons in novel manners and shape the wave front of electron beams at will.

Achieving large magnetoresistance (MR) effects in atomic-scale junctions is of great importance in the field of molecular spintronics. In this work, using ab initio quantum transport calculations, we report on the spin transport properties of molecular junctions made of single transition metal benzene sandwich molecules $\left(\mathrm{TM}{\mathrm{Bz}}_2\right)$ bridged by ferromagnetic electrodes. We find that relative magnetic orientations of two electrodes and the molecule can dramatically affect low-bias transport properties of the junction. While the $\mathrm{TM}{\mathrm{Bz}}_2$ are insulating in the gas phase, their corresponding molecular junctions bridged between the two electrodes are half-metallic minority-spin conductors (i.e., almost 100% spin polarization) and display very large MR. More interestingly, even larger molecular MR, corresponding to a reversal of molecular spin moment, are found due to the spin-valve behavior of the molecule controlled by the magnetic impurity. Such a particular performance is attributed to strong spin-selective hybridization of electronic states at the molecule/metal interface due to orbital-symmetry mismatch between the electrode's and molecular states. We also discuss the importance of the shape of electrodes on spin transport efficiency.

We explore the spin superfluid transport in exchange interaction-dominated three-sublattice antiferromagnets. The system in the long-wavelength regime is described by an $\mathrm{S}\mathrm{O}\left(3\right)$ invariant field theory. Additional corrections from Dzyaloshinskii-Moriya interactions or anisotropies can break the symmetry; however, the system still approximately holds a $\mathrm{U}\left(1\right)$-rotation symmetry. Thus, the power-law spatial decay signature of spin superfluidity is identified in a nonlocal-measurement setup where the spin injection is described by the generalized spin-mixing conductance. We suggest iron jarosites as promising material candidates for realizing our proposal.

We study the structural and electronic properties of phase III of solid hydrogen using accurate many-electron theories and compare the results to state-of-the-art experimental findings. The atomic structures of phase III modeled by $C2/c\text{−}24$ crystals are fully optimized on the level of second-order perturbation theory, demonstrating that previously employed structures optimized on the level of approximate density functionals exhibit errors in the ${\mathrm{H}}_2$ bond lengths that cause significant discrepancies in the computed quasiparticle band gaps and vibrational frequencies compared to experiment. Using these optimized atomic structures, we study the band gap closure and change in vibrational frequencies as a function of pressure. Our findings are in good agreement with recent experimental observations and may prove useful in resolving long-standing discrepancies between experimental estimates of metallization pressures possibly caused by disagreeing pressure calibrations.

Superpositions of spin helices can yield topological spin textures, such as two-dimensional vortices and skyrmions and three-dimensional hedgehogs. Their topological nature and spatial dimensionality depend on the number and relative directions of the constituent helices. This allows mutual transformation between the topological spin textures by controlling the spatial anisotropy. Here, we theoretically study the effect of anisotropy of the magnetic interactions in an effective spin model for chiral magnetic metals. By variational calculations for both triple and quadruple superpositions, we find that the hedgehog lattices, which are stable in the isotropic case, are deformed by the anisotropy and eventually changed into other spin textures with reduced dimension, such as helices and vortices. We also clarify the changes in topological properties by tracing the real-space positions of emergent magnetic monopoles and antimonopoles as well as the emergent magnetic field generated by the noncoplanar spin textures. Our results suggest possible control of the topological spin textures, e.g., by uniaxial pressure and chemical substitution in chiral materials.

Spin wave and magnetic texture are two elementary excitations in magnetic systems, and their interaction leads to rich magnetic phenomena. By describing spin wave and magnetic texture using their own collective coordinates, we find that they interact like classical particles traveling in mutual electromagnetic fields. Based on this unified collective coordinate model, we find that both skew scattering and side jump may occur as spin wave passing through magnetic textures. The skew scattering is associated with the magnetic topology of the texture, while the side jump correlates with the total magnetization of the texture. We illustrate the concepts of skew scattering and side jump by investigating the spin wave trajectories across the topological magnetic skyrmion and the topologically trivial magnetic bubble, respectively.

A method consisting of simultaneous measurements of a Hall effect and a magneto-optic Kerr effect is used to study a topological Hall effect in thin ferromagnetic films. The method is based on the idea that the values of the topological effects are different at dc and optical frequencies. The topological Hall effect in the artificial lattices of magnetic bubbles in nanostructured Co/Pt multilayers with perpendicular magnetic anisotropy is investigated using this method. The Lorentz transmission electron microscopy measurements demonstrate skyrmionic topology of the magnetic bubbles. The measured topological Hall effect is proportional to the skyrmion density in the system.

