Identifying a suitable set of descriptors for modeling physical systems often utilizes either deep physical insights or statistical methods such as compressed sensing. In statistical learning, a class of methods known as structured sparsity regularization seeks to combine both physics- and statistics-based approaches. Used in bioinformatics to identify genes for the diagnosis of diseases, group lasso is a well-known example. Here in physics, we present group lasso as an efficient method for obtaining robust cluster expansions (CEs) of multicomponent systems, a popular computational technique for modeling such systems and studying their thermodynamic properties. Via convex optimization, group lasso selects the most predictive set of atomic clusters as descriptors in accordance with the physical insight that if a cluster is selected, its subclusters should be too. These selection rules avoid spuriously large fitting parameters by redistributing them among lower-order terms, resulting in more physical, accurate, and robust CEs. We showcase these features of group lasso using the CE of the bcc ternary alloy Mo-V-Nb. These results are timely given the growing interest in applying CE to increasingly complex systems, which demand a more reliable machine learning methodology to handle the larger parameter space.

The properties of hydrogen at high pressure have wide implications in astrophysics and high-pressure physics. Its phase change in liquid is variously described as metallization, ${\mathrm{H}}_2$ dissociation, density discontinuity, or plasma phase transition. It has been tacitly assumed that these phenomena coincide at a first-order liquid-liquid transition. In this work, the relevant pressure-temperature conditions are thoroughly explored with first-principles molecular dynamics. We show that there is a large dependency on the exchange-correlation functional and significant finite-size effects. We use hysteresis in a number of measurable quantities to demonstrate a first-order transition up to a critical point, above which molecular and atomic liquids are indistinguishable. At higher temperature beyond the critical point, ${\mathrm{H}}_2$ dissociation becomes a smooth crossover in the supercritical region that can be modeled by a pseudotransition, where the ${\mathrm{H}}_2\leftrightarrow 2\mathrm{H}$ transformation is localized and does not cause a density discontinuity at metallization. Thermodynamic anomalies and the counterintuitive transport behavior of protons are also discovered even far beyond the critical point, making this dissociative transition highly relevant to the interior dynamics of Jovian planets. Below the critical point, simulation also reveals a dynamic ${\mathrm{H}}_2\to 2\mathrm{H}$ chemical equilibrium with rapid interconversion, showing that ${\mathrm{H}}_2$ and H are miscible. The predicted critical temperature lies well below the ionization temperature. Our calculations unequivocally demonstrate that there are three distinct regimes in the liquid-liquid transition of warm dense hydrogen: A first-order thermodynamic transition with density discontinuity and metallization in the subcritical region, a pseudotransition crossover in the supercritical region with metallization without density discontinuity, and finally a plasma transition characterized by the ionization process at very high temperatures. This feature and the induced anomalies originate in the dissociative transition nature that has a negative slope in the phase boundary, which is not unique to hydrogen but is a general characteristic shared by most dense molecular liquids. The revealed multifaceted nature of this dissociative transition could have an impact on the modeling of gas planets, as well as the design of H-rich compounds.

We study the response of a quantum Ising chain to transverse field oscillations in the asymptotic state attained after a quantum quench. We show that for quenches across a quantum phase transition, the dissipative part of the response at low frequencies is negative, corresponding to energy emission up to a critical frequency ${\omega }^{*}$. The latter is found to be connected to the time period $t^{*}$ of the singularities in the Loschmidt echo $\left(t^{*}=2\pi /{\omega }^{*}\right)$ signaling the presence of a dynamical quantum phase transition. This result suggests that a linear-response experiment can be used to detect this kind of phenomenon.

Hexaferrites with noncollinear conical magnetic structures are among the most interesting and promising single-phase multiferroics. One puzzle remaining unsolved is why the spin-induced electric polarization can be either reversible or nonreversible by applying magnetic fields. We have unraveled a solution to this puzzle by a systematic study in the multiferroic hexaferrite ${\mathrm{Ba}}_{0.3}{\mathrm{Sr}}_{1.7}{\mathrm{Co}}_2{\mathrm{Fe}}_{11}{\mathrm{AlO}}_{22}$ where the electric polarization is reversible at low temperatures but nonreversible at high temperatures. Neutron diffraction results reveal that the rotation of spin cones with applied $ab$-plane magnetic field takes distinct paths at 150 and 305 K: in-plane and out-of-plane rotation, respectively. A theoretical analysis based on the spin current model confirms that the reversal of electric polarization is caused by the in-plane rotation. Our study clarifies the mechanism underlying the reversibility of spin-induced electric polarization in multiferroic hexaferrites.

