We have investigated the Fermi surface properties of the Dirac type-II semimetal candidates (Ni,Zr)${\mathrm{Te}}_2$ using torque magnetometry with applied fields up to 35 T. Magnetization shows clear de Haas–van Alphen (dHvA) oscillations above 20 T. The dHvA oscillations are smooth and well defined and consist of one distinct frequency ($F_{\alpha }\sim 530$ T) in $\mathrm{Zr}{\mathrm{Te}}_2$ and three (${\overline{F}}_{\alpha }\sim 72$ T, ${\overline{F}}_{\beta }\sim 425$ T, and ${\overline{F}}_{\gamma }\sim 630$ T) in $\mathrm{Ni}{\mathrm{Te}}_2$. The Berry phase $\varphi$ was determined by constructing the Landau level fan diagram. It is found that $\varphi \sim$ 0 and $\pi$ for $F_{\alpha }$ and ${\overline{F}}_{\beta }$, respectively, for $\mathrm{Zr}{\mathrm{Te}}_2$ and $\mathrm{Ni}{\mathrm{Te}}_2$. This strongly suggests that the Dirac fermions make a dominant contribution to the transport properties of $\mathrm{Ni}{\mathrm{Te}}_2$, whereas topologically trivial fermions dominate those in $\mathrm{Zr}{\mathrm{Te}}_2$. The presence of lighter effective mass $m^{*}=0.13m_e$ in $\mathrm{Ni}{\mathrm{Te}}_2$ compared to $m^{*}=0.26m_e$ in $\mathrm{Zr}{\mathrm{Te}}_2$, where $m_e$ is an electron's rest mass, further confirms the presence of Dirac fermions in $\mathrm{Ni}{\mathrm{Te}}_2$. Our density functional theory calculations find that while both systems host type-II Dirac dispersions along the out-of-plane direction, their relative positions and the natures of the dispersions are different. The Dirac cone is closer to the Fermi energy $E_F$ ($\sim 100$ meV above) in $\mathrm{Ni}{\mathrm{Te}}_2$, whereas it is far ($\sim 500$ meV) above $E_F$ for $\mathrm{Zr}{\mathrm{Te}}_2$. This is consistent with our experimental finding of a nontrivial Berry phase and dominant contribution from lighter electrons in the quantum oscillation signal for only $\mathrm{Ni}{\mathrm{Te}}_2$. These findings suggest that the proximity of the Dirac cone to $E_F$ in topological compounds is crucial for observing the effect from Dirac quasiparticles in their electrical transport or magnetic properties.

Magnetic topological materials have attracted much attention because of their exotic topological quantum physics arising from the interplay between spintronics, crystallography, magnetism, and topology. Based on first-principles calculations and crystal symmetry analysis, we present a lot of materials with fully spin-polarized nodal boxes in a ferromagnetic cubic structure. This nodal box is formed by six butterflylike nodal lines in the Brillouin zone when neglecting the weak spin-orbit coupling (SOC) mainly from F atoms. Such a fully spin-polarized nodal box is reported here. The $M_{110}$ mirror symmetry is not broken in [110] magnetization direction, thus leaving a symmetrically protected butterflylike nodal line on the (110) surface when the SOC is present. The band gaps of the nodal box induced by SOC are less than 1 meV due to the weak SOC effect. Our discovery fills the gap in the study of a fully spin-polarized nodal box in the magnetic system and provides a good research platform for studying the combination of new topological states and spintronics. Such a fully spin-polarized nodal box with unique Fermi surfaces and surface states traversing the Brillouin zone holds great promise for applications in catalysis and spin transport.

