We investigate the electronic structure of ${\mathrm{SrMnO}}_3$ with density functional theory plus dynamical mean-field theory (DMFT). Within this scheme the selection of the correlated subspace and the construction of the corresponding Wannier functions is a crucial step. Due to the crystal-field splitting of the Mn-$3d$ orbitals and their separation from the $\mathrm{O}\text{−}2p$ bands, ${\mathrm{SrMnO}}_3$ is a material where on first sight a three-band $d$-only model should be sufficient. However, in the present work we demonstrate that the resulting spectrum is considerably influenced by the number of correlated orbitals and the number of bands included in the Wannier function construction. For example, in a $d\text{−}dp$ model we observe a splitting of the $t_{2g}$ lower Hubbard band into a more complex spectral structure, not observable in $d$-only models. To illustrate these high-frequency differences we employ the recently developed fork tensor product state (FTPS) impurity solver, as it provides the necessary spectral resolution on the real-frequency axis. We find that the spectral structure of a five-band $d\text{−}dp$ model is in good agreement with PES and XAS experiments. Our results demonstrate that the FTPS solver is capable of performing full five-band DMFT calculations directly on the real-frequency axis.

In this paper, the well-known Silbey-Harris (SH) polaron ansatz for the spin-boson model is improved by adding orthogonal displaced Fock states. The obtained results for the ground state in all baths converge very quickly within finite displaced Fock states and corresponding SH results are corrected considerably. Especially for the sub-Ohmic spin-boson model, the converging results are obtained for $0<s<1/2$ in the fourth-order correction and very accurate critical coupling strengths of the quantum phase transition are achieved. Converging magnetization in the biased spin-boson model is also arrived at. Since the present improved SH ansatz can yield very accurate, even almost exact results, it should have wide applications and extensions in various spin-boson model and related fields.

Chemical doping is a reliable method of modification of the electronic properties of transition metal compounds. In manganese perovskites, it leads to charge transfer and peculiar ordering phenomena. However, depending on the interplay of the local crystal structure and electronic properties, synthesis of stable compounds in the entire doping range is often impossible. Here, we show results of high-energy resolution x-ray absorption and emission spectroscopies on a ${\mathrm{La}}_{2-2x}{\mathrm{Sr}}_{1+2x}{\mathrm{Mn}}_2{\mathrm{O}}_7$ family of bilayered manganites in a broad doping range $\left(0.5\le x\le 1\right)$. We established a relation between local Mn charge and Mn-O distances as a function of doping. Based on a comparison of such relation with other manganites, we suggest why stable structures cannot be realized for certain doping levels of bilayered compounds.

Simulations in the warm dense matter regime using finite temperature Kohn-Sham density functional theory (FT-KS-DFT), while frequently used, are computationally expensive due to the partial occupation of a very large number of high-energy KS eigenstates which are obtained from subspace diagonalization. We have developed a stochastic method for applying FT-KS-DFT, that overcomes the bottleneck of calculating the occupied KS orbitals by directly obtaining the density from the KS Hamiltonian. The proposed algorithm scales as $O\left(N{T}^{-1}\right)$ and is compared with the high-temperature limit scaling $O\left(N^3{T}^3\right)$ of the deterministic approach, where $N$ is the system size (number of electrons, volume, etc.) and $T$ is the temperature. The method has been implemented in a plane-waves code within the local density approximation (LDA); we demonstrate its efficiency, statistical errors, and bias in the estimation of the free energy per electron for a diamond structure silicon. The bias is small compared to the fluctuations and is independent of system size. In addition to calculating the free energy itself, one can also use the method to calculate its derivatives and obtain the equations of state.