We study the XXZ Heisenberg model in a longitudinal magnetic field using a tensor renormalization method. Built into the tensor representation of the XXZ model is the U(1) symmetry, which is systematically maintained at each renormalization step. This enables rather large tensor representations. We extract ground-state properties as well as the low-lying spectrum from the fixed-point tensors. With rather moderate numerical effort we achieve a very good accuracy as demonstrated by comparison with Bethe ansatz calculations. The phase structure of the model can be accurately reproduced just from the largest fixed-point tensor elements.

We present an extension of the relativistic electron transport theory for the standard (charge) conductivity tensor of random alloys within the tight-binding linear muffin-tin orbital method to the so-called spin-dependent conductivity tensor, which describes the Kubo linear response of spin currents to external electric fields. The approach is based on effective charge- and spin-current operators that correspond to intersite electron transport and that are nonrandom, which simplifies the configuration averaging by means of the coherent potential approximation. Special attention is paid to the Fermi sea term of the spin-dependent conductivity tensor, which contains a nonzero incoherent part, in contrast to the standard conductivity tensor. The developed formalism is applied to the spin Hall effect in binary random nonmagnetic alloys, both on a model level and for Pt-based alloys with an fcc structure. We show that the spin Hall conductivity consists of three contributions (one intrinsic and two extrinsic) which exhibit different concentration dependences in the dilute limit of an alloy. Results for selected Pt alloys (Pt-Re, Pt-Ta) lead to the spin Hall angles around 0.2; these sizable values are obtained for compositions that belong to thermodynamically equilibrium phases. These alloys can thus be considered as an alternative to other systems for efficient charge to spin conversion, which are often metastable crystalline or amorphous alloys.

We propose controlling an electron-hole superfluid in semiconductor coupled quantum wells and double layers of a two-dimensional (2D) material by an external periodic field. This can be created either by the gates periodically located and attached to the quantum wells or double layers of the 2D material or by the moiré pattern of two twisted layers. The dependence of the electron-hole pairing order parameter on the temperature, the charge carrier density, and the gate parameters is obtained by minimization of the mean-field free energy. The second-order phase transition between superfluid and electron-hole plasma, controlled by the external periodic gate field, is analyzed for different parameters.

We report on a combined theoretical and numerical study of counterflow turbulence in superfluid ${}^4\mathrm{He}$ in a wide range of parameters. The energy spectra of the velocity fluctuations of both the normal-fluid and superfluid components are strongly anisotropic. The angular dependence of the correlation between velocity fluctuations of the two components plays the key role. A selective energy dissipation intensifies as scales decrease, with the streamwise velocity fluctuations becoming dominant. Most of the flow energy is concentrated in a wave-vector plane which is orthogonal to the direction of the counterflow. The phenomenon becomes more prominent at higher temperatures as the coupling between the components depends on the temperature and the direction with respect to the counterflow velocity.

We have found that the NMR coherence lifetime $T_2$ of the symmetric central $\left(\pm \frac{1}{2}\to \mp \frac{1}{2}\right)$ transition for ${}^{17}\mathrm{O}$ nuclei through a $\pi$ pulse train can be extended by over two orders of magnitude in a lattice dilute with NMR active nuclei through the use of highly selective (low-power) radio-frequency pulses. Crucial to this lifetime extension is the avoidance of coherence transfer to short-lived nonsymmetric transitions. For ${}^{17}\mathrm{O}$ in $\alpha$-quartz, we obtain $T_2=262\pm 1\text{s}$. This translates into enormous sensitivity gains for echo train acquisition schemes such as Carr-Purcell-Meiboom-Gill (CPMG). By combining satellite population transfer schemes with a low-power (2.73 kHz) CPMG on ${}^{17}\mathrm{O}$ in quartz, we obtain over a 1000-fold sensitivity enhancement compared to a spectrum from a free induction decay acquired at a more typical rf field strength of $32.5\text{kHz}$. For ${}^{33}\mathrm{S}$ in ${\mathrm{K}}_2{\mathrm{SO}}_4$ the same approach yields $T_2=8.8\pm 0.4\mathrm{s}$ and a sensitivity enhancement of 63. In both examples, these enhancements enable the acquisition of NMR spectra at 9.4 T, despite their low natural abundance and spin-lattice relaxation times of $\sim 900$ and $25\text{s}$, respectively, with signal-to-noise ratios of $\sim 30$ in 1 h.

