In this study, we report results of the high-pressure Hall coefficient $\left(R_H\right)$ measurements in the putative topological Kondo insulator $\mathrm{Sm}{\mathrm{B}}_6$ up to 37 GPa. Below 10 GPa, our data reveal that $R_H\left(T\right)$ exhibits a prominent peak upon cooling below 20 K. Remarkably, the temperature at which surface conduction dominates coincides with the temperature of the peak in $R_H\left(T\right)$. The temperature dependent resistance and Hall coefficient can be well fitted by a two-channel model with contributions from the metallic surface and the thermally activated bulk states. When the bulk of $\mathrm{Sm}{\mathrm{B}}_6$ becomes metallic and magnetic at $\sim 10\mathrm{GPa}$, both the $R_H\left(T\right)$ peak and the resistance plateau disappear simultaneously. Our results indicate that the $R_H\left(T\right)$ peak is a fingerprint to diagnose the presence of a metallic surface state in $\mathrm{Sm}{\mathrm{B}}_6$. The high-pressure magnetic state of $\mathrm{Sm}{\mathrm{B}}_6$ is robust to 180 GPa, and no evidence of superconductivity is observed in the metallic phase.

Optical responses in an excitonic insulating (EI) system with strong electron correlation are studied. We adopt the two-orbital Hubbard model with a finite energy difference between the two orbitals where the spin state degree of freedom exists. This model is analyzed by the variational cluster approach. In order to include the local electron correlation effect, the vertex correction is taken into account in the formulation of the optical conductivity spectra. We calculate a finite-temperature phase diagram in which an EI phase appears between a low-spin band insulating state and a high-spin Mott insulating state. Characteristic components of the optical conductivity spectra consisting of a sharp peak and continuum appear in the EI phase. Integrated intensity almost follows the order parameter of the EI state, suggesting that this component is available to identify the EI phases and transitions.

In this paper, we propose a new topological classification for Zeeman split Fermi surfaces of centrosymmetric metals under external magnetic field, where the Zeeman effect is described by the momentum-dependent $g$-factor tensor. With three important experimental effects, modification of spin-zero effect, Zeeman-effect-induced Fermi surface Chern number, and the in-plane anomalous Hall effect, we reveal that the additional Zeeman-induced Berry curvature plays a crucial role in the electron dynamics for metals under magnetic field, which has been overlooked for decades. By first-principles calculations, we study all these effects on two typical material, ${\mathrm{ZrTe}}_5$ and ${\mathrm{TaAs}}_2$ and the results are in good agreement with the existing experiments.

We present simulated x-ray diffraction patterns (XRD) from molecular dynamics studies of phase transformations in hydrogen at room temperature. Phase changes can be easily identified in simulation, by directly imaging the atoms and measuring correlation functions. We show that the room-temperature XRD patterns for hydrogen phases I, III, IV, and V are very similar. The signatures of the transformations in XRD are weak peaks and superlattice reflections denoting symmetry breaking from the hexagonal-closed-packed (hcp) phase I, and a pronounced change in the $c/a$ ratio. The XRD patterns implied by molecular dynamics calculations are very different from those arising from the static minimum enthalpy structures found by structure searching. Simulations also show that within phase I, the molecules become increasingly confined to the basal plane and suggest the possibility of an unusual critical point terminating the phase I-III boundary line. With these results, we propose a paradigm shift, i.e., that the predictions from density functional theory calculations should be seen as the most likely hypothesis. Specifically, we show that recent experimental results support the picture advanced by molecular dynamics simulations, and are inconsistent with the interpretation of an isostructural hcp transformation.