A detailed analysis of the Kondo effect of a magnetic impurity in a zigzag graphene nanoribbon is addressed. An adatom is coupled to the graphene nanoribbon via a hybridization amplitude ${\mathrm{\Gamma }}_{\mathrm{imp}}$ in a hollow- or top-site configuration. In addition, the adatom is also weakly coupled to a metallic scanning tunnel microscope (STM) tip by a hybridization function ${\mathrm{\Gamma }}_{\mathrm{tip}}$ that provides a Kondo screening of its magnetic moment. The entire system is described by an Anderson-like Hamiltonian whose low-temperature physics is accessed by employing the numerical renormalization-group approach, which allows us to obtain the thermodynamic properties used to compute the Kondo temperature of the system. We find two screening regimes when the adatom is close to the edge of the zigzag graphene nanoribbon: (1) a weak-coupling regime $\left({\mathrm{\Gamma }}_{\mathrm{imp}}\ll {\mathrm{\Gamma }}_{\mathrm{tip}}\right)$, in which the edge states produce an enhancement of the Kondo temperature $T_K$, and (2) a strong-coupling regime $\left({\mathrm{\Gamma }}_{\mathrm{imp}}\gg {\mathrm{\Gamma }}_{\mathrm{tip}}\right)$, in which a local singlet is formed, to the detriment of the Kondo screening by the STM tip. These two regimes can be clearly distinguished by the dependence of their characteristic temperature $T^{*}$ on the coupling between the adatom and the carbon sites of the graphene nanoribbon $V_{\mathrm{imp}}$. We observe that in the weak-coupling regime $T^{*}$ increases exponentially with $V_{\mathrm{imp}}^2$. Differently, in the strong-coupling regime, $T^{*}$ increases linearly with $V_{\mathrm{imp}}^2$.

The investigation and control of quantum degrees of freedom (DoFs) of carriers lie at the heart of condensed-matter physics and next-generation electronics/optoelectronics. van der Waals heterostructures stacked from distinct two-dimensional (2D) crystals offer an unprecedented platform for combining the superior properties of individual 2D materials and manipulating spin, layer, and valley DoFs. $\mathrm{Mo}{\mathrm{S}}_2$/graphene heterostructures, harboring prominent spin-transport properties of graphene, giant spin-orbit coupling, and spin-valley polarization of $\mathrm{Mo}{\mathrm{S}}_2$, are predicted as a perfect venue for optospintronics. Here, we report the epitaxial growth of commensurate $\mathrm{Mo}{\mathrm{S}}_2$ on graphene with high quality by chemical vapor deposition, and demonstrate robust temperature-independent spin-valley polarization at off-resonant excitation. We further show that the helicity of B exciton is larger than that of A exciton, allowing the manipulation of spin bits in the commensurate heterostructures by both optical helicity and wavelength. Our results open a window for controlling spin DoF by light and pave a way for taking spin qubits as information carriers in the next-generation valley-controlled optospintronics.

A three-dimensional Dirac semimetal has bulk Dirac cones in all three momentum directions and Fermi arc like surface states, and can be converted into a Weyl semimetal by breaking time-reversal symmetry. However, the highly conductive bulk state usually hides the electronic transport from the surface state in Dirac semimetal. Here, we demonstrate the supercurrent carried by bulk and surface states in $\mathrm{Nb}-{\mathrm{Cd}}_3{\mathrm{As}}_2$ nanowire-Nb short and long junctions, respectively. For the $\sim 1-\mu \mathrm{m}$-long junction, the Fabry-Pérot interferences-induced oscillations of the critical supercurrent are observed, suggesting the ballistic transport of the surface states carried supercurrent, where the bulk states are decoherent and the topologically protected surface states still stay coherent. Moreover, a superconducting dome is observed in the long junction, which is attributed to the enhanced dephasing from the interaction between surface and bulk states as tuning gate voltage to increase the carrier density. The superconductivity of topological semimetal nanowire is promising for braiding of Majorana fermions toward topological quantum computing.

We probe the accuracy limit of ab initio calculations of carrier mobilities in semiconductors, within the framework of the Boltzmann transport equation. By focusing on the paradigmatic case of silicon, we show that fully predictive calculations of electron and hole mobilities require many-body quasiparticle corrections to band structures and electron-phonon matrix elements, the inclusion of spin-orbit coupling, and an extremely fine sampling of inelastic scattering processes in momentum space. By considering all these factors we obtain excellent agreement with experiment, and we identify the band effective masses as the most critical parameters to achieve predictive accuracy. Our findings set a blueprint for future calculations of carrier mobilities, and pave the way to engineering transport properties in semiconductors by design.