Atomistic first-principles-based effective Hamiltonian simulations are conducted in some ferroelectric systems to predict the existence of a novel topological state. Such state is coined here as “topological eclecton” because it is made of a plethora of electrical topological defects, including (i) vortices and antivortices in different planes, (ii) hedgehogs and antihedgehogs, and (iii) a few skyrmions. Such state can be a ground state and readapt itself to form other striking topological states or phases when, e.g., heating the system or applying electric fields. In addition to its unique combined topological characteristics, it is also ferroelectric and chiral in nature.

Application of high pressure (HP) induces evolution from metallic to semimetallic state in the layered superconducting $\mathrm{La}{\mathrm{O}}_{1\text{−}x}{\mathrm{F}}_x\mathrm{BiS}{\mathrm{e}}_2$ phase, as concluded from ab initio calculations based on experimental $P$-dependent structural data. These changes in conduction properties are associated with a reported decrease in the superconducting transition temperature, $T_{\mathrm{c}}$. However, further increase in pressure induces structural transformation in $\mathrm{La}{\mathrm{O}}_{1\text{−}x}{\mathrm{F}}_x\mathrm{BiS}{\mathrm{e}}_2$, which results in a strong enhancement of its two-dimensionality with a related improvement in superconducting performance. In addition, the HP structural anisotropy induces a loss of long-range order correlations along the stacking direction of the LaO/F and $\mathrm{BiS}{\mathrm{e}}_2$ layers yielding formation of stacking faults. A similar but even more pronounced structural transition was observed in the related $\mathrm{La}{\mathrm{O}}_{1\text{−}x}{\mathrm{F}}_x\mathrm{Bi}{\mathrm{S}}_2$ phase which is also accompanied by an increase in $T_{\mathrm{c}}$. This demonstrates that enhancement of two-dimensionality in layered superconducting systems acts in favor of their superconducting performance.

We study the nonequilibrium dynamics of the extended toric-code model (both ordered and disordered) to probe the existence of dynamical quantum phase transitions (DQPTs). We show that in the case of the ordered toric-code model, the zeros of Loschmidt overlap (generalized partition function) occur at critical times when DQPTs occur, which is confirmed by the nonanalyticities in the dynamical counterpart of the free-energy density. Moreover, we show that DQPTs occur for any nonzero field strength if the initial state is the excited state of the toric-code model. In the disordered case, we show that it is imperative to study the behavior of the first time derivative of the dynamical free-energy density averaged over all the possible configurations to characterize the occurrence of DQPTs in the disordered toric-code model since the disorder parameter itself acts as a new artificial dimension. We also show that for the case where anyonic excitations are present in the initial state, the conditions for DQPTs to occur are the same as what happens in the absence of any excitation.

We use numerical linked cluster expansions (NLC) and exact diagonalization to study confinement transitions out of the quantum spin liquid phase in the pyrochlore-lattice Ising antiferromagnet with random transverse fields. We calculate entanglement entropies associated with local regions defined by single tetrahedron to observe these transitions. The randomness-induced confinement transition is marked by a sharp reduction in the local entanglement and a concomitant increase in Ising correlations. In NLC, it is studied through the destruction of loop resonances due to random transverse-fields. The confining phase is characterized by a distribution of local entanglement entropies, which persists to large random fields.

Perpendicular magnetic anisotropy (PMA) in ultrathin magnetic structures is a key ingredient for the development of electrically controlled spintronic devices. Due to their relatively large spin polarization, high Curie temperature, and low Gilbert damping, the Co-based full Heusler alloys are of special importance from a scientific and applications point of view. Here, we study the mechanisms responsible for the PMA in Pt/Co-based full Heusler alloy/MgO thin-film structures. We show that the ultrathin Heusler films exhibit strong PMA even in the absence of magnetic annealing. By means of ferromagnetic resonance experiments, we demonstrate that the effective magnetization shows a two-regime behavior depending on the thickness of the Heusler layers. Using Auger spectroscopy measurements, we evidence interdiffusion at the underlayer/Heusler interface and the formation of an interfacial CoFe-rich layer which causes the two-regime behavior. In the case of the ultrathin films, the interfacial CoFe-rich layer promotes the strong PMA through the electronic hybridization of the metal alloy and oxygen orbitals across the ferromagnet/MgO interface. In addition, the interfacial CoFe-rich layer is also generating an increase of the Gilbert damping for the ultrathin films beyond the spin-pumping effect. Our results illustrate that the strong PMA is not an intrinsic property of the Heusler/MgO interface but it is actively influenced by the interdiffusion, which can be tuned by a proper choice of the underlayer material, as we show for the cases of the Pt, Ta, and Cr underlayers.

