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Recent experimental results have demonstrated ferroelectricity in thin films of ${\mathrm{SrTiO}}_3$ induced by antisite ${\mathrm{Ti}}_{\mathrm{Sr}}$ defects. This opens a possibility to use ${\mathrm{SrTiO}}_3$ as a barrier layer in ferroelectric tunnel junctions (FTJs)—emerging electronic devices promising for applications in nanoelectronics. Here using density functional theory combined with quantum-transport calculations applied to a prototypical $\mathrm{Pt}/\mathrm{SrTi}{\mathrm{O}}_3/\mathrm{Pt}$ FTJ, we demonstrate that the localized in-gap energy states produced by the antisite ${\mathrm{Ti}}_{\mathrm{Sr}}$ defects are responsible for the enhanced electron tunneling conductance which can be controlled by ferroelectric polarization. Our tight-binding modeling, which takes into account multiple defects, shows that the predicted defect-assisted tunneling electroresistance effect is greatly amplified when the defect energy levels are brought to the Fermi energy by one of the polarization states. Our results have implications for FTJs based on conventional ferroelectric barriers with defects and can be employed for the design of new types of FTJs with enhanced performance.

Studies of the current-induced spin polarization (CISP) have been recently reinvigorated due to the discoveries of CISP in some burgeoning materials such as oxide interfaces, van der Waals, and topological quantum materials. Here, we investigate the CISP in two-dimensional systems for different types of spin-orbit coupling (SOC) using the Boltzmann transport theory. We find an anisotropic response of CISP to the current direction which strongly depends on the type of SOC. We demonstrate that the CISP is nonlinear with respect to the SOC magnitude, depends on the Fermi energy, and exhibits two different transport regimes for low or high carrier density. Finally, we propose a magnetoresistance device which can exploit the predicted CISP anisotropy.

Longitudinal nonlinear response plays a crucial role in nonreciprocal charge transport, and it may provide a simple electrical means to probe spin-orbit coupling, magnetic order, and polarization states, etc. Here, we report on a study on the polarization and magnetic field control of longitudinal nonlinear transport in two-dimensional (2D) ferroelectrics with in-plane polarization. Based on the Boltzmann transport theory, we first study this using a general Hamiltonian model, and we show that the nonlinear conductivity can be significantly tuned by the polarization and magnetic field. In addition, the nonlinear conductivity reveals a strong spatial anisotropy. We further derive the analytical formulas for the anisotropic nonlinear conductivity in exact accordance with numerical results. Then, we exemplify those phenomena in the 2D ferroelectric SnTe monolayer in the presence of an external magnetic field based on density functional theory calculations. It is also revealed that the polarity of nonlinear conductivity is locked in the direction of polarization, thus pointing to the feasibility of the nonlinear detection of polarization states. Our work uncovers intriguing features of the longitudinal nonlinear transport in 2D ferroelectrics, and it provides guidelines for designing the polarization control of rectifying devices.

The control and generation of spin-polarized current (SPC) without magnetic materials and an external magnetic field is a big challenge in spintronics and normally requires a spin-flip mechanism. In this Rapid Communication, we show the theoretical discovery of all-electrical generation of SPC without relying on spin-flip spin-orbit coupling (SOC). We find that the SPC can be produced as long as an energy-dependent phase difference between the spin up and down electrons can be established. We verify this through quantum transport calculations on a gated stanene zigzag nanoribbon, which is a quantum spin Hall (QSH) insulator. Our calculations indicate that the transient current as well as ac conductance are significantly spin polarized, which results from the genetic phase difference between spin up and down electrons after traversing the system. Our results are robust against edge imperfections and generally valid for other QSH insulators, such as silicene and germanene, etc. These findings establish a different route for generating SPCs by purely electrical means and open the door for interesting applications of semiconductor spintronics.

