We explore the effects of geometric frustration within a one-dimensional Bose-Hubbard model using a chain of rhombi subject to a magnetic flux. The competition of tunneling, self-interaction, and magnetic flux gives rise to the emergence of a pair-superfluid (pair-Luttinger liquid) phase besides the more conventional Mott-insulator and superfluid (Luttinger liquid) phases. We compute the complete phase diagram of the model by identifying characteristic properties of the pair-Luttinger liquid phase such as pair correlation functions and structure factors and find that the pair-Luttinger liquid phase is very sensitive to changes away from perfect frustration (half-flux). We provide some proposals to make the model more resilient to variants away from perfect frustration. We also study the bipartite entanglement properties of the chain. We discover that, while the scaling of the block entropy pair-superfluid and of the single-particle superfluid leads to the same central charge, the properties of the low-lying entanglement spectrum levels reveal their fundamental difference.

We characterize the properties of ${\mathrm{Ce}}_{1-x}{\mathrm{Yb}}_x{\mathrm{RhIn}}_5$ single crystals with $0\le x\le 1$ using measurements of powder x-ray diffraction, energy dispersive x-ray spectroscopy, electrical resistivity, magnetic susceptibility, specific heat, x-ray absorption near edge structure (XANES), and neutron diffraction. The Yb valence $v_{\text{Yb}}$, calculated from the magnetic susceptibility and measured using XANES, decreases from $3+$ at $x=0$ to $\sim 2.1+$ at $x_{\mathrm{act}}=0.2$, where $x_{\mathrm{act}}$ is the measured Yb concentration. A transition from incommensurate to commensurate antiferromagnetism is observed in neutron diffraction measurements along $Q=(0.5,0.5,l)$ between $0.2\le x_{\mathrm{act}}\le 0.27$; this narrative is supported by specific-heat measurements in which a second robust feature appears at a temperature $T_I\left(T_I<T_N\right)$ for the same concentration range. Magnetic susceptibility measurements also reveal features which provide additional evidence of magnetic ordering. The results of this study suggest that the evolution of the Yb valence plays a critical role in tuning the magnetic ground state of ${\mathrm{Ce}}_{1-x}{\mathrm{Yb}}_x{\mathrm{RhIn}}_5$.

${\mathrm{CeIr}}_3{\mathrm{Ge}}_7$ is an antiferromagnetic metal with a remarkably low ordering temperature $T_{\mathrm{N}}=0.63\mathrm{K}$, while most Ce-based magnets order between 2 and 15 K. Thermodynamic and transport properties as a function of magnetic field or pressure do not show signatures of Kondo correlations, interaction competition, or frustration, as had been observed in a few antiferromagnets with comparably low or lower $T_{\mathrm{N}}$. The averaged Weiss temperature measured below 10 K is comparable to $T_{\mathrm{N}}$, suggesting that the Ruderman-Kittel-Kasuya-Yosida exchange coupling is very weak in this material. The unusually low $T_{\mathrm{N}}$ in ${\mathrm{CeIr}}_3{\mathrm{Ge}}_7$ can therefore be attributed to the large Ce-Ce bond length of about 5.7 Å, which is about 1.5 Å larger than in the most Ce-based intermetallic systems.

The crystalline electric field (CEF) of ${\mathrm{Ce}}^{3+}$ in trigonal symmetry has recently become of some relevance, for instance, in the search of frustrated magnetic systems. Fortunately, it is one of the CEF cases in which a manageable analytic solution can be obtained. Here, we present this solution for the general case, and use this result to determine the CEF scheme of the new compound ${\mathrm{CeIr}}_3{\mathrm{Ge}}_7$ with the help of $T$-dependent susceptibility and isothermal magnetization measurements. The resulting CEF parameters $B_2^0=34.4\mathrm{K}, B_4^0=0.82\mathrm{K}$, and $B_4^3=67.3\mathrm{K}$ correspond to an exceptionally large CEF splitting of the first and second excited levels, 374 K and 1398 K, and a large mixing between the $\left|\pm \frac{5}{2}\right.〉$ and the $\left|\mp \frac{1}{2}\right.〉$ states. This indicates a very strong easy plane anisotropy with an unusually small $c$-axis moment. Using the same general expressions, we show that the properties of the recently reported system ${\mathrm{CeCd}}_3{\mathrm{As}}_3$ can also be described by a similar CEF scheme, providing a much simpler explanation for its magnetic properties than the initial proposal. Moreover, a similar strong easy plane anisotropy has also been reported for the two compounds CeAuSn and ${\mathrm{CePdAl}}_4{\mathrm{Ge}}_2$, indicating that the CEF scheme elaborated here for ${\mathrm{CeIr}}_3{\mathrm{Ge}}_7$ corresponds to an exemplary case for ${\mathrm{Ce}}^{3+}$ in trigonal symmetry.

