Significant advances in the use of atomistic simulation techniques, such as ab initio density functional theory and the molecular dynamics method, made it possible to predictively calculate properties of materials. In parallel, low-energy neutron scattering instruments and data analysis tools available in different institutes become mature for providing high-quality data for experimental validation purposes. Despite such experimental and theoretical improvements, the accurate modeling of experimental neutron-weighted multiphonon spectra for $\mathrm{U}{\mathrm{O}}_2$ over a broad temperature range still remains an issue. Combining prior phonon density of states (PDOS) from density functional theory and Monte Carlo inelastic neutron scattering calculations in a Bayesian fitting procedure is a valuable alternative approach to assess the partial PDOS of uranium and oxygen in $\mathrm{U}{\mathrm{O}}_2$ from 294 to 1675 K for improving the comparison to experiment and exploring the first-principles calculation hypothesis.

We investigate the interplay of Kondo and Ruderman-Kittel-Kasuya-Yosida coupling in the presence of disorder by chemically substituting local moment ${\mathrm{Cr}}^{3+}$ at the Ir sublattice in the metallic ${\mathrm{Pr}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$. We find evidence of non-Fermi liquid behavior in the transport and thermodynamic measurements at low temperature. Specifically, the magnetic susceptibility exhibits power law divergence at $T=0$. The nonanalytic temperature and magnetic-field dependence of magnetic susceptibility and the associated scaling suggest the existence of a two-fluid system consisting of a Kondo-screened paramagnetic metal coexisting with magnetically ordered rare regions dominated by interimpurity interaction, similar to the quantum critical Griffiths phase.

Motivated by recent technical developments on selective intercalation of inert gases just beneath the surface graphene of graphite, we perform path-integral Monte Carlo calculations to study structural and superfluid properties of ${}^4\mathrm{He}$ clusters encapsulated between a bulged surface graphene layer and the graphite (0001) substrate. The intercalated ${}^4\mathrm{He}$ atoms initially decorate the inner surface of the bulged graphene to form a shell structure, leaving a void inside it. Then, the additional ${}^4\mathrm{He}$ atoms form a disk-shaped liquid platelet in the void, which shows finite superfluid response at temperatures below 1 K. The temperature-dependent superfluid fractions of the platelet follow modified Kosterlitz-Thouless recursion relations, indicating that two-dimensional superfluidity can be realized in an intercalated ${}^4\mathrm{He}$ system.

Higher-fold chiral fermions that go beyond twofold Weyl fermions have recently been reported in crystalline systems. Here, we focus on such excitations in several binary, ternary, and quaternary alloys/compounds with CoGe, BiSbPt, and ${\mathrm{KMgBO}}_3$ as the representative examples that belong to the crystal space group 198. We found distinct threefold, fourfold, and sixfold chiral fermions in the bulk via density-functional computations. We provide general symmetry arguments for the protection of these degeneracies with special emphasis on the fourfold fermions. Our surface spectra simulations show that the size of Fermi arcs resulting from these chiral fermions are large, robust, and untouched from the bulk states due to the near absence of trivial bulk Fermi pockets. All these features make these systems—especially CoGe and ${\mathrm{KMgBO}}_3$—promising topological semimetal candidates to realize higher-fold fermions in future photoemission and transport experiments.

Quantum Monte Carlo (QMC) methods are useful for studies of strongly correlated materials because they are many body in nature and use the physical Hamiltonian. Typical calculations assume as a starting point a wave function constructed from single-particle orbitals obtained from one-body methods, e.g., density functional theory. However, mean-field-derived wave functions can sometimes lead to systematic QMC biases if the mean-field result poorly describes the true ground state. Here, we study the accuracy and flexibility of QMC trial wave functions using variational and fixed-node diffusion QMC estimates of the total spin density and lattice distortion of antiferromagnetic iron oxide (FeO) in the ground state $B1$ crystal structure. We found that for relatively simple wave functions the predicted lattice distortion was controlled by the choice of single-particle orbitals used to construct the wave function, rather than by subsequent wave function optimization techniques within QMC. By optimizing the orbitals with QMC, we then demonstrate starting-point independence of the trial wave function with respect to the method by which the orbitals were constructed by demonstrating convergence of the energy, spin density, and predicted lattice distortion for two qualitatively different sets of orbitals. The results suggest that orbital optimization is a promising method for accurate many-body calculations of strongly correlated condensed phases.