Two types of domain walls exist in magnetically soft cylindrical nanowires: the transverse-vortex wall (TVW) and the Bloch-point wall (BPW). The latter is expected to prevent the usual Walker breakdown and thus enable high domain wall speed. We showed recently [M. Schöbitz et al., Phys. Rev. Lett. 123, 217201 (2019)] that the previously overlooked Oersted field associated with an electric current is a key in experiments to stabilize the BPW and reach speed above $600\mathrm{m}/\mathrm{s}$ with spin transfer. Here, we investigate in detail this situation with micromagnetic simulations and modeling. The switching of the azimuthal circulation of the BPW to match that of the Oersted field occurs above a threshold current scaling with $1/R^3$ ($R$ is the wire radius), through mechanisms that may involve the nucleation and/or annihilation of Bloch points. The domain wall dynamics then remains of a below-Walker type, with speed largely determined by spin-transfer torque alone.

Quantum chaos imposes universal spectral signatures that govern the thermofield dynamics of a many-body system in isolation. The fidelity between the initial and time-evolving thermofield double states exhibits as a function of time a decay, dip, ramp, and plateau. Sources of decoherence give rise to a nonunitary evolution and result in information loss. Energy dephasing gradually suppresses quantum noise fluctuations and the dip associated with spectral correlations. Decoherence further delays the appearance of the dip and shortens the span of the linear ramp associated with chaotic behavior. The interplay between signatures of quantum chaos and information loss is determined by the competition among the decoherence, dip, and plateau characteristic times, as demonstrated in the stochastic Sachdev-Ye-Kitaev model.

${\mathrm{Fe}}_4{\mathrm{Si}}_2{\mathrm{Sn}}_7{\mathrm{O}}_{16}$ hosts an undistorted kagome lattice of ${\mathrm{Fe}}^{2+}$ ($3d^6, S=2$) ions. We present results of bulk magnetization and Sn nuclear magnetic resonance (NMR) measurements on an oriented ${\mathrm{Fe}}_4{\mathrm{Si}}_2{\mathrm{Sn}}_7{\mathrm{O}}_{16}$ powder sample oriented in geometries parallel ($\parallel$) and perpendicular ($\perp$) to the external applied magnetic field used for orienting the powder ($B_{\text{ori}}$). The bulk susceptibility $\chi$ shows a broad peak at $T_N\sim 3$ K associated with antiferromagnetic ordering. NMR spectra indicate the presence of planar anisotropy in the kagome planes. From an analysis of the static NMR shift ($K$) and dynamic spin-lattice relaxation rate ($1/T_1$) we conclude the presence of dominant magnetic fluctuations in the kagome planes. For the $\parallel$ orientation, $K$ scales linearly with the bulk susceptibility for temperatures down to $\sim 4$ K, while in the $\perp$ orientation $K$ starts to deviate strongly below $T\sim 30$ K. We associate this deviation with the onset of spin-tilting towards the kagome planes. These correlations are also reflected in the $1/T_1$ data for the $\parallel$ orientation, which starts to decrease below $T\sim 30$ K. In this correlated regime, $T_N<T< \sim 30$ K, we discuss the formation of positive chiral spin correlations in the kagome planes.

Vanadium sesquioxide (${\mathrm{V}}_2{\mathrm{O}}_3$) is a Mott insulator exhibiting a temperature-dependent metal-insulator transition (MIT) at 165 K accompanied by both a magnetic and structural transition. Although it is expected to be a metal under conventional band theory, electron interactions at low temperature cause it to behave like an insulator, making it difficult to accurately model its electronic properties with standard ab initio methods. As such, accurate theoretical assessments of the MIT with point defects requires special attention to the type of functionals used. In this study, we conclude that the $\mathrm{PBE}+U$ functional provides the best compromise between accuracy and efficiency in calculating the properties related to the MIT between low-temperature and high-temperature ${\mathrm{V}}_2{\mathrm{O}}_3$. We use this functional to explore the various influences that intrinsic point defects will have on the MIT in ${\mathrm{V}}_2{\mathrm{O}}_3$.

We explore the effect of doping on the correlated electronic structure of strained graphene. It is shown that the interplay between sizable multiorbital Coulomb interactions and electron-hole doping induces an orbital-selective electronic state, characterized by the coexistence of $\pi$-band Dirac-Kondo quasiparticles and emergent $\sigma$-band metallicity. The underlying orbital selectivity in the presence of spin-polarized electron bands, relevant to experiments of strained graphene proximitized to magnetic ions, shows coexistent Mott-localized and semimetal electronic states with, respectively, $\pi$- and $\sigma$-orbital character. Our results provide the theoretical foundations for understanding the intricate and interdependent changes in orbital degrees of freedom in strained carbon-based materials, and they open up an avenue to systematic studies of quantum many-body effects in correlated Dirac fermion systems.

