Transition metals in body-centered cubic (bcc) structures under compression can display several novel physical properties because of their complex electronic structures and electron-phonon interactions. Here, we used inelastic x-ray scattering experiments in a diamond-anvil cell up to ∼45 GPa and density-functional theory calculations up to 210 GPa to investigate the phonon dispersions, and electronic and elastic properties of single-crystal molybdenum (Mo). Our results show a pressure-induced Kohn anomaly at $q\sim 0.5$ along the [ξ00] direction in the longitudinal acoustic mode at ∼45 GPa; this anomaly is triggered by the pressure-enhanced Fermi-surface nesting effect. Theoretical calculations show that electron redistributions in the $s$-to-$d$ orbitals of bcc-Mo contribute to the shear modulus anomaly at ∼50 GPa. In contrast, the Young's modulus anomaly in bcc-Mo at ∼210 GPa results from a Lifshitz-type electronic topological transition. Our results shed light on the complex electronic behaviors that are associated with macroscopic elastic properties in typical bcc $d$-block transition metals under compression.

We explore the usage of pulse sequence optimization to boost the quantum properties of topological defects in molecular graphenoids at high temperatures. We reach spin-lattice relaxation times on the same order as those of the best quantum devices in the literature, $\sim 1$ ms at room temperature. The coherence time is shown to be heavily affected by the hyperfine interaction and by the high concentration of hydrogen atoms in particular. We test and compare the applicability and performance of different decoupling sequences in enhancing the coherence, identifying the best-performing sequences for the purposes of robust state initialization and coherence optimization. Coherence times up to $30 \mu \mathrm{s}$ are reached, and we provide insight into the system-environment interaction mechanisms, with a semiclassical model that considers the nuclear bath as a source of a classical random noise and the dynamical decoupling as a filter function. Full deconvolution of the noise spectrum of the bath is obtained, and we show the noise density has a Lorentzian shape whose parameters describe the nuclear-bath dynamics.

Nonequilibrium dynamics of many-body quantum systems under the effect of measurement protocols is attracting an increasing amount of attention. It has been recently revealed that measurements may induce an abrupt change in the scaling law of the bipartite entanglement entropy, thus suggesting the existence of different nonequilibrium regimes. However, our understanding of how these regimes appear and whether they survive in the thermodynamic limit is much less established. Here we investigate these questions on a one-dimensional quadratic fermionic model: this allows us to reach system sizes relevant in the thermodynamic sense. We show that local projective measurements induce a qualitative modification of the time growth of the entanglement entropy which changes from linear to logarithmic. However, in the stationary regime, the logarithmic behavior of the entanglement entropy does not survive in the thermodynamic limit and, for any finite value of the measurement rate, we numerically show the existence of a single area-law phase for the entanglement entropy. Finally, exploiting the quasiparticle picture, we further support our results by analyzing the fluctuations of the stationary entanglement entropy and its scaling behavior.

The Mott insulator ${\mathrm{YVO}}_3$ with $T_N=118\mathrm{K}$ is revisited to explore the role of spin, lattice, and orbital correlations across the multiple structural and magnetic transitions observed as a function of temperature. Upon cooling, the crystal structure changes from orthorhombic to monoclinic at 200 K, and back to orthorhombic at 77 K, followed by magnetic transitions. From the paramagnetic high-temperature phase, C-type magnetic ordering is first observed at 118 K, followed by a G-type spin re-orientation transition at 77 K. The dynamics of the transitions were investigated via inelastic neutron scattering and first-principles calculations. An overall good agreement between the neutron data and calculated spectra was observed. From the magnon density of states, the magnetic exchange constants were deduced to be $J_{ab}=J_c=-5.8\mathrm{meV}$ in the G-type spin phase, and $J_{ab}=-3.8\mathrm{meV}$, $J_c=7.6\mathrm{meV}$ at 80 K and $J_{ab}=-3.0\mathrm{meV}$, $J_c=6.0\mathrm{meV}$ at 100 K in the C-type spin phase. Paramagnetic scattering was observed in the spin ordered phases, well below the C-type magnetic transition temperature, which continuously increased above the transition. Fluctuations in the temperature dependence of the phonon density of states were observed between 50 and 80 K as well, coinciding with the G-type to C-type transition. These fluctuations are attributed to optical oxygen modes above 40 meV, from first-principles calculations. In contrast, little change in the phonon spectra is observed across $T_N$.