The quasi-one-dimensional van der Waals compound ${\mathrm{Bi}}_4{\mathrm{Br}}_4$ was recently found to be a promising high-order topological insulator with exotic electronic states. In this paper, we study the electrical transport properties of ${\mathrm{Bi}}_4{\mathrm{Br}}_4$ bulk crystals. Two electron-type samples with different electron concentrations are investigated. Both samples exhibit a saturation behavior of resistivity in low temperature. In the low-concentration sample, two-dimensional quantum oscillations are clearly observed in the magnetoresistance measurements, which are attributed to the band-bending-induced surface state on the (001) surface. In the high-concentration sample, the angular magnetoresistance exhibits two pairs of symmetrical sharp valleys with an angular difference close to the angle between the crystal planes (001) and (100). The additional valley can be explained by the contribution of the boundary states on the (100) facet. Besides, Hall measurements at low temperatures reveal an anomalous decrease of electron concentration with increasing temperature, which can be explained by the temperature-induced Lifshitz transition. These results shed light on the abundant boundary state transport signals and the temperature-induced Lifshitz transition in ${\mathrm{Bi}}_4{\mathrm{Br}}_4$.

We systematically study three-terminal InSb-Al nanowire devices by using radio-frequency reflectometry. Tunneling spectroscopy measurements on both ends of the hybrid nanowires are performed while systematically varying the chemical potential, magnetic field, and junction transparencies. Identifying the lowest-energy state allows for the construction of the lowest- and zero-energy state diagrams, which show how the states evolve as a function of the aforementioned parameters. Importantly, comparing the diagrams taken for each end of the hybrids enables the identification of states which do not coexist simultaneously, ruling out a significant amount of the parameter space as candidates for a topological phase. Furthermore, altering junction transparencies filters out zero-energy states sensitive to a local gate potential. Such a measurement strategy significantly reduces the time necessary to identify a potential topological phase and minimizes the risk of falsely recognizing trivial bound states as Majorana zero modes.

We have analyzed by means of scanning tunneling microscopy and spectroscopy the atomic and electronic structure of monolayers (1L) of $1T-{\mathrm{V}}_x{\mathrm{Pt}}_{1-x}{\mathrm{Se}}_2$ alloys grown by molecular beam epitaxy on epitaxial graphene substrates. We have focused on the composition range ($0.1\le x\le 0.35$) where ferromagnetic behavior has recently been demonstrated. For low Pt concentration ($x=0.07$ and 0.21), small domains (a few nanometers in diameter) exhibiting the characteristic superstructure of the charge density wave (CDW) state of a pristine ${\mathrm{VSe}}_2$ 1L remain visible on most of the sample surface. Thus, alloying preserves the short-range order of the CDW phase, although it destroys its long-range order. For higher Pt concentration ($x\approx 0.35$), a disordered alloy forms. It presents a fully developed gap (a few tens of millielectronvolts in width) at the Fermi level and is thus a disordered insulator. This gap exhibits strong variations at the nanometer scale, reflecting the local fluctuations in the composition. An unexpectedly large interaction of the transition-metal-dichalcogenide layer with the graphene substrate sets in for this composition range.

We compute the frequency-dependent conductivity of the two-dimensional square lattice Hubbard model at zero temperature as a function of density to second order in the interaction strength, and compare the results to the predictions of single-site dynamical mean field theory computed at the same order. We find that despite the neglect of vertex corrections, the single-site dynamical mean field approximation produces semiquantitatively accurate results for most carrier concentrations, but fails qualitatively for the nearly empty or nearly filled band cases where the model exhibits an emergent Galilean invariance. The DMFT approximation also becomes qualitatively inaccurate very near half filling if nesting is important.

Zirconium pentatelluride (${\mathrm{ZrTe}}_5$) has recently attracted intense research interest, mainly due to its potential topological nontriviality and the extraordinary quantum phenomena it displays. As an exemplary layered compound, ${\mathrm{ZrTe}}_5$ is expected to exhibit thickness-sensitive physical properties that vary with thinning towards the two-dimensional (2D) limit, which has not been thoroughly investigated yet. In this work, we successfully prepare sizable ${\mathrm{ZrTe}}_5$ thin flakes down to the monolayer for the first time. By examining the evolution of magnetotransport properties and the Shubnikov–de Haas effect in ${\mathrm{ZrTe}}_5$ flakes with various layer numbers, we reveal a pronounced thickness dependence of the electronic structure of ${\mathrm{ZrTe}}_5$ characterized by a downward shift of the Fermi level as large as ∼160 meV upon thickness reduction from bulk to two-unit cells (four atomic layers). Furthermore, an external electric field effectively modifies the magnetoresistance and quantum oscillation frequency in the few-layered ${\mathrm{ZrTe}}_5$. Our study proves that ${\mathrm{ZrTe}}_5$ thin flake can be an excellent platform for exploring the novel properties proposed for 2D topological materials as well as their tunability.