A phenomenon appears in a few examples of the chalcopyrites (space group $I$-42 $d$) where heavier atoms do not necessarily lead to lower lattice thermal conductivity, in contradiction with Keyes expression that formulates an inverse relation of thermal conductivity with mean atomic mass. Herewith, the thermal conductivity of $\mathrm{CuIn}{\mathrm{Se}}_2, \mathrm{CuIn}{\mathrm{Te}}_2, \mathrm{AgIn}{\mathrm{Se}}_2$, and $\mathrm{AgIn}{\mathrm{Te}}_2$ was calculated and compared at room temperature from the linearized Boltzmann transport equation using ab initio density functional theory. $\mathrm{CuIn}{\mathrm{Se}}_2$ and $\mathrm{AgIn}{\mathrm{Se}}_2$ solids exhibit lower lattice thermal conductivity than that of $\mathrm{CuIn}{\mathrm{Te}}_2$ and $\mathrm{AgIn}{\mathrm{Te}}_2$, respectively, despite the fact that Te atoms are significantly heavier than Se. A comparison between dispersion relation, the Grüneisen parameter, and projected density of states leads to the conclusion that anharmonic transverse acoustic modes in the form of anomalous vibrations of Cu and Ag cause the lower values of the thermal conductivity. By analyzing the electronic structure, the compounds under study fit perfectly into a recently defined region of the metavalent bonding well known for its pronounced anharmonicity. The insight gained from the current results deepens our understanding of the unusual heat transfer phenomenon related to the metavalent bonding and sheds light on design and discovery of thermally functional materials that break the prediction by the conventional theory.

Structural phase transitions between semiconductors and topological insulators have rich applications in nanoelectronics but are rarely found in two-dimensional (2D) materials. In this work, by combining ab initio computations and evolutionary structure search, we investigate two stable 2D forms of gold(I) telluride (${\mathrm{Au}}_2\mathrm{Te}$) with square symmetry, noted as s(I)- and s(II)-${\mathrm{Au}}_2\mathrm{Te}$. s(II)-${\mathrm{Au}}_2\mathrm{Te}$ is the global minimum structure and is a room-temperature topological insulator. s(I)-${\mathrm{Au}}_2\mathrm{Te}$ is a direct-gap semiconductor with high carrier mobilities and unusual in-plane negative Poisson's ratio. Both s(I) and s(II) phases have ultralow Young's modulus, implying high flexibility. By applying a small tensile strain, s(II)-${\mathrm{Au}}_2\mathrm{Te}$ can be transformed into s(I)-${\mathrm{Au}}_2\mathrm{Te}$. Hence, a structural phase transition from a room-temperature topological insulator to an auxetic semiconductor is found in the 2D forms of ${\mathrm{Au}}_2\mathrm{Te}$, which enables potential applications in phase-change electronic devices. Moreover, we elucidate the mechanism of the phase transition with the help of phonon spectra and group theory analysis.

Monolayer transition-metal dichalcogenide (TMDC) semiconductors host strongly bound two-dimensional excitonic complexes, and form an excellent platform for probing many-body physics through manipulation of Coulomb interaction. The quantum confined Stark effect is one of the routes to dynamically tune the emission line of these excitonic complexes. In this paper, using a high-quality graphene/hexagonal boron nitride $\left(\mathrm{hBN}\right)/{\mathrm{WS}}_2/\mathrm{hBN}/\mathrm{Au}$ vertical heterojunction, we demonstrate an out-of-plane electric-field driven change in the sign of the Stark shift from blue to red for four different excitonic species, namely, the neutral exciton, the charged exciton (trion), the charged biexciton, and the defect-bound exciton. Such universal nonmonotonic Stark shift with electric field arises from a competition between the conventional quantum confined Stark effect driven redshift and a suppressed binding-energy driven anomalous blueshift of the emission lines, with the latter dominating in the low-field regime. We also find that the encapsulating environment of the monolayer TMDC plays an important role in wave-function spreading, and hence in determining the magnitude of the blue Stark shift. The results for neutral and charged excitonic species are in excellent agreement with calculations from the Bethe-Salpeter equation that use a seven-band per spin tight-binding Hamiltonian. The findings have important implications in probing many-body interaction in two dimensions as well as in developing layered semiconductor based tunable optoelectronic devices.

Nanoscale channels realized at the conducting interface between ${\mathrm{LaAlO}}_3$ and ${\mathrm{SrTiO}}_3$ provide a perfect playground to explore the effect of dimensionality on the electronic properties of complex oxides. Here we compare the electric transport properties of devices realized using the atomic force microscope-writing technique and conventional photolithography. We find that the lateral size of the conducting paths has a strong effect on their transport behavior at low temperature. We observe a crossover from a metallic to an insulating regime occurring at about 50 K for channels narrower than 100 nm. The insulating upturn can be suppressed by the application of a positive backgate. We compare the behavior of nanometric constrictions in lithographically patterned channels with the result of model calculations, and we conclude that the experimental observations are compatible with the physics of a quantum point contact.