We report observation of ultrarobust linear magnetoelectric memory and significant training effect in a honeycomb antiferromagnet ${\mathrm{Co}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$, which is controllable by magnetic and electric fields. The memory states show distinct linear magnetoelectric coefficients over a broad magnetic field range. Antiferromagnetic domain evolution is believed to be responsible for the versatile memory behaviors promisingly accessible in multiferroics and other magnetoelectric materials such as topological insulators. The compensated antiferromagnetic phase essential to the magnetoelectric memory may allow to further integrate the unique merits of antiferromagnetic spintronics such as ultrahigh density and ultrafast switching.

Layer Li-B-C compounds have been shown to have feasible superconductivity. Using the adaptive genetic algorithm, we predict the structures of the Li-B-C system at 100 GPa. We identify several low-enthalpy metallic phases with stoichiometries of $\mathrm{Li}{\mathrm{B}}_2\mathrm{C}$, $\mathrm{Li}{\mathrm{B}}_3\mathrm{C}$, ${\mathrm{Li}}_2\mathrm{B}{\mathrm{C}}_2$, ${\mathrm{Li}}_3{\mathrm{B}}_2{\mathrm{C}}_3$, ${\mathrm{Li}}_3\mathrm{BC}$, and ${\mathrm{Li}}_5\mathrm{BC}$. Using a fast screening method of electron-phonon interaction, we find that $\mathrm{Li}{\mathrm{B}}_3\mathrm{C}$ is a promising candidate for superconductivity. The consecutive calculations using the full Brillouin zone confirm the existence of the strong electron-phonon coupling (EPC) in this system. The anharmonic B-C phonon modes near the zone center provide the major contribution to the EPC. The EPC constant is 1.40, and the estimated critical temperature is 22 K. In this paper, we indicate that superconductivity can also happen without a layered structural motif in the Li-B-C system. We also demonstrate an effective strategy for crystal structure prediction of superconducting materials.

The phonon dispersion of a $\mathrm{Bi}{\mathrm{S}}_2$-based superconductor $\mathrm{La}\mathrm{Bi}{\mathrm{S}}_2{\mathrm{O}}_{0.5}{\mathrm{F}}_{0.5}$ is investigated by first-principles calculations and inelastic x-ray scattering experiments. The origin of superlattice (SL) reflections arising from transverse-type lattice modulation, which were recently reported in [J. Kajitani et al., J. Phys. Soc. Jpn. 90, 103601 (2021)], is discussed in terms of lattice dynamics. Our first-principles calculations of phonon dispersion and the Fermi surfaces (FSs) demonstrate that the phonon mode corresponding to the transverse-type lattice modulation is unstable, and the propagation vector corresponding to the SL reflections is close to the FS nesting vector, which suggests that the phonon softening originates from the FS nesting. Against these calculated expectations, measured phonon dispersion in $\mathrm{La}\mathrm{Bi}{\mathrm{S}}_2{\mathrm{O}}_{0.5}{\mathrm{F}}_{0.5}$ along the Z-A direction, where the SL point is located, shows no remarkable temperature dependence, and there are no steeply declining branches accompanied with a softening around the SL point. Based on these results, we discuss the two possibilities for the transverse lattice modulation in $\mathrm{La}\mathrm{Bi}{\mathrm{S}}_2{\mathrm{O}}_{0.5}{\mathrm{F}}_{0.5}$: the order-disorder-type structural transition and the displacive structural transition with an overdamped mode, for both of which the local structure distortion or the short-range correlation within the $\mathrm{Bi}{\mathrm{S}}_2$ plane would be essential.