The altermagnet exhibits the nonrelativistic spin splitting that enables all-electrical generation of spin-polarized currents beyond the spin-orbit coupling. Here, we report on a study on the anisotropic spin-polarized conductivity in collinear altermagnets. Based on the Boltzmann transport theory, we first study this effect using the general group-theoretical analysis and identify the spin point groups sustaining the finite spin polarization given in terms of spin-polarized conductivity. We show that the spin polarization vanishes along any direction for the $g$-wave and $i$-wave altermagnets while the spin polarization is significantly anisotropic for the $d$-wave altermagnet. We further derive the analytical expressions for the anisotropic spin polarization in the $d$-wave altermagnets. Then, we exemplify those phenomena in several representative altermagnets based on the density functional theory calculations. Our work enriches the altermagnetic spintronics and paves the practical way to produce large spin polarization in collinear altermagnets.

A High-quality single crystal of $\mathrm{CaF}{\mathrm{e}}_3{\mathrm{O}}_5$ was grown using the floating-zone method, enabling a precise investigation of its electronic phase transitions. Temperature-dependent resistivity measurements reveal two successive semiconductor-metal-insulator transitions at $\sim 299$ and $\sim 296\mathrm{K}$, which are identified via specific heat and thermal expansion data as the emergence of a charge-averaged (CA) phase and the formation of a $\mathrm{F}{\mathrm{e}}^{3+}\text{−}\mathrm{F}{\mathrm{e}}^{2+}\text{−}\mathrm{F}{\mathrm{e}}^{3+}$ trimeron in the charge-ordered (CO) phase in $\mathrm{CaF}{\mathrm{e}}_3{\mathrm{O}}_5$, respectively. Our results demonstrate that the occurrences of CO and CA phases in $\mathrm{CaF}{\mathrm{e}}_3{\mathrm{O}}_5$ exhibit a competitive relationship instead of forming simultaneously. Under applied pressure, the metal behavior vanishes above 0.13 GPa, with $\mathrm{CaF}{\mathrm{e}}_3{\mathrm{O}}_5$ exhibiting pure semiconductor/insulator behavior. This means that pressure stabilizes the CO phase in $\mathrm{CaF}{\mathrm{e}}_3{\mathrm{O}}_5$ by prompting the formation of a trimeron, offering an alternative route to control competing electronic orders in mixed-valent oxides.

Ferroelectric tunnel junctions (FTJs) utilizing an in-plane head-to-head ferroelectric domain wall (DW) have recently been realized, showing interesting physics and new functionalities. However, the DW state in these junctions was found to be metastable and not reversible after applying an electric field. In this work, we demonstrate that a stable and reversible head-to-head DW state can be achieved in FTJs by proper engineering of polar interfaces. Using density functional theory (DFT) calculations and phenomenological modeling, we explore the DW stability by varying stoichiometry of the ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x\mathrm{O}/{\mathrm{TiO}}_2$ interfaces in FTJs with ${\mathrm{La}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_3$ electrodes and a ferroelectric ${\mathrm{BaTiO}}_3$ tunnel barrier. For, $x\le 0.4$ we find that the DW state becomes a global minimum and the calculated hysteresis loops exhibit three reversible polarization states. For such FTJs, our quantum transport calculations predict the emergence of a DW tunneling electroresistance effect—reversible switching of the tunneling conductance between the highly conductive DW state and two much less conductive uniform polarization states.

The type-II Dirac fermions that are characterized by a tilted Dirac cone and anisotropic magnetotransport properties have been recently proposed theoretically and confirmed experimentally. Here, we predict the emergence of two-dimensional (2D) type-II Dirac fermions in $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{LaNi}{\mathrm{O}}_3/\mathrm{LaAl}{\mathrm{O}}_3$ quantum-well structures. Using first-principles calculations and model analyses, we show that the Dirac points are formed at the crossing between the $d_{x^2-y^2}$ and $d_{z^2}$ bands protected by the mirror symmetry. The energy position of the Dirac points can be tuned to appear at the Fermi energy by changing the quantum-well width. For the quantum-well structure with a two-unit-cell-thick $\mathrm{LaNi}{\mathrm{O}}_3$ layer, we predict the coexistence of the type-II Dirac point and the closed nodal line. The results are analyzed and interpreted using a tight-binding model and symmetry arguments. Our findings offer a practical way to realize 2D type-II Dirac fermions in oxide heterostructures.