Martensitic transformations are strain driven displacive transitions governing the mechanical and physical properties in intermetallic materials. This is the case in ${\mathrm{Ni}}_2\mathrm{MnGa}$, where the martensite transition is at the heart of the striking magnetic shape memory and magnetocaloric properties. Interestingly, the martensitic transformation is preceded by a premartensite phase, and the role of this precursor and its influence on the martensitic transition and properties is still a matter of debate. In this work we report on the influence of Co doping (${\mathrm{Ni}}_{50-x}{\mathrm{Co}}_x{\mathrm{Mn}}_{25}{\mathrm{Ga}}_{25}$ with $x=3$ and 5) on the martensitic transformation path in stoichiometric ${\mathrm{Ni}}_2\mathrm{MnGa}$ by neutron diffraction. The use of the superspace formalism to describe the crystal structure of the modulated martensitic phases, joined with a group theoretical analysis, allows unfolding the different distortions featuring the structural transitions. Finally, a general Landau thermodynamic potential of the martensitic transformation, based on the symmetry analysis, is outlined. The combined use of phenomenological and crystallographic studies highlights the close relationship between the lattice distortions at the core of the ${\mathrm{Ni}}_2\mathrm{MnGa}$ physical properties and, more in general, on the properties of the martensitic transformations in the Ni-Mn based Heusler systems.

We explore several classes of quadrupolar ordering in a system of antiferromagnetically coupled quantum spin-1 dimers, which are stacked in the triangular lattice geometry forming a bilayer. Low-energy properties of this model are described by an $\mathcal{S}=1$ hard-core bosonic degrees of freedom defined on each dimer bond, where the singlet and triplet states of the dimerized spins are interpreted as the vacuum and the occupancy of boson, respectively. The number of bosons per dimer and the magnetic and density fluctuations of bosons are controlled by the interdimer Heisenberg interactions. In a solid phase where each dimer hosts one boson and the interdimer interaction is weak, a conventional spin nematic phase is realized by the pair fluctuation of bosons. Larger interdimer interaction favors Bose-Einstein condensates (BEC) carrying quadrupolar moments. Among them, we find one exotic phase where the quadrupoles develop a spatially modulated structure on the top of a uniform BEC, interpreted in the original dimerized spin-1 model as coexistent $p$-type nematic and ${120}^{\circ }$ magnetic correlations. This may explain an intriguing nonmagnetic phase found in ${\mathrm{Ba}}_3{\mathrm{ZnRu}}_2{\mathrm{O}}_9$.

The transition of molecular hydrogen to atomic ionized state with the increase of temperature and pressure poses still unresolved problems for experimental methods and theory. Here we analyze the dynamics of this transition and show its nonequilibrium nonadiabatic character overlooked in both interpreting experimental data and in theoretical models. The nonadiabatic mechanism explains the strong isotopic effect [M. Zaghoo, R. J. Husband, and I. F. Silvera, Phys. Rev. B 98, 104102 (2018).] and the large latent heat [M. Houtput, J. Tempere, and I. F. Silvera, Phys. Rev. B 100, 134106 (2019).] reported recently. We demonstrate the possibility of the formation of intermediate excitonlike molecular states at heating of molecular hydrogen that can explain the puzzling experimental data on reflectivity and conductivity during the insulator-to-metal transition.

Type-III nodal ring associated with a flat band without dispersion, whose nodes are the critical states between type-I and type-II Dirac/Weyl nodes, is of great interest. Here, we propose a strategy to explore and design materials that can realize the type-III nodal-ring phonons by introducing two-dimensional (2D) lattices with ideal flat band. As a concrete example, we show by first-principles calculations that the Laves phase $A{B}_2$ with $C14$ structure possesses ideal type-III nodal-ring phonons. The flat phonon band related to type-III nodal-ring phonons originates from their 2D triangular and kagome layers in the Laves phase $A{B}_2$. The type-III nodal-ring phonons with nontrivial Berry phase lie on a reflection-invariant plane, which are protected by the inversion and time-reversal symmetries. In addition, the drumhead surface states and unique surface arcs are clearly visible, which facilitate experimental observations. Our work enriches the classification of topological quantum phases, and provides a feasible strategy in designing flat-band related topological phases.

We report on the study of the noncentrosymmetric ternary carbide ${\mathrm{YbCoC}}_2$. Our magnetization, specific heat, resistivity, and neutron diffraction measurements consistently show that the system behaves as a heavy-fermion compound, displaying an amplitude-modulated magnetic structure below the Néel temperature reaching $T_{\mathrm{N}}=33$ K under pressure. Such a large value, being the highest among the Yb-based systems, is explained in the light of our ab initio calculations, which show that the $4f$ electronic states of Yb have a dual nature, i.e., due to their strong hybridization with the $3d$ states of Co, $4f$ states expose both localized and itinerant properties.