Neutron scattering measurements have been performed on deuterated powder samples of claringbullite and Zn-doped claringbullite ${\mathrm{[Zn}}_x{\mathrm{Cu}}_{4-x}{\left(\mathrm{OD}\right)}_6\mathrm{FCl}$]. At low temperatures, claringbullite ${\mathrm{Cu}}_4{\left(\mathrm{OD}\right)}_6\mathrm{FCl}$ forms a distorted pyrochlore lattice with long-range magnetic order and spin-wave-like magnetic excitations. Partial Zn doping leads to the nominal ${\mathrm{ZnCu}}_3{\left(\mathrm{OD}\right)}_6\mathrm{FCl}$ compound, a geometrically frustrated spin-1/2 kagome antiferromagnet that shows no transition to magnetic order down to 1.5 K. The magnetic excitations form a gapless continuum, a signature of fractional excitations in a quantum spin liquid.

Monte Carlo simulations based on a first-principles-derived Hamiltonian are conducted to study the finite-temperature properties of chiral-lattice multiferroic insulator ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$. The use of this numerical technique (i) reveals basic features of the phase diagram as a function of temperature and external magnetic field, including the long-range helical phase at low temperature and zero magnetic field and the skyrmion lattice phase, in which skyrmions are arranged in a two-dimensional hexagonal lattice, and (ii) leads to the discovery of an overlooked vortex lattice phase in a narrow pocket of the phase diagram near the fluctuations' disordered-helical phase transition. The scheme also provides strong numerical evidence that the transition to a helical state in ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ is of first order driven by critical fluctuations.

We have prepared and investigated superconducting ${\mathrm{Nb}}_{67}{\mathrm{Hf}}_{11}{\mathrm{Ti}}_{11}{\mathrm{Zr}}_{11}$ and ${\mathrm{Nb}}_{35}{\mathrm{Ta}}_{35}{\mathrm{Hf}}_{10}{\mathrm{Ti}}_{10}{\mathrm{Zr}}_{10}$ medium- and high-entropy alloys in form of thin films with thicknesses of 600, 100, and 30 nm, and compared their properties with bulk counterparts. We show that the superconducting transition temperature $T_c$ as well as the upper critical magnetic field $B_{c2}\left(0\right)$ decrease with decreasing thickness. Application of hydrostatic pressure up to 33 kbar on the 600-nm ${\mathrm{Nb}}_{35}{\mathrm{Ta}}_{35}{\mathrm{Hf}}_{10}{\mathrm{Ti}}_{10}{\mathrm{Zr}}_{10}$ film shows a decrease of $T_c$ with pressure, which differs from that observed on bulk sample. However, no clear $T_c$ dependence was observed if pressure was applied on the 100-nm film. This result is most likely related to increasing disorder (tendency to structure amorphization) in thinner films. Moreover, we performed point-contact spectroscopy measurements on the 600-nm ${\mathrm{Nb}}_{67}{\mathrm{Hf}}_{11}{\mathrm{Ti}}_{11}{\mathrm{Zr}}_{11}$ and ${\mathrm{Nb}}_{35}{\mathrm{Ta}}_{35}{\mathrm{Hf}}_{10}{\mathrm{Ti}}_{10}{\mathrm{Zr}}_{10}$ films and were able to observe directly the temperature development of the superconducting energy gap $\mathrm{\Delta }\left(T\right)$ and determine the superconducting coupling strength $2\mathrm{\Delta }/k_B{T}_c=3.54$ and $2\mathrm{\Delta }/k_B{T}_c=4.21$, respectively, which is consistent with that of conventional $s$-wave phonon-mediated Bardeen-Cooper-Schrieffer superconductors.

A variational method for studying the ground state of strongly interacting quantum many-body bosonic systems is presented. Our approach constructs a class of extensive variational non-Gaussian wave functions which extend Gaussian states by means of nonlinear canonical transformations (NLCTs) on the fields of the theory under consideration. We illustrate this method with the one-dimensional Bose-Hubbard model for which the proposal presented here provides a family of approximate ground states at arbitrarily large values of the interaction strength. We find that, for different values of the interaction, the non-Gaussian NLCT-trial states sensibly improve the ground-state energy estimation when the system is in the Mott phase.