We use determinant quantum Monte Carlo simulations to study the role of electron-electron interactions on three-dimensional (3D) Dirac fermions based on the $\pi$-flux model on a cubic lattice. We show that the Hubbard interaction drives the 3D Dirac semimetal to an antiferromagnetic (AF) insulator only above a finite critical interaction strength and the long-range AF order persists up to a finite temperature. We evaluate the critical interaction strength and temperatures using finite-size scaling of the spin structure factor. The critical behaviors are consistent with the (3+1)-dimensional Gross-Neveu universality class for the quantum critical point and 3D Heisenberg universality class for the thermal phase transitions. We further investigate correlation effects in the birefringent Dirac fermion system. It is found that the critical interaction strength $U_c$ is decreased by reducing the velocity of the Dirac cone, quantifying the effect of velocity on the critical interaction strength in 3D Dirac fermion systems. Our findings unambiguously uncover correlation effects in 3D Dirac fermions and may be observed using ultracold atoms in an optical lattice.

For a particle confined to the two-dimensional helical surface embedded in four-dimensional (4D) Euclidean space, the effective Hamiltonian is deduced in the thin-layer quantization formalism. We find that the gauge structure of the effective dynamics is determined by torsion, which plays the role of U(1) gauge potential, and find that the topological structure of associated states is defined by orbital spin which originates from 4D space. Strikingly, the response to torsion contributes a quantum Hall effect, and the response to the deformation of torsion contributes Hall viscosity that is perfectly presented as a simultaneous occurrence of multiple channels for the quantum Hall effect. This result directly provides a way to probe Hall viscosity.

We study the nonlinear thermoelectric cooling performance of a quantum spin Hall system. The setup consists of a nanomagnet contacting a Kramers pair of helical edge states, resulting in a transmission probability with a rich structure containing peaks, well-type, and step-type features. We present a detailed analysis of the impact of all these features on the cooling performance, based to a large extent on analytical results. We analyze the cooling power as well as the coefficient of performance of the device. Since the basic features we study may be present in the transmission function of other mesoscopic conductors, our conclusions provide useful insights to analyze the nonlinear thermoelectric behavior of a wide class of quantum devices. The combination of all these properties defines the response of the quantum spin Hall setup, for which we provide some realistic estimates for the conditions limiting and optimizing its operation as a cooling device.

By means of magnetic, specific heat, and pyroelectric measurements, we report on magnetic ferroelectricity in the quadruple perovskite $\mathrm{Na}{\mathrm{Mn}}_7{\mathrm{O}}_{12}$, characterized by a canted antiferromagnetic (AFM) CE structure. Surprisingly, ferroelectricity is concomitant to a dramatic broadening of the magnetic hysteresis loop, well below the AFM ordering temperature. This unconventional behavior shows that the formation of ferroelectric domains is induced by the symmetric exchange interaction in the local scale, e.g., at magnetic domain boundaries or defects. The value of electric polarization, $P=0.027\mu \mathrm{C}{\mathrm{cm}}^{-2}$, measured in polycrystalline samples is comparatively large as compared to other magnetic multiferroics, suggesting that the above scenario is promising indeed for the rational design of practical multiferroic materials.

In this study, we report on the observation of de Haas–van Alphen–type quantum oscillations (QOs) in the ultrasound velocity of NbP as well as “giant QOs” in the ultrasound attenuation in pulsed magnetic fields. The difference in the QO amplitude for different acoustic modes reveals a strong anisotropy of the effective deformation potential, which we estimate to be as high as $9\mathrm{eV}$ for certain parts of the Fermi surface. Furthermore, the natural filtering of QO frequencies and the tracing of the individual Landau levels to the quantum limit allows for a more detailed investigation of the Fermi surface of NbP, as was previously achieved by means of analyzing QOs observed in magnetization or electrical resistivity.

The discovery of two-dimensional (2D) materials possessing switchable spontaneous polarization with atomic thickness opens up exciting opportunities to realize ultrathin, high-density electronic devices with potential applications ranging from memories and sensors to photocatalysis and solar cells. First-principles methods based on density functional theory (DFT) have facilitated the discovery and design of 2D ferroelectrics (FEs). However, DFT calculations employing local and semilocal exchange-correlation functionals failed to predict accurately the band gaps for this family of low dimensional materials. Here, we present a $\mathrm{DFT}+U+V$ study on 2D FEs represented by $\alpha \text{−}{\mathrm{In}}_2{\mathrm{Se}}_3$ and its homologous ${\mathrm{III}}_2\text{−}{\mathrm{VI}}_3$ compounds with both out-of-plane and in-plane polarization, using Hubbard parameters computed from first principles. We find that ACBN0, a pseudohybrid density functional that allows self-consistent determination of $U$ parameters, improves the prediction of band gaps for all investigated 2D FEs with a computational cost much lower than the Heyd-Scuseria-Ernzerhof hybrid density functional. The intersite Coulomb interaction $V$ becomes critical for accurate descriptions of the electronic structures of van der Waals heterostructures such as bilayer ${\mathrm{In}}_2{\mathrm{Se}}_3$ and ${\mathrm{In}}_2{\mathrm{Se}}_3$/InTe. Pertinent to the study of FE-based catalysis, we find that the application of self-consistent $U$ corrections can strongly affect the adsorption energies of open-shell molecules on the polar surfaces of 2D FEs.