The study of warm dense matter, widely existing in nature and laboratories, is challenging due to the interplay of quantum and classical fluctuations. We develop a variational diagrammatic Monte Carlo method and determine the exchange-correlation kernel $K_{\mathrm{XC}}(q;T)$ of a warm dense electron gas for a broad range of temperature $T$ and momentum $q$. We observe several interesting physics, including the $T$-dependent evolution of the hump structure and the large-$q$ tail and the emergence of a scaling relation. Particularly, by deriving an analytical form for $q\to \infty$, we obtain large-$q$ tails of $K_{\mathrm{XC}}$ with high precision. It is shown that the $K_{\mathrm{XC}}$ data can be reliably transformed into real space, which can be directly used in density-functional-theory calculations of real warm dense matter.

Potassium (K) atoms are predicted to form a stable and ordered structure on silicene with high coverage. However, not much is known about the atomic structures of K atoms on silicene/Ag(111). Here, the adsorption of K atoms on monolayer silicene-$\left(3\times 3\right)/\mathrm{Ag}\left(111\right)$ has been studied by combined scanning tunneling microscopy and density functional theory (DFT) calculations. An intriguing structural phase transition from a dispersed phase to an ordered close-packed phase is found with the increasing of K density. DFT calculations reveal three metastable bridge adsorption sites for K on silicene-$(3\times 3)$. K atoms show high mobilities in both the dispersed phase and around the vacancy defects or domain boundaries in the ordered phase, indicating a weak interaction between K and silicene-$(3\times 3)$. A nearly $\mathsf{V}$-shaped density of states has been found in scanning tunneling spectroscopy obtained on the ordered K/silicene-$(3\times 3)$, which is suggested to be the Dirac signature of silicene-$(3\times 3)$ by electronic band structure calculations. This study not only reveals the adsorption mechanism of K atoms on silicene, but also provides an approach to study the adsorption of alkali-metal atoms on other graphenelike two-dimensional materials.

Transition-metal light-element (TMLE) compounds possess high mechanical strength and hardness desirable for wide-ranging applications; but these materials tend to suffer large loss of shear strength in the presence of a compressive stress normal to the shear plane, which greatly impairs structural integrity and hinders mechanical performances under many practical loading conditions. There is pressing need to explore stress-strain relations to set benchmarks and find mechanisms for property optimization among this broad class of materials. Here, we show by first-principles calculations that titanium nitride (TiN) exhibits strain-induced stress enhancement and homogeneity, rendering enhanced and more isotropic stress responses under diverse compression constrained shear deformation; TiN also exhibits contrasting ductile/brittle stress responses under indentation and wear strains with distinct normal-to-shear stress ratios. Moreover, we assess crystal-orientation dependent strengths to account for experimentally observed directional variations of hardness. Analysis of bonding evolution unveils intricate atomistic mechanisms that dictate the deformation modes and resulting stress responses under various practical loading conditions. Such exceptional strain strengthening and tuning of major mechanical properties makes TiN an exemplary case among TMLE compounds for elucidating strain induced strengthening and load-sensitive ductile/brittle characteristics. These results provide an accurate description and in-depth understanding of diverse calculated and measured properties of TiN and offer insights for further exploration of versatile loading and crystal-orientation dependent mechanical properties among the vast class of TMLE compounds. The present findings raise the prospects of rational design and optimization of these prominent materials tailored for wide-ranging experimental implementation and diverse application.