Using path-integral Monte Carlo sampling and a lattice Wannier function based effective Hamiltonian obtained from first principles, we show that the quantum fluctuations of the nuclei play a central role in the paraelectric phase of ${\text{BaFe}}_{12}{\text{O}}_{19}$ at low temperatures $(T$'s). Contribution from the geometrical frustrations, on the other hand, is negligible. The $T$ dependence of the dielectric function and the displacements of the Fe ions reported in previous experiments were reproduced. The nature of the quantum paraelectrics (QPEs) was assigned to order-disorder or displacive, using the relative magnitudes of the on-site/intersite energies of the electric dipoles, and the quantum fluctuations of the nuclei in the path-integral samplings. This study provides a unified atomic-level picture of QPEs, which could be used to understand different experiments in which new QPE materials were found.

We study the interplay of superconductivity and disorder by solving numerically the Bogoliubov–de Gennes equations in a two-dimensional lattice of size $80\times 80$, which makes investigating the weak-coupling limit possible. In contrast with results in the strong-coupling region, we observe enhancement of superconductivity and intriguing multifractal-like features such as a broad log-normal spatial distribution of the order parameter, a parabolic singularity spectrum, and level statistics consistent with those of a disordered metal at the Anderson transition. The calculation of the superfluid density, including small phase fluctuations, reveals that, despite this intricate spatial structure, phase coherence still holds for sufficiently weak disorder. It breaks down only for stronger disorder but before the insulating transition takes place.

The three-dimensional and temperature-dependent electronic structures of the heavy-fermion superconductor ${\mathrm{CePt}}_2{\mathrm{In}}_7$ are investigated. Angle-resolved photoemission spectroscopy using variable photon energy establishes the existence of quasi-two- and three-dimensional Fermi surface topologies. Temperature-dependent $4d\text{−}4f$ on-resonance photoemission spectroscopies data reveal that heavy quasiparticle bands begin to form at a temperature well above the characteristic (coherence) temperature $T^{*}$. The emergence of low-lying crystal electric field excitation may be responsible for the “relocalization” or the precursor to the establishment of heavy electrons coherence in heavy-fermion compounds. These findings provide critical insight into understanding the hybridization in heavy-fermion systems.

Measurements destroy entanglement. Building on ideas used to study ‘quantum disentangled liquids,’ we explore the use of this effect to characterize states of matter. We focus on systems with multiple components, such as charge and spin in a Hubbard model or local moments and conduction electrons in a Kondo lattice model. In such systems, measurements of (a subset of) one of the components can leave behind a quantum state of the other that is easy to understand, for example in terms of scaling of entanglement entropy of subregions. We bound the outcome of this protocol, for any choice of measurement, in terms of more standard information-theoretic quantities. We apply this quantum disentangling protocol to several problems of physical interest, including gapless topological phases, heavy fermions, and scar states in the Hubbard model.

It is demonstrated that supervised machine learning of local environments of the atoms in a molecule can be very effectively combined with the density functional-based tight-binding method to provide a fast and size-extensive scheme for electronic structure calculations. We train our machine learning model on small and basic molecules and then run it successfully for large and complicated molecules. This facilitates investigations of structural, electronic, and optical properties of large model systems for which the conventional iterative self-consistent procedure becomes too costly. The fruitfulness of this shortsightedness-based scheme is shown for describing the energy landscape of the ground and low-lying excited states of several model molecules of variant sizes and complexity. The achieved accuracy in the tests supports the locality view to the electronic redistribution in a molecule and promises the efficiency of the machine learning equipped divide-and-conquer approach for solving the Schrödinger equation.