We report a first-principles theoretical investigation of spin-polarized quantum transport in organic magnetic tunnel junctions (OMTJs) to provide a microscopic understanding on the sign of the tunnel magnetoresistance ratio (TMR). We consider two different OMTJs, formed by sandwiching 1-stearic acid radicals (1-SAR) or 1,18-stearic diacid radicals (1,18-SDR) between two Ni electrodes. Even though the main difference between them is only on one of the Ni/molecule contacts, such a structure difference is found to induce a significant sign change of the TMR. The TMR is negative for 1-SAR at $-19.6\%$, but is positive for 1,18-SDR at $13.7\%$. By investigating the concept of scattering density of states (SDOS), we found that scattering processes of $p$ electrons at the Ni/molecule interface determines the sign of TMR. Based on spin polarization of the SDOS, we extend the Julliere model to explain both the sign and the value of the TMR qualitatively and semiquantitatively. It is concluded that understanding spin-polarized quantum transport in organic magnetic tunnel junction requires a comprehensive knowledge of the electronic structures of the molecule, the metal electrode, and the metal-molecule contacts.

We investigated spin-lattice coupling in orthorhombic pseudobrookite ${\mathrm{FeTi}}_2{\mathrm{O}}_5$ single crystal with highly ordered $\mathrm{F}{\mathrm{e}}^{2+}$/$\mathrm{T}{\mathrm{i}}^{4+}$ occupation, which consists of quasi-one-dimensional (1D) $S=2$ chains running along the $a$ axis. Both the magnetization and specific heat measurements confirm that the antiferromagnetic phase transition of ${\mathrm{FeTi}}_2{\mathrm{O}}_5$ occurs at $T_N=42\mathrm{K}$. The structural distortions were also observed around $T_N$ in the thermal expansion $\mathrm{\Delta }L/L\left(T\right)$ data. Moreover, the magnetic field was found to strongly affect the thermal expansion both along chains and in the perpendicular direction clearly signaling a substantial magnetoelastic coupling, which was recently proposed to be the origin of a rare spin Jahn-Teller effect, when frustration is lifted via additional lattice distortions. Experimentally observed change in the thermal conductivity slope around $T_N$ is usually associated with the orbital ordering, but density functional theory (DFT)+$U$ calculations do not detect modification of the orbital structure across the transition. However, the first-principles calculation results confirm that ${\mathrm{FeTi}}_2{\mathrm{O}}_5$ is a quasi-1D magnet with a ratio of frustrating interchain to intrachain exchanges $J^{\prime }/J=0.03$ and a substantial single-ion anisotropy $(A=4\mathrm{K})$ of easy-axis type making this material interesting for studying quantum criticality in transverse magnetic fields.

The efficient detection of the Néel vector in antiferromagnets is one of the prerequisites toward antiferromagnetic spintronic devices and remains a challenging problem. Here, we propose that the layer Hall effect can be used to efficiently detect the Néel vector in centrosymmetric magnetoelectric antiferromagnets. Thanks to the robust surface magnetization of magnetoelectric antiferromagnets, the combination of sizable exchange field and an applied electric field results in the layer-locked spin-polarized band edges. Moreover, the Berry curvature can be engineered efficiently by an electric field, which consequently gives rise to the layer-locked Berry curvature responsible for the layer Hall effect. Importantly, it is demonstrated that the layer Hall conductivity strongly depends on the Néel vector orientation and exhibits rich electromagnetic responses, which can be used to detect the Néel vector reversal. Based on density functional theory calculations, we exemplify those phenomena in the prototypical ${\mathrm{Cr}}_2{\mathrm{O}}_3$ compound. A complete list of the magnetic point groups sustaining the layer Hall effect is presented, aiding the search for realistic materials. Our work proposes a novel approach to detect the Néel vector and holds great promise for antiferromagnetic spintronic applications.