The femtosecond transient optical spectroscopy is employed to study the relaxation dynamics of the equilibrium and hidden metastable charge-density-wave states in single crystals of $1T\text{−}{\mathrm{TaS}}_{2-x}{\mathrm{Se}}_x$ as a function of the Se doping $x$. Similar to pristine $1T\text{−}{\mathrm{TaS}}_2$, the transition to a hidden phase is observed at low temperature after a quench with a single 50 fs laser pulse, in the commensurate Mott phase up to $x=0.6$. The photo-induced hidden-phase formation is accompanied by a notable change in the coherent phonon spectra, and particularly the collective amplitude mode. While the temperature stability of the hidden phase is only slightly dependent on the Se content, the creation-threshold fluence strongly increases with Se content from 1 to $\sim 4$ mJ/${\mathrm{cm}}^2$, which is attributed predominantly to the change in optical absorption coefficient at the laser excitation wavelength, and not an increased barrier of the hidden state with increasing Se.

We report anomalous Hall effect (AHE) and transport properties of trigonal ${\mathrm{Cr}}_5{\mathrm{Te}}_8$ (tr-${\mathrm{Cr}}_5{\mathrm{Te}}_8)$ single crystals. The electrical resistivity as well as the Seebeck coefficient show a clear kink at the paramagnetic-ferromagnetic transition of tr-${\mathrm{Cr}}_5{\mathrm{Te}}_8$, which is also confirmed by the heat capacity measurement. The scaling behavior between anomalous Hall resistivity ${\rho }_{xy}^A$ and longitudinal resistivity ${\rho }_{xx}$ is linear below $T_c$. Further analysis suggests that the AHE in tr-${\mathrm{Cr}}_5{\mathrm{Te}}_8$ is dominated by the skew-scattering mechanism rather than the intrinsic or extrinsic side-jump mechanism.

An electron spin qubit in a silicon donor atom is a promising candidate for quantum information processing because of its long coherence time. To be sensed with a single-electron transistor, the donor atom is usually located near an interface, where the donor states can be coupled with interface states. Here we study the phonon-assisted spin-relaxation mechanisms when a donor is coupled to confined (quantum-dot-like) interface states. We find that both Zeeman interaction and spin-orbit interaction can hybridize spin and orbital states, each contributing to phonon-assisted spin relaxation in addition to the spin relaxation for a bulk donor or a quantum dot. When the applied magnetic field $B$ is weak (compared to orbital spacing), the phonon assisted spin relaxation shows the $B^5$ dependence. We find that there are peaks (hot spots) in the $B$-dependent and detuning dependent spin relaxation due to strong hybridization of orbital states with opposite spin. We also find spin relaxation dips (cool spots) due to the interference of different relaxation channels. Qubit operations near spin relaxation hot spots can be useful for the fast spin initialization and near cool spots for the preservation of quantum information during the transfer of spin qubits.

We report a comprehensive study on the surface structural and electronic properties of $\mathrm{TaT}{\mathrm{e}}_2$ at room temperature. The surface structure was investigated using both low energy electron diffraction intensity versus voltage and density functional theory calculations. The relaxed structures obtained from the two methods are in good agreement, which is very similar to the bulk, maintaining double zigzag trimer chains. The calculated density of states indicates that such structure originates from the trimer bonding states of the Ta $d_{xz}$ and $d_{xy}$ orbitals. This work will further provide new insights towards the understanding of the charge density wave phase transition in $\mathrm{TaT}{\mathrm{e}}_2$ at low temperature.

The creation of magnetism on nonmagnetic semiconductor surfaces is of importance for the realization of spintronics devices. In particular, the coupling of electron spins within quantum nanostructures can be utilized for nanomagnetism applications. Here, we demonstrate, based on first-principles density-functional theory calculations, that the adsorption of H atoms on the Si(111)-$\left(7\times 7\right)$ surface induces the spin polarization of surrounding Si dangling bonds (DBs) and their spin orderings. It is revealed that the H adsorption on a rest-atom site exhibits a Jahn-Teller-like distortion that accompanies a charge transfer from the rest atom to the nearest-neighboring adatoms. This charge transfer increases the local density of states of three such adatoms at the Fermi level, thereby inducing a Stoner-type instability to produce a ferrimagnetic order of adatom DBs around the adsorbed H atom. Meanwhile, the H adsorption on an adatom site cannot induce spin polarization, but, as adsorbed H atoms increase, the ferrimagnetic order of rest-atom DBs emerges through the charge transfer from rest atoms to adatoms. Our findings provide a microscopic mechanism of the H-induced spin orderings of Si DBs at the atomic scale, which paves the way for the design of nanoscale magnetism in the representative semiconductor surface.