Geometrically frustrated quantum impurities coupled to metallic leads have been shown to exhibit rich behavior with a quantum phase transition separating Kondo screened and local moment phases. Frustration in the quantum impurity can alternatively be introduced via Kitaev couplings between different spins of the impurity cluster. We use the numerical renormalization group (NRG) to study a range of systems where the quantum impurity comprising a Kitaev cluster is coupled to a bath of noninteracting fermions. The models exhibit a competition between Kitaev and Kondo dominated physics depending on whether the Kitaev couplings are greater or less than the Kondo temperature. We characterize the ground-state properties of the system and determine the temperature dependence of the crossover scale for the emergence of fractionalized degrees of freedom in the model. We also demonstrate qualitatively as well as quantitatively that in the Kondo limit, the complex impurity can be mapped to an effective two-impurity system, where the emergent spin $1/2$ comprises of both Majorana and flux degrees of freedom. For a tetrahedral-shaped Kitaev cluster, an extra orbital degree of freedom closely related to a flux degree of freedom remains unscreened even in the presence of both Heisenberg and Kondo interactions.

We report on a theoretical study on the rise of strong peaks at the harmonics of the cyclotron resonance in the irradiated magnetoresistance in ultraclean two-dimensional electron systems. The motivation is the experimental observation of a totally unexpected strong resistance peak showing up at the second harmonic. We extend the radiation-driven electron orbit model (previously developed to study photocurrent oscillations and zero resistance states) to an ultraclean scenario that implies a longer scattering time and longer mean free path. Thus, when the mean free path is equivalent, in terms of energy, to twice the cyclotron energy ($2\hslash w_c$), the electron behaves as under an effective magnetic field that is twice the one actually applied. Then, at high radiation power and/or low temperature, a resistance spike can be observed at the second harmonic. For even cleaner samples the energy distance could increase to three or four times the cyclotron energy giving rise to resistance peaks at higher harmonics (third, fourth, etc.), i.e., a resonance peak shift to lower magnetic fields as the quality of the sample increases. Thus, by selecting the sample mobility, one automatically would select the radiation resonance response without altering the radiation frequency.

Interaction between adsorbed hydrogen atoms in graphene is studied using a combination of DFT and the path integral formalism. Our results reveal a complex nonmonotonic interaction profile. We show that the strength and sign of the interaction are dictated by the adsorbate arrangement, as well as the system doping. The path integral approach given here allows one to compute energies and densities in an efficient manner without relying on exact diagonalization.

Carrying information using generation and detection of the orbital current, instead of the spin current, is an emerging field of research, where the orbital Hall effect (OHE) is an important ingredient. Here, we propose a mechanism of the OHE that occurs in noncentrosymmetric materials. We show that the broken inversion symmetry in the two-dimensional transition metal dichalcogenides (TMDCs) causes a robust orbital moment, which flows in different directions due to the opposite Berry curvatures under an applied electric field, leading to a large OHE. This is in complete contrast to the inversion-symmetric systems, where the orbital moment is induced only by the external electric field. We show that the valley-orbital locking as well as the OHE both appear even in the absence of the spin-orbit coupling. The nonzero spin-orbit coupling leads to the well-known valley-spin locking and the spin Hall effect, which we find to be weak, making the TMDCs particularly suitable for direct observation of the OHE, with potential application in orbitronics.

We discover an $e_g$-orbital ($d_{z^2},d_{x^2-y^2}$) model within the diamond lattice ($e_g$-diamond model) that hosts novel topological states. Specifically, the $e_g$-diamond model yields a three-dimensional (3D) nodal cage (3D-NC), which is characterized by a $d-d$ band inversion protected by two types of degenerate states (i.e., $e_g$-orbital and diamond-sublattice degeneracies). We demonstrate materials realization of this model in the well-known spinel compounds ($A{B}_2{X}_4$), where the tetrahedron-site cations ($A$) form the diamond sublattice. An ideal half metal with one metallic spin channel formed by well-isolated and half-filled $e_g$-diamond bands, accompanied by a large spin gap (4.36 eV) is discovered in one 4-2 spinel compound (${\mathrm{VMg}}_2{\mathrm{O}}_4$), which becomes a magnetic Weyl semimetal when spin-orbit coupling effect is further considered. Our discovery greatly enriches the physics of diamond structure and spinel compounds, opening a door to their application in spintronics.