The antiferromagnetic skyrmion is a promising building block for future antiferromagnetic spintronic devices, which has several advantages, including ultrafast dynamics and zero skyrmion Hall angle. The understanding of the pinning and depinning of an antiferromagnetic skyrmion to defects is a prerequisite for skyrmion-based in-line motion applications, such as racetrack memory. Here, we numerically study the effect of two types of defects caused by the local decrease/increase of perpendicular magnetic anisotropy on the current-induced dynamics of an antiferromagnetic skyrmion, where the critical depinning current density of the skyrmion motion for different defects is determined under the framework of micromagnetics. We also provide an explanation for the complex behaviors of the antiferromagnetic skyrmion via energy landscape, which shows that the skyrmion always prefers to stay at a specific place with minimal energy. Furthermore, the current-induced motion of a skyrmion in an antiferromagnetic racetrack with defects is compared to that in a ferromagnetic racetrack. Our results are useful for the understanding of antiferromagnetic skyrmion physics at low temperatures and could provide guidelines for designing applications based on antiferromagnetic skyrmions.

We investigate the emergence of both quantum anomalous Hall and disorder-induced Anderson-Chern insulating phases in two-dimensional hexagonal lattices, with an antiferromagnetically ordered 3Q state and in the absence of spin-orbit coupling. Using tight-binding modeling, we show that such systems display not only a spin-polarized edge-localized current, the chirality of which is energy dependent, but also an impurity-induced transition from trivial metallic to topological insulating regimes, through one edge mode plateau. We compute the gaps' phase diagrams and demonstrate the robustness of the edge channel against deformation and disorder. Our study hints at the 3Q state as a promising building block for dissipationless spintronics based on antiferromagnets.

Here we report a quasi-one-dimensional $S=1$ chain compound ${\mathrm{NiTe}}_2{\mathrm{O}}_5$. From the comprehensive study of the structure and magnetic properties on high-quality single-crystalline ${\mathrm{NiTe}}_2{\mathrm{O}}_5$, it is revealed that ${\mathrm{NiTe}}_2{\mathrm{O}}_5$ undergoes a transition into an intriguing long-range antiferromagnetic order at $T_{\mathrm{N}}=30.5\mathrm{K}$, in which longitudinal magnetic moments along the chain direction are ferromagnetically ordered, while their transverse components have an alternating ferromagnetic-antiferromagnetic coupling. Even though the temperature dependence of magnetic susceptibility represents an archetypal anisotropic antiferromagnetic order, we found that critical behavior of unconventional nature with ${\alpha }^{\prime }\sim 0.25$ and $\beta \sim 0.18$ is accompanied by the temperature evolution of the antiferromagnetic order parameter.

We find systematic signatures suggesting a different superconducting nature for a triple-layered cuprate ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{Ca}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{10+\delta }$ with respect to a double layer through the properties of intrinsic Josephson junctions (IJJs). Our measurements on the current-voltage characteristics reveal that the $c$-axis maximum Josephson current density is sensitive to the superfluid density in outer planes while the critical temperature and the superconducting gap remain unaffected. Switching dynamics of stacked IJJs exhibit that the fluctuation in the gauge-invariant phase difference of an IJJ implies that the inner plane completely shields the capacitive coupling between adjacent IJJs, which is essential for mono- and bilayered cuprates.

We examine the perennial quantum spin liquid candidate $S=\frac{1}{2}$ Heisenberg antiferromagnet on the kagome lattice. Our paper is based on achieving Lanczos diagonalization of the Hamiltonian on a 48 site cluster in sectors with dimensions as large as $5\times {10}^{11}$. The results reveal intricate structures in the low-lying energy spectrum. These structures by no means unambiguously support a ${\mathbb{Z}}_2$ spin liquid ground state, but instead appear compatible with several scenarios, including fourfold topological degeneracy, inversion symmetry breaking, and a combination thereof. We discuss finite-size effects, such as the apparent absence of the eigenstate thermalization hypothesis, and note that, while considerably reduced, some are still present for the largest cluster. Finally, we observe that an XXZ model in the Ising limit reproduces remarkably well the most striking features of finite-size spectra.