We report high-pressure photoluminescence, Raman scattering, and x-ray diffraction measurements on a lead-free halide perovskite ${\mathrm{Cs}}_3{\mathrm{Sb}}_2{\mathrm{Br}}_9$. At about 3 GPa, an electronic transition manifests itself through a broad minimum in linewidth, a maximum in the intensity of $E_g$, $A_{1g}$ Raman modes, and the unusual change in the $c/a$ ratio of the trigonal lattice. The large compressibility and observed Raman anomalies indicate to a soft material with strong electron-phonon coupling. The observed below-bandgap broadband emission in the photoluminescence measurement indicates the recombination of self-trapped excitons. The initial blueshift of the photoluminescence peak reinforces itself to the redshift at around 3 GPa due to the change in the electronic landscape. A first-order trigonal to a monoclinic structural transition is also seen at 8 GPa. The first-principles density functional theory (DFT) calculations reveal that the electronic transition is associated with direct-to-indirect bandgap transition due to changes in the hybridization of $\text{Sb-5}s$ and $\text{Br-4}p$ orbitals near the Fermi level in the valence band. The experimentally observed Raman modes are assigned to their symmetry using the density functional perturbation theory. In addition, the DFT calculations predict a 27.5% reduction of the bandgap in the pressure range 0–8 GPa.

We investigate photoassisted electronic transport in a normal-metal/BCS-superconductor junction with a microscopic Hamiltonian approach, for several types of periodic voltage drives applied on the normal-metal side. The time-dependent current and the photoassisted noise are computed to all orders of the tunneling Hamiltonian using a Keldysh-Nambu-Floquet approach. An excess noise analysis allows one to determine to what extent pure electronic excitations with a small number of electrons per period can be generated by the different drives. When the superconducting gap is small compared to the drive frequency, the junction behaves like a normal-metal junction and minimal excess noise is reached for Lorentzian voltage drives carrying an integer charge (levitons). In the opposite regime of a large gap, the excess noise vanishes for half-quantized levitons, giving rise to the perfect transmission of a Cooper pair on the superconducting side. This microscopic approach also allows us to address the intermediate regime, when the drive frequency is comparable to the gap, allowing us to study the nontrivial interplay between Andreev reflection and quasiparticle transfer processes. Our analysis also shows the appearance of Tien-Gordon–type relations connecting the current and noise in the AC-driven junction to their DC counterpart, which we investigate in details. Finally, the possibility to build a reliable on-demand source of Cooper pairs with this setup is examined using realistic experimental parameters.

Strain tuning ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ through the Lifshitz point, where the Van Hove singularity of the electronic spectrum crosses the Fermi energy, is expected to cause a change in the temperature dependence of the electrical resistivity from its Fermi liquid behavior $\rho \sim T^2$ to $\rho \sim T^2\log (1/T)$, a behavior consistent with experiments by Barber et al. [Phys. Rev. Lett. 120, 076602 (2018)]. This expectation originates from the same multiband scattering processes with large momentum transfer that were recently shown to account for the linear in $T$ resistivity of the strange metal ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$. In contrast, the thermal resistivity ${\rho }_Q\equiv T/\kappa$, where $\kappa$ is the thermal conductivity, is governed by qualitatively distinct processes that involve a broad continuum of compressive modes, i.e., long-wavelength density excitations in Van Hove systems. While these compressive modes do not affect the charge current, they couple to thermal transport and yield ${\rho }_Q\propto T^{3/2}$. As a result, we predict that the Wiedemann-Franz law in strained ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ should be violated with a Lorenz ratio $L\propto T^{1/2}\log (1/T)$. We expect this effect to be observable in the temperature and strain regime where the anomalous charge transport was established.

Phonon anharmonicity is critical for accurately predicting the material's thermal conductivity ($\kappa$). However, its calculation based on the perturbation theory is a difficult and time-consuming task, especially for the high-order phonon scattering process. In this work, using cubic boron arsenide (BAs) and diamond as examples, we combine the machine learning potential (MLP) with molecular dynamics simulations to predict $\kappa$ and assess the effect of anharmonicity on thermal transport properties. A MLP based on the matrix tensor algorithm is developed in this work, which can accurately describe lattice dynamics behaviors in both BAs and diamond. The phonon spectral energy density analysis reveals that MLP can effectively capture both the phonon mode softening and the linewidth broadening induced by the anharmonicity at finite temperatures in both materials. Compared to diamond, BAs exhibits a stronger anharmonicity revealed by the larger deviation from equilibrium position and more pronounced phonon broadening effect, especially at high temperatures. Furthermore, based on the phonon Boltzmann transport equation and three-phonon scattering process, our calculation results demonstrate that the accuracy of the MLP in predicting the $\kappa$ is comparable to that of density-functional theory calculations for both diamond and BAs. However, this framework can only predict $\kappa$ of diamond in agreement with experimental measurements, but significantly overestimates the $\kappa$ of BAs compared to the experimental results, due to the significant impact of high-order phonon scattering process in BAs. In contrast, the $\kappa$ values predicted by equilibrium molecular dynamics simulations combined with MLP agree well with experimental values for both BAs and diamond. Our study suggests that molecular dynamics simulation combined with MLP is a reliable and computationally efficient tool to account for full orders of anharmonicity and provide accurate predictions of material's thermal conductivity without any a priori knowledge of the importance of high-order phonon anharmonicity.