We describe a practical and efficient approach to represent physically realistic long-range interactions in two-dimensional tensor network algorithms via projected entangled-pair operators (PEPOs). We express the long-range interaction as a linear combination of correlation functions of an auxiliary system with only nearest-neighbor interactions. To obtain a smooth and radially isotropic interaction across all length scales, we map the physical lattice to an auxiliary lattice of expanded size. Our construction yields a long-range PEPO as a sum of ancillary PEPOs, each of small, constant bond dimension. This representation enables efficient numerical simulations with long-range interactions using projected entangled pair states.

We investigate the influence of spin-orbit coupling $\lambda$ in strongly-correlated multiorbital systems that we describe by a three-orbital Hubbard-Kanamori model on a Bethe lattice. We solve the problem at all integer fillings $N$ with the dynamical mean-field theory using the continuous-time hybridization expansion Monte Carlo solver. We investigate how the quasiparticle renormalization $Z$ varies with the strength of spin-orbit coupling. The behavior can be understood for all fillings except $N=2$ in terms of the atomic Hamiltonian (the atomic charge gap) and the polarization in the $j$ basis due to spin-orbit induced changes of orbital degeneracies and the associated kinetic energy. At $N=2, \lambda$ increases $Z$ at small $U$ but suppresses it at large $U$, thus eliminating the characteristic Hund's metal tail in $Z\left(U\right)$. We also compare the effects of the spin-orbit coupling to the effects of a tetragonal crystal field. Although this crystal field also lifts the orbital degeneracy, its effects are different, which can be understood in terms of the different form of the interaction Hamiltonian expressed in the respective diagonal single-particle basis.

In recent years, the role of crystal symmetries in enriching the variety of TIs has been actively investigated. Higher-order TIs are a new type of topological crystalline insulators that exhibit gapless boundary states whose dimensionality is lower than those on the surface of conventional TIs. In this paper, relying on a concrete tight-binding model, we show that higher-order TIs can be smoothly connected to conventional TIs in a lower dimension without the bulk-gap closing or symmetry breaking. Our result supports the understanding of higher-order TIs as a stacking of lower-dimensional TIs in a way respecting all the crystalline symmetry.

In this work, we report the structural, magnetic, electrical and thermal transport properties of the Heusler-type alloy ${\mathrm{Ru}}_2\mathrm{NbAl}$. From the detailed analysis of magnetization data, we infer the presence of superparamagnetically interacting clusters with a Pauli paramagnetic background, while short-range ferromagnetic interaction is developed among the clusters below 5 K. The presence of this ferromagnetic interaction is confirmed through heat capacity measurements. The relatively small value of electronic contribution to specific heat $\gamma$ ($\sim 2.7\mathrm{mJ}{\mathrm{mol}}^{-1}{\mathrm{K}}^{-2}$) as well as the linear nature of temperature dependence of the Seebeck coefficient indicate a semimetallic ground state with a pseudogap, which is also supported by our electronic structure calculations. The activated nature of resistivity is reflected in the observed negative temperature coefficient and has its origin in the charge carrier localization due to antisite defects, inferred from magnetic measurements as well as structural analysis. Although the absolute value of thermoelectric figure of merit is rather low ($ZT=5.2\times {10}^{-3}$) in ${\mathrm{Ru}}_2\mathrm{NbAl}$, it is the largest among all the reported nondoped full Heusler alloys.

We have measured the reflectivity spectra of the barium iridate $9R{\mathrm{BaIrO}}_3$, the crystal structure of which consists of characteristic ${\mathrm{Ir}}_3{\mathrm{O}}_{12}$ trimers. In the high-temperature phase above the transition temperature $T_c\simeq 180$ K, we find that the optical conductivity involves two temperature-dependent optical transitions with an ill-defined Drude response. These features are reminiscent of the optical spectra in the organic dimer Mott insulators, implying a possible emergence of an unusual electronic state named trimer Mott insulator in ${\mathrm{BaIrO}}_3$, where the carrier is localized on the trimer owing to the strong Coulomb repulsion. Along with a pronounced splitting of the phonon peak observed below $T_c$, which is a hallmark of charge disproportionation, we discuss a possible phase transition from the trimer Mott insulator to a charge-ordered insulating phase in ${\mathrm{BaIrO}}_3$.