The electric polarization as a bulk quantity is described by the modern theory of polarization in insulating systems and cannot be defined in conducting systems. Upon a gradual change of a parameter in the system, the polarization always varies smoothly as long as the gap remains open. In this paper, we focus on the two-dimensional Weyl semimetal, which hosts Weyl nodes protected by symmetries, and study the behavior of the polarization when a symmetry-breaking term $M$ is introduced and a gap opens. We show that there can be a jump between $M\to 0^{+}$ and $0^{-}$ limits. We find that the jump is universally described by the “Weyl dipole” representing how the Weyl nodes with monopole charges are displaced in the reciprocal space. Our result is applicable to general two-dimensional Weyl semimetals.

Although entanglement is a key resource for quantum-enhanced metrology, not all entanglement is useful. For example, in the process of many-body thermalization, bipartite entanglement grows rapidly, naturally saturating to a volume law. This type of entanglement generation is ubiquitous in nature but has no known application in most quantum technologies. The generation, stabilization, and exploitation of genuine multipartite entanglement, on the other hand, is far more elusive yet highly desirable for metrological applications. Recently, it has been shown that quantum many-body scars can have extensive multipartite entanglement. However, the accessibility of this structure for real application has been so far unclear. In this work, we show how systems containing quantum many-body scars can be used to dynamically generate stable multipartite entanglement and describe how to exploit this structure for phase estimation with a precision that beats the standard quantum limit. Key to this is a physically motivated modification of a Hamiltonian that generates a variety of multipartite entangled states through the dynamics in the scar subspace.

We devise a number-conserving scheme for the realization of Majorana zero modes in an interacting fermionic ladder coupled by Aharonov-Bohm cages. The latter provide an efficient mechanism to cancel single-particle hopping by destructive interference. The crucial parity symmetry in each wire is thus encoded in the geometry of the setup, in particular, its translation invariance. A generic nearest-neighbor interaction generates the desired correlated hopping of pairs. We exhibit the presence of an extended topological region in parameter space, first in a simplified effective model via bosonization techniques, and subsequently in a larger parameter regime with matrix-product-states numerical simulations. We demonstrate the adiabatic connection to previous models, including exactly solvable ones, and we briefly comment on possible experimental realizations in synthetic quantum platforms, like cold atomic samples.

We address here general issues and show how to properly extract the local charge order in two-dimensional systems from scanning tunneling microscopy/spectroscopy (STM/STS) measurements. When the charge order presents spatial variations at the atomic scale inside the unit cell and is energy dependent, particular care should be taken. We show that the widely used lock-in technique performed while acquiring an STM topography in a closed feedback loop cannot be used to extract this local charge order from STS $dI/dV$ differential conductance maps. In such situations, the use of the lock-in technique leads to systematically incorrect $dI/dV$ measurements, giving a false local charge order. We show that a correct method is either to perform a constant height measurement or to perform a full grid of $dI/dV\left(V\right)$ spectroscopies, using a set point for the bias voltage outside the bandwidth of the correlated material where the local density of states (LDOS) is expected to be spatially homogeneous. We take as a paradigmatic example of two-dimensional material the 1/3 single-layer Pb/Si(111) phase. As large areas of this phase cannot be grown, charge ordering in this system is not accessible to angular resolved photoemission or grazing x-ray diffraction. Two previous investigations by STM/STS supplemented by ab initio density functional theory (DFT) calculations concluded that this material undergoes a phase transition to a low-temperature $3\times 3$ reconstruction where one Pb atom moves up, the two remaining Pb atoms shifting down. A third STM/STS study by Adler et al. [F. Adler, S. Rachel, M. Laubach, J. Maklar, A. Fleszar, J. Schäfer, and R. Claessen, Phys. Rev. Lett. 123, 086401 (2019)] came to the opposite conclusion, i.e., that two Pb atoms move up, while one Pb atom shifts down. We show that this latter erroneous conclusion comes from an artifact induced by the lock-in technique. In contrast, using a full grid of $dI/dV$($V$) spectroscopy measurements, our results show that the energy-dependent LDOS maps agree very well with state-of-the-art DFT calculations, confirming the one-up two-down charge ordering. We show that this structural and charge reordering inside the $3\times 3$ unit cell is equally driven by electron-electron interactions and the coupling to the substrate.