Spatially structured light fields applied to semiconductor quantum dots yield fundamentally different absorption spectra than homogeneous beams. In this paper, we provide a detailed theoretical discussion of the resulting spectra for different light beams using a cylindrical multipole expansion. For the description of the quantum dots we employ a model based on the envelope function approximation including Coulomb interaction and valence band mixing. The combination of a single spatially structured light beam and state mixing allows all exciton states in the quantum dot to become optically addressable. Furthermore, we demonstrate that the beams can be tailored such that single states are selectively excited, without the need of spectral separation. Using this selectivity, we propose a method to measure the exciton wave function of the quantum dot eigenstate. The measurement goes beyond electron density measurements by revealing the spatial phase information of the exciton wave function. Such an extraction of phase information is known from polarization-sensitive measurements; however, here the infinitely large spatial degree of freedom can be accessed by the beam profile in addition to the two-dimensional polarization degree of freedom.

As an allotrope of black phosphorus, two-dimensional blue phosphorus (2D bP) has gained significant attention since the first realization of its growth through molecular beam epitaxy in 2016. In this paper, self-encapsulated blue phosphorus nanoscrolls (bPNSs) formed by rolling up 2D bP are found to be energetically favorable at room temperature, based on ab initio calculations. Our results demonstrate that these quasi-one-dimensional bPNSs are all semiconductors with wide-range band gaps varying from 0.02 to 1.8 eV for different scroll thicknesses and rolling directions. The electron mobility reaches as high as 6620 ${\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ in armchair double-layer bPNSs, which is approximately 20-fold greater than that of a 2D bP monolayer. This study discovers the bPNSs as another class of semiconductors and proposes the scrolling strategy as an excellent candidate for the application of other 2D materials.

Using high-energy x-ray scattering and large-scale three-dimensional (3D) structure modeling, we investigate the relationship between the crystal lattice, charge density wave (CDW), and superconducting (SC) orders in transition metal dichalcogenides (TMDs). In particular, we systematically substitute Te for Se in Ta-Se-Te solid solutions, determine changes in their crystal lattice, and relate them to changes in the CDW transition temperature, $T_{\mathrm{CDW}}$, and SC critical temperature, $T_{\mathrm{c}}$. We find that strong lattice distortions such as buckling of Ta layers are detrimental to the CDW and SC orders. The presence of a perfect lattice order in two dimensions is a prerequisite to the emergence of CDWs but insufficient to achieve a SC ordered state. For the SC order to emerge, the Ta sublattice should also appear periodic in 3D. Local chemical disorder may promote the SC order, and the perfectness of Ta coordination polyhedra is a factor contributing to its strength. A hierarchical relationship among the crystal lattice, CDW, and SC orders thus appears to exist in TMDs in a sense that different degrees of crystal lattice order-disorder promote and maintain the CDW and SC orders to a different extent, offering an opportunity to control the latter through modifying the former by rational design. Our findings are a step towards a better understanding of the correlation between lattice and electronic degrees of freedom in TMDs. We also demonstrate an efficient experimental approach to study them in fine detail.

We predict a potassium-nickel intermetallic compound ${\mathrm{K}}_2\mathrm{Ni}$ at high pressure and identify it as the long-sought structure of the only known K-Ni compound to date [Parker et al., Science 273, 95 (1996)]. Although both constituent elements are metallic, ${\mathrm{K}}_2\mathrm{Ni}$ exhibits a semiconducting ground state with an indirect band gap of 0.65 eV. Electron instability due to the degeneracy at the Fermi level arises from the particular motif of the structure, which in turn induces symmetry-breaking Peierls distortion and a nonmetallic ground state. The results indicate that the chemical properties of elements can change dramatically under extreme conditions and have significant implications for the postulation that potassium is incorporated in Earth's core.

The discovery of topological features of quantum states plays an important role in modern condensed matter physics and various artificial systems. Due to the absence of local order parameters, the detection of topological quantum phase transitions remains a challenge. Machine learning may provide effective methods for identifying topological features. In this work we show that the unsupervised manifold learning can successfully retrieve topological quantum phase transitions in momentum and real space. Our results show that the Chebyshev distance between two data points sharpens the characteristic features of topological quantum phase transitions in momentum space, while the widely used Euclidean distance is in general suboptimal. Then a diffusion map or isometric map can be applied to implement the dimensionality reduction, and to learn about topological quantum phase transitions in an unsupervised manner. We demonstrate this method on the prototypical Su-Schrieffer-Heeger (SSH) model, the Qi-Wu-Zhang (QWZ) model, and the quenched SSH model in momentum space, and further provide implications and demonstrations for learning in real space, where the topological invariants could be unknown or hard to compute. The interpretable good performance of our approach shows the capability of manifold learning, when equipped with a suitable distance metric, in exploring topological quantum phase transitions.