The long-studied Hubbard model is one of the simplest models of copper-oxide superconductors. However, the connection between the model and the experimental phase diagram is still under debate, in particular regarding the existence and extent of the $d$-wave superconducting phase. Recent rapid progress in improving the accuracy of numerical solvers has opened a way to answer this question reliably. Here, we study the hole-doping concentration ($\delta$) dependence of the Hubbard model in the ground states on a square lattice at strong coupling $U/t=10$, for the on-site interaction $U$ and the transfer $t$, using a variational Monte Carlo method. The method, which combines tensor network and Lanczos methods on top of Pfaffian wave functions, reveals a rich phase diagram, in which many orders compete severely and degenerate within the energy range of $0.01t$. We have identified distinct phases including a uniform $d$-wave superconducting phase for $0.17\lesssim \delta \lesssim 0.22$ and a stripe charge/spin ordered phase for $\delta \lesssim 0.17$ with the stripe period depending on $\delta$, together with presumable spatially coexisting antiferromagnetic and stripe order for $\delta \lesssim 0.07$ and coexisting stripe and $d$-wave superconductivity for $0.07\lesssim \delta \lesssim 0.17$. The present, improved method revealed a wider region of a charge uniform superconducting phase than the previous studies and shows a qualitative similarity to the phase diagram of the cuprate superconductors. The superconducting order parameter is largest at doping of around $\delta =0.17$ in the ground state, which undergoes phase transitions from an inhomogeneous to a uniform state.

We propose a platform to realize nodal topological superconductors in a superconducting monolayer of ${\mathrm{MoX}}_2(\mathrm{X}=\mathrm{S},\text{Se},\text{Te})$ using an in-plane magnetic field. The bulk nodal points appear where the spin splitting due to spin-orbit coupling vanishes near the $\pm \mathit{K}$ valleys of the Brillouin zone and are six or twelve per valley in total. In the nodal topological superconducting phase, the nodal points are connected by flat bands of zero-energy Andreev edge states. These flat bands, which are protected by chiral symmetry, are present for all lattice-termination boundaries except zigzag.

Crystal structure prediction with theoretical methods is particularly challenging when unit cells with many atoms need to be considered. Here we employ a symmetry-driven structure search (SYDSS) method and combine it with density functional theory (DFT) to predict novel crystal structures at high pressure. We sample randomly from all 1506 Wyckoff positions of the 230 space groups to generate a set of initial structures. During the subsequent structural relaxation with DFT, existing symmetries are preserved, but the symmetries and the space group may change as atoms move to more symmetric positions. By construction, our algorithm generates symmetric structures with high probability without excluding any configurations. This improves the search efficiency, especially for large cells with 20 atoms or more. We apply our SYDSS algorithm to identify stoichiometric (${\mathrm{H}}_2{\mathrm{O})}_n$-(${\mathrm{NaCl})}_m$ and ${\mathrm{C}}_n{\mathrm{O}}_m$ compounds at high pressure. We predict a novel ${\mathrm{H}}_2\mathrm{O}$-NaCl structure with Pnma symmetry to form at 3.4 Mbar, which is within the range of diamond anvil experiments. In addition, we predict a novel ${\mathrm{C}}_2\mathrm{O}$ structure at 19.8 Mbar and ${\mathrm{C}}_4\mathrm{O}$ structure at 44.0 Mbar with Pbca and C2/m symmetry, respectively.

We investigate dispersion relation of spin waves in a thin homogeneous permalloy film decorated with a periodic array of elliptically shaped permalloy dots and separated by a nonmagnetic Pt spacer. We demonstrate formation of the magnonic band structure for Damon-Eshbach waves propagating in a permalloy film with the band gaps opened at the Brillouin zone border and at smaller wave numbers, due to the Bragg interference and interaction of propagating waves of the continuous film with a standing resonant mode of the nanoellipses, respectively. We show that the predominant role in the formation of the magnonic band structure and opening band gaps is played by a vertical dynamic coupling between propagating waves and magnetization oscillations in the nanodots. The shape anisotropy of the permalloy nanodots allows us to control the spin wave dynamics through the switch between two states of the magnetization with respect to the underneath film magnetization.