The ground-state behavior of a spinless fermion chain with pairing (the Kitaev chain) and an interaction between fermions at neighboring sites is studied for free open boundaries. Using the exact quantum solution, it has been shown that there can exist boundary bound states for many values of the interaction. The interaction produces charging of the vacua of the model and charging of the boundary bound states. The theory also describes the behavior of an XYZ spin-1/2 chain with free open edges.

We investigate the effects of long-range Coulomb interactions on the low-temperature properties of a second-order Dirac semimetal in terms of the renormalization group. In contrast to the first-order Dirac semimetal, the full rotation symmetry is broken by further additional terms in the Hamiltonian, and thus the low-energy physics is controlled by two dimensionless parameters: the dimensionless coupling constant and the ratio of the anisotropy parameters. We show that the first parameter flows to zero and the second flows to a fixed value at low energies. Thus, one may calculate physical quantities in terms of the renormalized perturbation theory. As an application, we determine the temperature dependence of the specific heat by solving the renormalization-group equations. Following from the breaking of the full rotation symmetry, there exists a crossover temperature scale $T_c$ (and a lengthscale $L_c$). Physical quantities approach the values for the first-order Dirac semimetal only when the temperature is much smaller than $T_c$. Similarly, the screened Coulomb potential will become anisotropic when the distance is smaller than $L_c$, while the unscreened form is recovered at its tail.

We discuss the systematic engineering of quasicrystals in open quantum systems where quasiperiodicity is introduced through purely dissipative processes. While the resulting short-time dynamics is governed by non-Hermitian variants of the Aubry-André-Harper model, we demonstrate how phases and phase transitions pertaining to the non-Hermitian quasicrystals fundamentally change the long-time, steady-state-approaching dynamics under the Lindblad master equation. Our schemes are based on an exact mapping between the eigenspectrum of the Liouvillian superoperator with that of the non-Hermitian Hamiltonian, under the condition of quadratic fermionic systems subject to linear dissipation. Our work suggests a systematic route toward engineering exotic quantum dynamics in open systems, based on insights of non-Hermitian physics.

Manipulating interlayer interactions in two-dimensional (2D) materials is a widely used and effective method for regulating heat transport, which is important for thermal management of electronics. In this paper, we find that thermal transport can be enhanced by strong interlayer interactions. For instance, 2D bilayer materials with interlayer bonding have larger thermal conductivity than their monolayer counterparts. To further verify the conjecture, we take bilayer graphene (BLG) as a study case to gradually strengthen the interlayer interactions in BLG by applying a series of compressive strains along the out-of-plane direction. It is found that the thermal conductivity of BLG first decreases and then anomalously increases when the strain is larger than the threshold of 12%. For the decreasing trend, it is mainly due to the strain-induced reduction of phonon group velocity and lifetime. While at larger strain (>12%), the anomalous increase of thermal conductivity is found to be caused by hydrodynamic phonon transport and weak phonon anharmonicity. Thus, enhanced interlayer interactions can enhance the hydrodynamic phonon transport and thus improve the heat transfer performance. In this paper, we not only report the hydrodynamic enhancement of thermal transport through enhanced interlayer interactions but also provide solid evidence. These findings will deepen the understanding of thermal transport in layered materials and heterostructures, which will benefit the applications in thermal management.

Quantum simulators hold the promise of probing central questions of high-energy physics in tunable condensed matter platforms, for instance, the physics of confinement. Local defects can be an obstacle in these setups, harming their simulation capabilities. However, defects in the form of impurities can also be useful as probes of many-body correlations and may lead to fascinating new phenomena themselves. Here, we investigate the interplay between impurity and confinement physics in a basic spin chain setup, showing the emergence of exotic excitations as impurity-meson bound states with a long lifetime. For weak confinement, semiclassical approximations can describe the capture process in a meson-impurity scattering event. In the strong-confining regime, intrinsic quantum effects are visible through the quantization of the emergent bound state energies which can be readily probed in quantum simulators.