Motivated by recent experimental observations, we explore electron transport properties of a ferroelectric tunnel junction (FTJ) with an embedded head-to-head ferroelectric domain wall, using first-principles density-functional theory calculations. We consider a FTJ with $\mathrm{L}{\mathrm{a}}_{0.5}\mathrm{S}{\mathrm{r}}_{0.5}\mathrm{Mn}{\mathrm{O}}_3$ electrodes separated by a $\mathrm{BaTi}{\mathrm{O}}_3$ barrier layer and show that an in-plane charged domain wall in the ferroelectric $\mathrm{BaTi}{\mathrm{O}}_3$ can be induced by polar interfaces. The resulting $\mathsf{V}$-shaped electrostatic potential profile across the $\mathrm{BaTi}{\mathrm{O}}_3$ layer creates a quantum well and leads to the formation of a two-dimensional electron gas, which stabilizes the domain wall. The confined electronic states in the barrier are responsible for resonant tunneling as is evident from our quantum-transport calculations. We find that the resonant tunneling is an orbital selective process, which leads to sharp spikes in the momentum- and energy-resolved transmission spectra. Our results indicate that domain walls embedded in FTJs can be used to control the electron transport.

Valleytronics is an emerging field of research utilizing the valley-pseudospin degree of freedom. In this work, we propose to exploit spin-valley locking in two-dimensional materials, such as silicene, germanene, stanene, and 1T′ transition-metal dichalcogenides, to realize the logic devices with multiple voltage-controlled gate contacts. These materials possess space-inversion and time-reversal symmetries and have two valleys at the non-time-reversal invariant momenta, K and K′, related by time-reversal symmetry. Due to these properties, the valley-spin polarization in these materials can be switched by electric field, and the device conductance and the output voltage can be controlled by the polarity of the input gate voltage. Based on the explicit quantum-transport calculations, we demonstrate the realization of seven logic gates, namely not, xnor, xor, and, nand, or, and nor.

Nonreciprocal charge transport (NCT) characterized by unequal resistances for opposite current direction enriches the fundamental physics of electron transport and is promising for potential device applications. Here, we report on a study on the magnetic control of the NCT in two-dimensional systems with different types of spin-orbit coupling (SOC). Based on the Boltzmann transport theory, we first study this effect using a general Hamiltonian model and show that the nonlinear conductivity can be efficiently tuned by the exchange field. In particular, the exchange-field orientation dependence of the nonlinear conductivity reveals significantly distinct relations for different SOCs. We further derive the analytical expressions with the weak-field approximation, which are in good accordance with numerical results. Then, we exemplify this phenomenon in the ${\mathrm{Ag}}_2\mathrm{Te}/{\mathrm{Cr}}_2{\mathrm{O}}_3$ surface based on density functional theory calculations. We predict that the nonlinear conductivity in such a system strongly depends on the surface magnetization orientation of the ${\mathrm{Cr}}_2{\mathrm{O}}_3$ layer. Those findings also suggest that measuring the NCT as a function of the exchange-field orientation provides a simple electrical means to quantify the SOC type. Our results are expected to provide a valuable insight into the NCT physics and open avenues for future rectifying devices.

Symmetries govern macroscopic physical quantities, and the electronic and spin properties of crystals. Here, we demonstrate that the spin-orbit-induced spin polarization at the Brillouin zone border for noncentrosymmetric and nonsymmorphic space groups is dictated by the wave function symmetries instead of, as commonly held, the point group symmetry of a wave vector. This is confirmed and exemplified by a group-theoretical analysis and density-functional theory calculations on several space groups and realistic materials. Our results are expected to enrich the fundamental understanding of symmetry-dictated spin-orbit physics and broaden the range of spintronic materials.

Spin relaxation results in spin decoherence and a finite spin lifetime, which are detrimental to spintronic devices. To achieve a long spin lifetime desirable for spintronic devices, elucidating the spin relaxation mechanism and factors influencing the spin lifetime is of vital importance. Here, we investigate the spin relaxation in $X$-wave magnets ($X=p,d,f,g,i$) with Rashba spin-orbit coupling within the framework of the D'yakonov-Perel' mechanism. We calculate the tensor of the spin relaxation rate for an arbitrary Néel vector direction in the regime of a weak spin-orbit field. It is found that the spin relaxation rate matrices are anisotropic and can be dictated by magnetic point group symmetry. As an illustration, we study the spin relaxation for a Néel vector along the [001] direction. For the $p$-wave magnet, we derive analytical expressions for the temporal evolution of spins. Moreover, we show that the spin relaxation is highly controllable by the Néel vector and the spin relaxation rate is proportional to the momentum relaxation time, Rashba, and spin-split strengths. Our results shine more light on the fundamental understanding of the spin relaxation mechanism in unconventional $X$-wave magnets.