Recent ultrafast magnetic-sensitive measurements [Johnson et al., Phys. Rev. B 92, 184429 (2015); Bothschafter et al., Phys. Rev. B 96, 184414 (2017)] have revealed a delayed melting of the long-range cycloid spin order in ${\mathrm{TbMnO}}_3$ following photoexcitation across the fundamental Mott-Hubbard gap. The microscopic mechanism behind this slow transfer of energy from the photoexcited carriers to the spin degrees of freedom is still elusive and not understood. Here, we address this problem by combining spectroscopic ellipsometry, ultrafast broadband optical spectroscopy, and ab initio calculations. Upon photoexcitation, we observe the emergence of a complex collective response, which is due to high-energy coherent optical phonons coupled to the out-of-equilibrium charge density. This response precedes the magnetic order melting and is interpreted as the fingerprint of the formation of anti-Jahn-Teller polarons. We propose that the charge localization in a long-lived self-trapped state hinders the emission of magnons and other spin-flip mechanisms, causing the energy transfer from the charge to the spin system to be mediated by the reorganization of the lattice. Furthermore, we provide evidence for the coherent excitation of a phonon mode associated with the ferroelectric phase transition.

Molecular hydrogen trapped within {111}-oriented platelets in silicon is studied by means of Raman scattering and first principles theory. The rotational transition $S_0\left(0\right)$ ($J=0\to J=2$) of para-${\mathrm{H}}_2$ (nuclear spin $I=0$) at 353 ${\mathrm{cm}}^{-1}$ is used as a probe. We find that for temperatures below 100 K the $S_0\left(0\right)$ Raman line starts to broaden asymmetrically, which is interpreted as the onset of a phase transition from a state with a short-range order (“gaseous” or “liquid” phase) to a two-dimensional molecular crystal lying in the {111} plane of silicon. The shape of the $S_0\left(0\right)$ line at helium temperatures strongly depends on the relative content of ortho- (nuclear spin $I=1$) and para-${\mathrm{H}}_2$ revealing the details of the intermolecular interaction. A comprehensive theoretical analysis based on ab initio calculations, molecular dynamics simulations, and rotational spectra modeling reveals that the phase transition to the crystalline state of the two-dimensional hydrogen does occur at temperatures substantially higher compared to those of bulk ${\mathrm{H}}_2$.

We investigate topological states of two-dimensional (2D) triangular lattices with multiorbitals. Tight-binding model calculations of a 2D triangular lattice based on $p_x$ and $p_y$ orbitals exhibit very interesting doubly degenerate energy points at different positions $(\mathrm{\Gamma }$ and $K/K^{\prime })$ in momentum space, with quadratic non-Dirac and linear Dirac band dispersions, respectively. Counterintuitively, the system shows a global topologically trivial rather than nontrivial state with consideration of spin-orbit coupling due to the “destructive interference effect” between the topological states at the $\mathrm{\Gamma }$ and $K/K^{\prime }$ points. The topologically nontrivial state can emerge by introducing another set of triangular lattices to the system (bitriangular lattices) due to the breakdown of the interference effect. With first-principles calculations, we predict an intrinsic Chern insulating behavior (quantum anomalous Hall effect) in a family of the 2D triangular lattice metal-organic framework of $\text{Co}\left({\mathrm{C}}_{21}{\mathrm{N}}_3{\mathrm{H}}_{15}\right)$ (TPyB-Co) from this scheme. Our results provide a different path and theoretical guidance for the search for and design of new 2D topological quantum materials.