We report a ${\mathrm{La}}_2{\mathrm{CuO}}_4$-like interlayer antiferromagnetic order in ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ films with large orthorhombic distortion $(>1.5\%)$. The biaxial lattice strain in epitaxial heterostructures of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4/{\mathrm{Ca}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ lowers the crystal symmetry of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ from tetragonal ($C_4$) to orthorhombic ($C_2$), guiding the Ir $5d J_{\mathrm{eff}}=1/2$ pseudospin moment parallel to the elongated $b$ axis via magnetic anisotropy. From resonant x-ray scattering experiments, we observed an antiferromagnetic order in the orthorhombic ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ film whose interlayer stacking pattern is inverted from that of the tetragonal ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ crystal. This interlayer stacking is similar to that of the orthorhombic ${\mathrm{La}}_2{\mathrm{CuO}}_4$, implying that the asymmetric interlayer exchange interactions between $a$ and $b$ directions exceed the anisotropic interlayer pseudodipolar interaction. Our result suggests that strain-induced distortion can provide a delicate knob for tuning the long-range magnetic order in quasi-two-dimensional systems by evoking the competition between the interlayer exchange coupling and the pseudodipolar interaction.

Transition metal borides (TMBs) have received great interest since these intriguing borides could be considered potential applications for designing exotic materials with distinguished properties such as superhardness. The search for TMBs regarding high hardness value remains urgent. Given that Sc is the lightest transition metal element that often exhibits unique properties, in this paper, we undertake a comprehensive investigation of the Sc-B system at high pressures using the state-of-the-art structure search method within the framework of first-principles electronic structure. As a result, our structure searches identified several stable Sc-B phases (i.e., ScB, ${\mathrm{ScB}}_3, {\mathrm{ScB}}_4$, and ${\mathrm{ScB}}_5$) as well as a metastable ${\mathrm{ScB}}_6$. Strikingly, among these scandium borides, $Pnma {\mathrm{ScB}}_3$ contains a boron framework with open channels, while ${\mathrm{ScB}}_6$ possesses interlinking boron trimer units. Remarkably, our simulations suggest the hard features of $Pnma {\mathrm{ScB}}_3$ and $P{6}_{3}mc {\mathrm{ScB}}_6$ with high simulated Vickers hardness values of 38.3 and 39.8 GPa, respectively, which are mainly attributed to their peculiar strong covalent boron network. The excellent stability and high hardness of ${\mathrm{ScB}}_3$ and ${\mathrm{ScB}}_6$ render them promising candidates for superhard materials. Additionally, an orthorhombic boron allotrope $o\text{−}{\mathrm{B}}_{12}$ is predicted by removing Sc from $Pnma {\mathrm{ScB}}_3$, where this structure exhibits an estimated hardness value of 24.9 GPa and superconducting critical temperature $T_{\mathrm{c}}$ of 2.9 K at 1 atm. In this paper, we advance the existing knowledge of TMBs as well as provide implications for the search for TMBs with unique properties.

The sol-gel technique is used in this paper to prepare Cr-doped $\mathrm{BiFe}{\mathrm{O}}_3$. The magnetism of $\mathrm{Bi}{\mathrm{Fe}}_{1\text{−}\mathrm{x}}{\mathrm{Cr}}_{\mathrm{x}}{\mathrm{O}}_3$ with a distorted rhombohedral perovskite structure has been studied for $x=0.1$. X-ray diffraction analysis reveals that the sample is rhombohedral with the space group $R_{3}C$ and contains some impurity phases. The Mössbauer spectra at 300 and 500 K are used to determine the magnetic and impurity phase proportion as well as the valence state, electric field gradient, internal magnetic field, and electron cloud distribution of ${\mathrm{Fe}}^{3+}\mathrm{ions}$ in $\mathrm{Bi}{\mathrm{Fe}}_{0.9}{\mathrm{Cr}}_{0.1}{\mathrm{O}}_3$. The magnetic properties of $\mathrm{Bi}{\mathrm{Fe}}_{0.9}{\mathrm{Cr}}_{0.1}{\mathrm{O}}_3$ are complex. Through the zero-field-cooled/field-cooled magnetization curve, the magnetism of $\mathrm{Bi}{\mathrm{Fe}}_{0.9}{\mathrm{Cr}}_{0.1}{\mathrm{O}}_3$ is analyzed. According to the first-order differential of magnetization to temperature, the Néel temperature is determined to be $\sim 134\mathrm{K}$. It is speculated that a partial relaxation structure may exist in the system. Then the Heisenberg model simulation and experimental data are approximated using the particle swarm optimization algorithm to further analyze the magnetism and calculate the more precise phase transformation temperature. It has been determined that the exchange constants for cubic structure and relaxation structure are $J_{\mathrm{Fe}\text{−}\mathrm{Cr}}=6.75\mathrm{meV}, J_{\mathrm{Fe}\text{−}\mathrm{Fe}}=-6.13\mathrm{meV}, J_{\mathrm{Fe}\text{−}\mathrm{Fe}}^{{}^{\prime }}=-1.16\mathrm{meV}$, and $J_{\mathrm{Fe}\text{−}\mathrm{Cr}}^{{}^{\prime }}=-1.54\mathrm{meV}$. The cubic and relaxation structures contribute 0.71 and 0.29 to magnetism, respectively. According to the Heisenberg model, the Curie temperature should be $\sim 606\mathrm{K}$, and the Néel temperature should be $\sim 116\mathrm{K}$. The ratio of the Curie temperature to the Weiss constant is significantly >5, implying that the entire spin system is in a state of high frustration.