A precise measurement of the proton flux in primary cosmic rays with rigidity (momentum/charge) from 1 GV to 1.8 TV is presented based on 300 million events. Knowledge of the rigidity dependence of the proton flux is important in understanding the origin, acceleration, and propagation of cosmic rays. We present the detailed variation with rigidity of the flux spectral index for the first time. The spectral index progressively hardens at high rigidities.

Among the various candidates for dark matter (DM), ultralight vector DM can be probed by laser interferometric gravitational wave detectors through the measurement of oscillating length changes in the arm cavities. In this context, KAGRA has a unique feature due to differing compositions of its mirrors, enhancing the signal of vector DM in the length change in the auxiliary channels. Here we present the result of a search for $U(1{)}_{B-L}$ gauge boson DM using the KAGRA data from auxiliary length channels during the first joint observation run together with GEO600. By applying our search pipeline, which takes into account the stochastic nature of ultralight DM, upper bounds on the coupling strength between the $U(1{)}_{B-L}$ gauge boson and ordinary matter are obtained for a range of DM masses. While our constraints are less stringent than those derived from previous experiments, this study demonstrates the applicability of our method to the lower-mass vector DM search, which is made difficult in this measurement by the short observation time compared to the auto-correlation timescale of DM.

The binary black hole signal GW250114, the loudest gravitational wave detected to date, offers a unique opportunity to test Einstein’s general relativity (GR) in the high-velocity, strong-gravity regime and probe whether the remnant conforms to the Kerr metric. Upon perturbation, black holes emit a spectrum of damped sinusoids with specific, complex frequencies. Our analysis of the postmerger signal shows that at least two quasinormal modes are required to explain the data, with the most damped remaining statistically significant for about one cycle. We probe the remnant’s Kerr nature by constraining the spectroscopic pattern of the dominant quadrupolar ($\ell =m=2$) mode and its first overtone to match the Kerr prediction to tens of percent at multiple postpeak times. The measured mode amplitudes and phases agree with a numerical-relativity simulation having parameters close to GW250114. By fitting a parametrized waveform that incorporates the full inspiral-merger-ringdown sequence, we constrain the fundamental $(\ell =m=4)$ mode to tens of percent and bound the quadrupolar frequency to within a few percent of the GR prediction. We perform a suite of tests—spanning inspiral, merger, and ringdown—finding constraints that are comparable to, and in some cases 2–3 times more stringent than those obtained by combining dozens of events in the fourth Gravitational-Wave Transient Catalog. These results constitute the most stringent single-event verification of GR and the Kerr nature of black holes to date, and outline the power of black-hole spectroscopy for future gravitational-wave observations.

We present a search for short-duration gravitational-wave transients in data from the first eight months of Advanced LIGO-Virgo-KAGRA’s fourth observing run, denoted O4a. We use four analyses which are sensitive to a wide range of potential signals lasting up to a few seconds in the 16–4096 Hz band. Excluding binary black hole merger candidates that were already identified by low-latency analyses, we find no statistically significant evidence for other gravitational-wave transients. We measure the sensitivity of the search for representative signals, including sine-Gaussians, Gaussian pulses, and white-noise bursts with different frequencies and durations, adopting a false alarm rate of 1 per 100 years as detection threshold. Depending on signal type, we find improvements over previous searches by factors of 2 to 10 in terms of sensitivity to strain amplitude and of 90% confidence upper limit on the rate density of sources. We also evaluate a variety of core-collapse supernova models and find that, for some models, the search could have detected gravitational waves from stellar core-collapse throughout the Milky Way. Finally, we consider neutron star $f$-modes associated with pulsar glitches and find that, assuming a source similar to the Vela Pulsar, the search could have detected a gravitational-wave signal from a glitch with fractional frequency change as small as $\sim 2$ to $6\times {10}^{-5}$ depending on the neutron star mass.