The interplay between itinerant electrons and local magnetic moments in quantum materials brings about rich and fascinating phenomena and stimulates various developments in the theoretical framework. In this work, thermodynamic, electric transport, and neutron diffraction measurements were performed on a newly synthesized honeycomb lattice magnet ${\mathrm{TmNi}}_3{\mathrm{Al}}_9$. Based on the experimental data, a magnetic-field–temperature phase diagram was constructed, exhibiting three essentially different magnetic regions. Below $T_{\mathrm{N}}=2.97\pm 0.02\mathrm{K}{\mathrm{Tm}}^{3+}$, moments order antiferromagnetically in zero field. We found that the ${\mathrm{Tm}}^{3+}$ ions form a pseudodoublet ground state with the Ising-like moments lying normal to the two-dimensional honeycomb layers. Application of a magnetic field along the easy axis gradually suppresses the antiferromagnetic order in favor of an induced ferromagnetic state above the critical field $B_c=0.92\pm 0.05\mathrm{T}$. In the vicinity of $B_c$, a strong enhancement of the quantum spin fluctuations was observed. The quantum Ising nature of the local moments and the coupling to itinerant electrons are discussed.

The search for electrides has recently attracted great interest owing to their unique physical properties, such as superconductivity. ${\mathrm{Li}}_5\mathrm{C}$ has been proposed as a candidate electride with a high superconducting critical temperature $T_{\text{c}}$ of 48 K originating mainly from the localized electrons. Here, a combination of structure searches and first-principles calculations has been performed to acquire the phase diagrams of a Li-C system at high pressure. The structure searches unraveled four successive structures under pressure for ${\mathrm{Li}}_5\mathrm{C}$ that are energetically more stable than the previously proposed superconducting one. The dimension of the localized electrons in ${\mathrm{Li}}_5\mathrm{C}$ decreases under compression due to the reduced interstitial voids. Electron-phonon calculations demonstrated that the four ${\mathrm{Li}}_5\mathrm{C}$ phases are superconductors with a maximum $T_{\text{c}}$ of 5.7 K, excluding its possibility of high superconductivity. Analysis showed that the host hybridized electrons, instead of interstitial electrons, contribute mainly to the superconductivity in ${\mathrm{Li}}_5\mathrm{C}$.

In periodically driven quantum systems, resonances can induce exotic nonequilibrium behavior and new phases of matter without static analog. We report on the emergence of fractional and integer resonances in a broad class of many-body Hamiltonians with a modulated hopping with a frequency that is either a fraction or an integer of the on-site interaction. We contend that there is a fundamental difference between these resonances when interactions bring the system to a Floquet prethermal state. Second-order processes dominate the dynamics in the fractional resonance case, leading to less entanglement and more localized quantum states than in the integer resonance case dominated by first-order processes. We demonstrate the dominating emergence of fractional resonances using the Magnus expansion of the effective Hamiltonian and quantify their effects on the many-body dynamics via quantum states' von Neumann entropy and Loschmidt echo. Our findings reveal novel features of the nonequilibrium quantum many-body system, such as the coexistence of Floquet prethermalization and localization, that may allow to development of quantum memories for quantum technologies and quantum information processing.

The $S=\frac{1}{2}$ quantum spin ladder system with the anisotropic ferromagnetic exchange interaction on the rung under magnetic field is investigated using the numerical diagonalization and the density matrix renormalization group (DMRG) analyses. It is found that the nematic-spin-correlation-dominant Tomonaga-Luttinger liquid (TLL) appears in some high magnetic field. It is included in the TLL phase where the two-magnon bound state is realized. For some suitable parameters, after the field-induced phase transition from this two-magnon-bound TLL phase to the single-magnon TLL one, the re-entrant transition to the two-magnon-bound TLL phase occurs, which is confirmed by the magnetization curves by the DMRG. Several phase diagrams on the plane of the coupling anisotropy versus the magnetization and the magnetic field are presented. The present result is a proposal of the candidate system which exhibits the spin nematic phase without the biquadratic interaction or the frustration.

Local atomic structures around Mn in $\left(\mathrm{Zn},\mathrm{Sn},\mathrm{Mn}\right){\mathrm{As}}_2$ thin films grown on InP (001) substrates were studied using x-ray fluorescence holography. The reconstructed three-dimensional atomic images showed that the addition of Mn resulted in further distortions of the As sublattice from the pure sphalerite ${\mathrm{ZnSnAs}}_2$. The first-principles calculations showed that the further distortion could be attributed to the shortening of the Mn-As bond length induced by the p-d exchange mechanism, which mediates the ferromagnetism in this material. In addition, we found that the positional fluctuation of the cations was maximized at the second nearest neighbor cation shell from detailed analysis of the atomic images. This anomaly in the fluctuation could be explained by the calculated forces acting on the cations.