Recently, the niobium (Nb) doped topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, in which the finite magnetic moments of the Nb atoms are intercalated in the van der Waals gap between the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ layers, has been shown to exhibit both superconductivity with $T_c\simeq 3$ K and topological surface states. Here we report on muon spin rotation experiments probing the temperature and field dependence of effective magnetic penetration depth ${\lambda }_{\mathrm{eff}}\left(T\right)$ in the layered topological superconductor candidate ${\mathrm{Nb}}_{0.25}{\mathrm{Bi}}_2{\mathrm{Se}}_3$. The exponential temperature dependence of ${\lambda }_{\mathrm{eff}}^{-2}\left(T\right)$ at low temperatures suggests a fully gapped superconducting state in the bulk with the superconducting transition temperature $T_c=2.9\mathrm{K}$ and the gap to $T_c$ ratio $2\mathrm{\Delta }/k_B{T}_c=3.95\left(19\right)$. We also reveal that the ratio $T_c/{\lambda }_{\mathrm{eff}}^{-2}$ is comparable to those of unconventional superconductors, which hints at an unconventional pairing mechanism. Furthermore, time-reversal symmetry breaking was excluded in the superconducting state with sensitive zero-field $\mu \mathrm{SR}$ experiments. We hope the present results will stimulate theoretical investigations to obtain a microscopic understanding of the relation between superconductivity and the topologically nontrivial electronic structure of ${\mathrm{Nb}}_{0.25}{\mathrm{Bi}}_2{\mathrm{Se}}_3$.

The anomalous proximity effect in dirty superconducting junctions is one of most striking phenomena highlighting the profound nature of Majorana bound states and odd-frequency Cooper pairs in topological superconductors. Motivated by the recent experimental realization of planar topological Josephson junctions, we describe the anomalous proximity effect in a superconductor/semiconductor hybrid, where an additional dirty normal-metal segment is extended from a topological Josephson junction. The topological phase transition in the topological Josephson junction is accompanied by a drastic change in the low-energy transport properties of the attached dirty normal metal. The quantization of the zero-bias differential conductance, which appears only in the topologically nontrivial phase, is caused by the penetration of the Majorana bound states and odd-frequency Cooper pairs into a dirty normal-metal segment. As a consequence, we propose a practical experiment for observing the anomalous proximity effect.

We investigate both pure and mixed states Floquet dynamical quantum phase transition (DQPT) in the periodically time-dependent extended XY model. We exactly show that the proposed Floquet Hamiltonian of interacting spins can be expressed as a sum of noninteracting quasispins imposed by an effective time dependent magnetic field (Schwinger-Rabi model). The calculated Chern number indicates that there is a topological transition from the nonadiabatic to adiabatic regime. In the adiabatic regime, the quasispins trace the time dependent effective magnetic field and then oscillate between spin up and down states. While in the nonadiabatic regime, the quasispins cannot follow the time dependent effective magnetic field and feel an average magnetic field. We find the range of driving frequency over which the quasispins experience adiabatic cyclic processes. Moreover, we obtain the exact expression of the Loschmidt amplitude and generalized Loschmidt amplitude of the proposed Floquet system. The results represent that both pure and mixed states dynamical phase transition occurs when the system evolves adiabatically. In other words, the minimum required driving frequency for the appearance of Floquet DQPT is equal to the threshold frequency needed for transition from the nonadiabatic to adiabatic regime.

In two-dimensional (2D) magnetic systems, large magnetic anisotropy is needed to stabilize the magnetic order according to Mermin-Wagner theorem. Based on density-functional theory (DFT) calculations, we propose that the magnetic anisotropic energy (MAE) of 2D ferromagnetic (FM) semiconductors can be strongly enhanced in van der Waals heterostructures by attaching a nonmagnetic semiconductor monolayer with large spin-orbit coupling. We studied ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6/{\mathrm{PtSe}}_2$ bilayer heterostructures, where each layer has been realized in recent experiments. The DFT calculations show that the MAE of ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6/{\mathrm{PtSe}}_2$ is enhanced by 70%, and the Curie temperature $T_C$ is increased far beyond room temperature. A model Hamiltonian is suggested to analyze the DFT results, showing that both the Dzyaloshinskii-Moriya interaction and single-ion anisotropy contribute to the enhancement of the MAE. Based on the superexchange picture, we find that the decreased energy difference between $3d$ orbitals of Cr and $5p$ orbitals of Te contributes partially to the increase of $T_C$. Our present work indicates a promising way to enhance the MAE and $T_C$ by constructing van der Waals semiconductor heterostructures, which will inspire further studies on the 2D magnetic semiconductor systems.