Most of the group IV, III-V, and II-VI compounds crystallize in semiconductor structures under ambient conditions. Upon application of pressure, they undergo structural phase transitions to more closely packed structures, sometimes metallic phases. We have performed density functional calculations using projector augmented wave (PAW) pseudopotentials to determine the transition pressures for these transitions within the local density approximation (LDA), the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA), and the strongly constrained and appropriately normed (SCAN) meta-GGA. LDA underestimates the transition pressure for most of the studied materials. PBE under- or overestimates in many cases. SCAN typically corrects the errors of LDA and PBE for the transition pressure. The accuracy of SCAN is comparable to that of computationally expensive methods like the hybrid functional HSE06, the random phase approximation (RPA), and quantum Monte Carlo (QMC), in cases where calculations with these methods have been reported, but at a more modest computational cost. The improvement from LDA to PBE to SCAN is especially clearcut and dramatic for covalent semiconductor-metal transitions, as for Si and Ge, where it reflects the increasing relative stabilization of the covalent semiconducting phases under increasing functional sophistication.

Molecular dynamics (MD) simulations play an important role in studying heat transport in complex materials. The lattice thermal conductivity can be computed either using the Green-Kubo formula in equilibrium MD (EMD) simulations or using Fourier's law in nonequilibrium MD (NEMD) simulations. These two methods have not been systematically compared for materials with different dimensions and inconsistencies between them have been occasionally reported in the literature. Here we give an in-depth comparison of them in terms of heat transport in three allotropes of Si: three-dimensional bulk silicon, two-dimensional silicene, and quasi-one-dimensional silicon nanowire. By multiplying the correlation time in the Green-Kubo formula with an appropriate effective group velocity, we can express the running thermal conductivity in the EMD method as a function of an effective length and directly compare it to the length-dependent thermal conductivity in the NEMD method. We find that the two methods quantitatively agree with each other for all the systems studied, firmly establishing their equivalence in computing thermal conductivity.

The combination of strong electron correlations in $3d$ transition-metal oxides and spin-orbit interactions in the $5d$ counterpart can give rise to exotic electronic and magnetic properties. Here, the nature of emerging phenomena at the interface between $\mathrm{SrIr}{\mathrm{O}}_3$ (SIO) and $\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{S}{\mathrm{r}}_{1/3}\mathrm{Mn}{\mathrm{O}}_3$ (LSMO) is presented. Nominally, SIO with strong spin-orbit interaction is metallic and nonmagnetic on the verge of a metal-insulator transition, whereas LSMO is metallic and ferromagnetic with itinerant character and high spin polarization. In the 1:1 LSMO/SIO superlattice, we observe ferromagnetic Mn moments with an insulating behavior, accompanied by antiferromagnetic ordering in SIO. Element-resolved x-ray magnetic circular dichroism proves that there is a weak net ferromagnetic Ir moment aligned antiparallel to the Mn counterpart. The branching ratio shows the formation of molecular orbitals between the Mn and Ir layers modifying the Ir $5d$ electronic configuration through the mixture of $t_{2g}$ and $e_g$ states, resulting in a deviation from $J_{\mathrm{eff}}=1/2$. This result demonstrates a pathway to manipulate the spin-orbit entanglement in $5d$ states with two-dimensional $3d$ spin-polarized electrons through heterostructure design.

Superconducting quantum multilevel systems coupled to resonators have recently been considered in some applications such as microwave lasing and high-fidelity quantum logical gates. In this work, using an rf-SQUID type phase qudit coupled to a microwave coplanar waveguide resonator, we study both theoretically and experimentally the energy spectrum of the system when the qudit level spacings are varied around the resonator frequency by changing the magnetic flux applied to the qudit loop. We show that the experimental result can be well described by a theoretical model that extends from the usual two-level Jaynes-Cummings system to the present four-level system. It is also shown that due to the small anharmonicity of the phase device a simplified model capturing the leading state interactions fits the experimental spectra very well. Furthermore we use the Lindblad master equation containing various relaxation and dephasing processes to calculate the level populations in the simpler qutrit-resonator system, which allows a clear understanding of the dynamics of the system under the microwave drive. Our results help to better understand and perform the experiments of coupled multilevel and resonator systems and can be applied in the case of transmon or Xmon qudits having similar anharmonicity to the present phase device.