Considerable research interest has been attracted to noncollinear magnetic structures for their intriguing physics and promising applications. In this work, based on relativistic density functional theory, we reveal the interesting magnetic order and relevant properties in monolayer ${\mathrm{RuF}}_4$, which can be exfoliated from its bulk phase. Although the spins on Ru ions are almost antiferromagnetically aligned between nearest neighbors, weak ferromagnetism is generated because of the antisymmetric Dzyaloshinskii-Moriya interaction as well as the single-ion anisotropy. A prominent magneto-optical Kerr effect can be observed for this antiferromagnet, similar to those of regular strong ferromagnets. In addition, a uniaxial strain can induce a ferroelastic switching together with the in-plane rotation of spin direction, giving rise to a strong intrinsic magnetoelasticity. Our work not only suggests an alternative direction for two-dimensional magnetic materials, but also provides hints to future devices based on antiferromagnetic magnetoelastic or magneto-optical materials.

We explore an exact duality in $(2+1)$ dimensions $\left[\right(2+1\left)\mathrm{D}\right]$ between the fermionization of a bosonic theory with a ${\mathbb{Z}}_2$ subsystem symmetry and a fermionic theory with a ${\mathbb{Z}}_2$ subsystem fermion parity symmetry. A typical example is the duality between the fermionization of the plaquette Ising model and the plaquette fermion model. We first revisit the standard boson-fermion duality in $(1+1)$ dimensions with a ${\mathbb{Z}}_2$ 0-form symmetry, presenting in a way generalizable to $(2+1)\mathrm{D}$. We proceed to $(2+1)\mathrm{D}$ with a ${\mathbb{Z}}_2$ subsystem symmetry and establish the exact duality on the lattice by using the generalized Jordan-Wigner map, with a careful discussion on the mapping of the twist and symmetry sectors. This motivates us to introduce the subsystem Arf invariant, which exhibits a foliation structure.

We report on the Fermi surfaces and superconducting parameters of ${\text{CaSb}}_2$ single crystals (superconducting below $T_{\text{c}}\sim 1.8\mathrm{K}$) grown by the self-flux method. The frequency of de Haas–van Alphen and Shubnikov–de Haas oscillations evidences a quasi-two-dimensional (quasi-2D) Fermi surface, consistent with one of the Fermi surfaces forming Dirac lines predicted by first-principles calculations. Measurements in the superconducting state reveal that ${\text{CaSb}}_2$ is close to a type-I superconductor with the Ginzburg-Landau parameter of around unity. The temperature dependence of the upper critical field $H_{\text{c2}}$ is well described by a model considering two superconducting bands, and the enhancement of the effective mass estimated from $H_{\text{c2}}\left(0\mathrm{K}\right)$ is consistent with the quasi-2D band observed by the quantum oscillations. Our results indicate that a quasi-2D band forming Dirac lines contributes to the superconductivity in ${\text{CaSb}}_2$.

We investigate the difference between the coupling of a bare carrier to phonons versus the coupling of a correlations-dressed quasiparticle to phonons, and we show that the latter may be weak even if the former is strong. Specifically, we analyze the effect of the hole-phonon coupling on the dispersion of the quasiparticle that forms when a single hole is doped into a cuprate layer. To model this, we start from the three-band Emery model supplemented by the Peierls modulation of the $p\text{−}d$ and $p\text{−}p$ hoppings due to the motion of O ions. We then project onto the strongly correlated $U_{dd}\to \infty$ limit where charge fluctuations are frozen on the Cu site. The resulting effective Hamiltonian describes the motion of a doped hole on the O sublattice, and its interactions with Cu spins and O phonons. We show that even though the hole-phonon coupling is moderate to strong, it leads to only a very minor increase of the quasiparticle's effective mass as compared to its mass in the absence of coupling to phonons, consistent with a weak coupling to phonons of the correlations-dressed quasiparticle. We explain the reasons for this suppression, revealing why it is expected to happen in any systems with strong correlations.