We investigate strong electron-electron correlation effects on two-dimensional van der Waals materials ${\mathrm{Nb}}_3{X}_8 (X=\mathrm{Cl},\mathrm{Br},\text{and}\mathrm{I})$. We find that the monolayers ${\mathrm{Nb}}_3{X}_8$ are ideal systems close to the strong correlation limit. They can be described by a half-filled single band Hubbard model in which the ratio between the Hubbard, $U$, and the bandwidth, $W, U/W\approx 5–10$. Both Mott and magnetic transitions of the material are calculated by the slave boson mean-field theory. Doping the Mott state, a $d_{x^2-y^2}+i{d}_{xy}$ superconducting pairing instability is found. We also construct a tunable bilayer Hubbard system for two sliding ${\mathrm{Nb}}_3{X}_8$ layers. The bilayer system displays a crossover between the band insulator and Mott insulator.

Platinum (Pt) has been very much used for spin-charge conversion in spintronics research due to its large intrinsic spin-orbit interaction. Magnetoconductance originating from weak antilocalization in the quantum interference regime is used as a powerful tool to obtain the microscopic information of spin-orbit interaction and the coherence phase breaking scattering process among itinerant electrons. To acquire the knowledge of different types of scattering processes, we have performed a magnetoconductance study on Pt thin films which manifests multiband (multichannel) conduction. An extensive analysis of quantum-interference-originated weak antilocalization reveals the existence of strong (weak) interband scattering between two similar (different) orbitals. Coherence phase breaking lengths $\left(l_{\varphi }\right)$ and their temperature dependence are found to be significantly different for these two conducting bands. The observed effects are consistent with the theoretical prediction that there exist three Fermi sheets with one $s$ and two $d$ orbital character. This study provides the evidence of two independent nonsimilar conducting channels and the presence of anisotropic spin-orbit interaction along with $e\text{−}e$ correlation in Pt thin films.

Hosting unique drumhead surface states enclosed by nodal lines, topological nodal-line semimetals exhibit novel transport phenomena. Thus, the exploration of topological semimetals with different nodal-line structures has attracted much attention. In this paper, we first find a second-order nodal line semimetal (SONLS), which has coexisting hinge Fermi arcs and drumhead surface states, in a $\mathcal{PT}$-symmetry broken system. Then, without changing the intrinsic parameters, we artificially create exotic hybrid-order nodal-line semimetals hosted by different quasienergy gaps and rich nodal-line structures including nodal chains, crossing ring nodal nets, crossing line nodes, and nodal nets by applying a periodic driving on our SONLS. Enriching the classification of topological semimetals, such Floquet engineered high tunability of the orders and nodal-line structures of the SONLS sets up a foundation for exploring its further applications.

We study many-body localization (MBL) in the quasiperiodic $t_1\text{−}{t}_2$ model, focusing on the role of next-nearest-neighbor (NNN) hopping $t_2$, which introduces a single-particle mobility edge. The calculated phase diagram can be divided into three distinct regimes, depending on the strength of the short-range interaction $U$. For weak interactions ($U\ll t_1$), this model is always nonthermal. For intermediate interactions ($U\sim t_1$), the thermal-MBL phase transition in this model is qualitatively the same as that of the Aubry-Andre (AA) model, which is consistent with existing experimental observations. For strong interactions $(U\gg t_1)$, the NNN hopping produces qualitatively new physics because it breaks down the Hilbert space fragmentation present in the AA model. The NNN hopping is thus irrelevant when the interaction is intermediate but relevant for strong interactions.