Recently, a host-guest clathrate $\mathrm{Sr}{\mathrm{B}}_3{\mathrm{C}}_3$ with $s{p}^3$-bonded boron-carbon framework was synthesized at $\sim 50\mathrm{GPa}$. Based on electron count, the structure is understood as guest ${\mathrm{Sr}}^{2+}$ cations intercalated in the ${\left({\mathrm{B}}_3{\mathrm{C}}_3\right)}^{3-}$ framework. Previous calculations suggest that $\mathrm{Sr}{\mathrm{B}}_3{\mathrm{C}}_3$ is a hole conductor with an estimated superconducting critical temperature $\left(T_{\mathrm{c}}\right)$ of 42 K at ambient pressure. If atoms with similar radius, such as Rb, can substitute ${\mathrm{Sr}}^{2+}$ in the lattice, the electronic as well as superconductivity properties of this material will be modified significantly. Here, we perform extensive simulations on the stability and physical properties of the Rb-Sr-${\mathrm{B}}_3{\mathrm{C}}_3$ system using first-principles density functional calculation in combination with cluster expansion and the calypso structure prediction method. We predict a phonon-mediated superconductor ${\mathrm{Rb}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{B}}_3{\mathrm{C}}_3$ with a remarkably high $T_{\mathrm{c}}$ of 75 K at ambient pressure, which is a significant improvement from the estimated value (42 K) in $\mathrm{Sr}{\mathrm{B}}_3{\mathrm{C}}_3$. The current results suggest that substitution of alkali atoms in synthesized clathrate $\mathrm{Sr}{\mathrm{B}}_3{\mathrm{C}}_3$ is a viable route toward high-$T_{\mathrm{c}}$ compounds.

Over the past decades, wave localization and a wide variety of related phenomena have come to the forefront of research. Here, we theoretically and experimentally investigate the localization and delocalization of the flat-band states in an acoustic cubic lattice. Under evanescent couplings, the band structure of the designed cubic lattice has two dispersive bands and two degenerate flat bands. According to the analyses, we find that the constructions of flat-band states only depend on the excitation pressures and the coupling coefficients, which are frequency independent. With the flat-band state excitation, the acoustic wave can be either localized or delocalized in the lattice, which is determined by the excitation frequency. When the excitation frequency is close to the flat-band frequency of the lattice, the flat-band state can spread into the whole cubic lattice due to the resonance energy transfer among primitive cells. On the other hand, when the excitation frequency deviates from the flat-band frequency, the flat-band state can be localized in any primitive cell of the designed lattice. This work lays the groundwork for exploring the high-dimensional bound states in acoustic systems and has potential impacts on the applications of acoustic sensing and sound energy harvesting.

The tripartite information is an observable-independent measure for scrambling and delocalization of information. Therefore, one can expect that the tripartite information is a good observable-independent indicator for distinguishing between many-body localized and delocalized regimes, which we confirm for the XXZ chain in a random field. Specifically, we find that the tripartite information signal spreads inside a light cone that grows only logarithmically in time in the many-body localized regime, similar to entanglement entropy. We also find that the tripartite information eventually reaches a plateau with an asymptotic value that is suppressed by strong disorder.

We propose a technique for polarizing and cooling finite many-body classical systems using feedback control. The technique requires the system to have one collective degree of freedom conserved by the internal dynamics. The fluctuations of other degrees of freedom are then converted into the growth of the conserved one. The proposal is validated using numerical simulations of classical spin systems in a setting representative of nuclear magnetic resonance experiments. In particular, we were able to achieve 90% polarization for a lattice of 1000 classical spins starting from an unpolarized infinite temperature state.