The magnetic field distribution of the vortex lattice of optimally doped ${\mathrm{La}}_{1.84}{\mathrm{Sr}}_{0.16}{\mathrm{CuO}}_4$ thin films of various thicknesses, grown on different substrates, was investigated. The influence of film thickness and biaxial strain on the magnetic penetration depth and the superconducting number density were studied using muon spin rotation and compared to single-crystal results. We found an effective superconducting layer thickness smaller than the total film thickness, implying that the interfaces of the films are not or less superconducting than the bulk of the film. The superfluid density diminished in thinner films whereas compressive strain enhanced it. This shows that the number density of superconducting carriers is strongly affected by the boundary conditions as well as by the strain. Furthermore, in fully relaxed optimally doped ${\mathrm{La}}_{1.84}{\mathrm{Sr}}_{0.16}{\mathrm{CuO}}_4$ films grown on ${\mathrm{SrTiO}}_3$, we found a low-temperature magnetic state which sets in at $T_c$. It is reasoned that defects at the surface slow down high-frequency magnetic fluctuations such that a “quasistatic” magnetic ground state results, which coexists with the diminished surface superconductivity. These results indicate that the properties of the surface of optimally doped ${\mathrm{La}}_{1.84}{\mathrm{Sr}}_{0.16}{\mathrm{CuO}}_4$ superconductors differ substantially from the bulk.

${}^{75}\mathrm{As}$ NMR measurements have been made on single crystals to study the nematic state in the iron arsenides NaFeAs, which undergoes a structural transition from a high-temperature (high-$T$) tetragonal phase to a low-$T$ orthorhombic phase at $T_{\mathrm{s}}=57$ K and an antiferromagnetic transition at $T_{\mathrm{N}}=42$ K, and LiFeAs having a superconducting transition at $T_{\mathrm{c}}=18$ K. We observe the in-plane anisotropy of the electric field gradient $\eta$ even in the tetragonal phase of NaFeAs and LiFeAs, showing the local breaking of tetragonal $C_4$ symmetry. Then, $\eta$ is found to obey the Curie-Weiss (CW) law as well as in $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$. The good agreement between $\eta$ and the nematic susceptibility obtained by electronic Raman spectroscopy indicates that $\eta$ is governed by the nematic susceptibility. From comparing $\eta$ in NaFeAs and LiFeAs with $\eta$ in $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$, we discuss the carrier-doping dependence of the nematic susceptibility. The spin contribution to nematic susceptibility is also discussed from comparing the CW terms in $\eta$ with the nuclear spin-lattice relaxation rate divided by temperature $1/T_{1}T$. Finally, we discuss the nematic transition in the paramagnetic orthorhombic phase of NaFeAs from the in-plane anisotropy of $1/T_{1}T$.

Binary borides has been a subject of extensive research. However, the exact compositions and crystal structures of sodium borides remained controversial. Here, using the ab initio variable-composition evolutionary algorithm, a new stable ${\mathrm{Na}}_2{\mathrm{B}}_{30}$ with $I{2}_1{2}_1{2}_1$ symmetry ($I{2}_1{2}_1{2}_1-{\mathrm{Na}}_2{\mathrm{B}}_{30}$) is found, which is $-7.38\mathrm{meV}/\mathrm{atom}$ lower in energy than the $Imma-{\mathrm{Na}}_2{\mathrm{B}}_{30}$ structure reported by experimentalists. Interestingly, the $Imma-{\mathrm{Na}}_2{\mathrm{B}}_{30}$ structure is predicted to be a topological nodal line semimetal, which may result in superior electronic transport. In contrast, $I{2}_1{2}_1{2}_1-{\mathrm{Na}}_2{\mathrm{B}}_{30}$ is an ultrahard semiconductor with an unprecedented open-framework structure, whose interstitial helical boron sublattice enhances its hardness and energetic stability.

We demonstrate that magnetocaloric measurements can rapidly reveal details of the phase diagrams of high-temperature skyrmion hosts, concurrently yielding quantitative latent heats of the field-driven magnetic phase transitions. Our approach addresses an outstanding issue in the phase diagram of the skyrmion host FeGe by showing that dc magnetic anomalies can be explained in terms of entropic signatures consistent with a phase diagram containing a single pocket of skyrmionic order and a Brazovskii transition.