The Landé or $g$ factors of charge carriers in solid state systems provide invaluable information about the response of quantum states to external magnetic fields and are key ingredients in the description of spin-dependent phenomena in nanostructures. We report on a comprehensive theoretical analysis of electron and hole $g$ factors in lead chalcogenide nanocrystals. By combining symmetry analysis, atomistic calculations, and extended $\mathbf{k}·\mathbf{p}$ theory, we relate calculated linear-in-magnetic field energy splittings of confined electron states in nanocrystals to the intravalley $g$ factors of the multivalley bulk materials, renormalized due to the quantum confinement. We demonstrate that this renormalization is correctly reproduced by analytical expressions derived in the framework of the extended $\mathbf{k}·\mathbf{p}$ model.

The cyclic sudden switch of a magnetic impurity from a Kondo to a non-Kondo state and back was recently proposed to involve an important dissipation of the order of several $k_B{T}_K$ per cycle. The possibility to reveal this and other electronic processes through nanomechanical dissipation by, e.g., ultrasensitive atomic force microscope (AFM) tools would represent an unusual and interesting form of spectroscopy. Here, we explore the dependence on the switching time of the expected dissipation, a quantity whose magnitude is physically expected to drop from maximum to zero between sudden and slow switching, respectively. As an application of a recently established matrix-product-state-based time-dependent variational algorithm, we study the magnetic-field-induced Kondo switching in an Anderson model of the magnetic impurity. We find, quite reasonably, that dissipation requires switching within the Kondo timescale $\hslash {\left(k_B{T}_K\right)}^{-1}$ or faster. While such a fast switching seems problematic for current AFM setups, the challenge remains open for future means to detect this dissipation by time-dependent magnetic fields, an electrostatic impurity level shift, or hybridization switching. The technical aspects revealed by this approach will be of interest for future nonequilibrium calculations.

Phase-change materials are ubiquitous in technology because of their ability to transition between amorphous and crystalline phases fast and reversibly, upon shining light or passing a current. Here we argue that to fully understand their electronic properties, it is necessary to define a novel electronic phase: the amorphous obstructed insulator. It differs from an obstructed insulator crystal in that it presents localized edge or surface states irrespective of the sample termination. Consequently, we show that obstructed amorphous insulators in three dimensions host a surface two-dimensional flat band, detectable using angle-resolved photoemission spectroscopy. Our work establishes basic models for materials where topological and obstructed properties can be switched on and off externally, including two-dimensional surface flat bands.

The transition-metal tetrachalcogenides are a model system to explore the conjunction of correlated electronic states such as charge density waves (CDWs) with topological phases of matter. Understanding the connection between these phases requires a thorough understanding of the individual states, which for the case of the CDW in this system, is still missing. In this paper we combine phonon-structure calculations and scanning tunneling microscopy measurements of ${\mathrm{NbTe}}_4$ in order to provide a full characterization of the CDW state. We find that, at short range, the superstructure formed by the CDW is fully commensurate with the lattice parameters. Moreover, our data reveals the presence of phase-slip domain walls separating regions of commensurate CDWs in the nanoscale, indicating that the CDW in this compound is discommensurate at long range. Our results solve a long-standing discussion about the nature of the CDW in these materials and provide a strong basis for the study of the interplay between this state and other novel quantum electronic states.

In this paper, we investigate Majorana correlations in quantum impurities coupled to a topological superconducting wire. By employing a density matrix renormalization group approach, we have calculated the Majorana correlation functions in the ground state of the system for different positions of the impurities along the wire. We observe that the correlations decrease exponentially with the distance between the impurities and the ends of the wire. Moreover, different electron-electron interactions on the wire and on the impurities were also analyzed, and the results showed a stronger effect as the electron occupation in the wire is increased. In addition, while changing the occupation of the impurities by tuning their energy levels, we observe that, for specific values of $\varepsilon$, the absolute value of correlation is highly peaked when the chemical potential lies within the window of the topological regime. These peaks are associated with Coulomb blockade phenomena in the impurities, revealing very clearly its effect on the Majorana bound states in the topological phase of the system.