Transport and magnetic properties of a $4f^2$ fcc lattice compound, ${\mathrm{PrCdNi}}_4$, were studied. The magnetic susceptibility $\chi \left(T\right)$ follows the Curie-Weiss law from 300 to 20 K, as expected for a free ${\mathrm{Pr}}^{3+}$ ion. As the temperature decreases below 5 K, $\chi \left(T\right)$ approaches a constant, indicating van Vleck paramagnetic behavior. The specific heat $C\left(T\right)$ displays a broad shoulder at around 4 K, which can be reproduced by a doublet-triplet two-level model with an energy gap of 12 K. These results suggest a nonmagnetic ${\mathrm{\Gamma }}_3$ doublet ground state of the ${\mathrm{Pr}}^{3+}$ ion in the cubic crystalline electric field. $C\left(T\right)$ exhibits a peak at $T_{\mathrm{O}}$ = 1.0 K and this peak remains robust against magnetic fields up to 5 T. In powder neutron diffraction measurements, no magnetic reflection was observed at 0.32 K $<T_{\mathrm{O}}$. Two anomalies at $B$ = 2.1 and 5.3 T in magnetoresistance $\rho \left(B\right)$ at 0.05 K likely originate from switching in the order parameter. These results suggest that the phase transition at $T_{\mathrm{O}}$ is ascribed to an antiferro-type order of the electric quadrupole or magnetic octupole of the ${\mathrm{\Gamma }}_3$ doublet in the $4f^2$ fcc lattice.

The stability of the decagonal quasicrystal $d$-Al-Ni-Co that exhibits two-dimensional quasiperiodicity is unresolved to date. Here, employing photoelectron spectroscopy performed with hard x-rays and density functional theory (DFT), we demonstrate existence of the pseudogap in the bulk electronic structure, which is associated with strong Al-transition metal bonding. However, the pseudogap in $d$-Al-Ni-Co is shallower compared to other icosahedral quasicrystals, which explains the lower resistivity of the former. The $W$-Al-Ni-Co approximant used to perform the DFT calculation represents $d$-Al-Ni-Co well, as demonstrated by the existence of a calculated pseudogap and good agreement with the overall shape of the experimental valence band. The surface electronic structure of $d$-Al-Ni-Co studied using surface-sensitive ultraviolet photoemission spectroscopy and slab model of the $W$-Al-Ni-Co approximant also show satisfactory agreement and indicate that the pseudogap is suppressed at the surface.

Transverse acoustic (TA) phonon waves are analogous to electromagnetic waves and can carry a certain angular momentum. In this paper, we study the electron-TA phonon couplings in three-dimensional pentatellurides and explore the conditions under which the TA phonon condensation is stable. We analyze the Lindhard response function, phonon softening, mean-field parameters, and renormalized dispersions, on the basis of which the phase diagrams of the electron-phonon couplings in ${\mathrm{ZrTe}}_5$ and ${\mathrm{HfTe}}_5$ are calculated. The phase diagrams show that, if the chemical potential lies near the Weyl nodes, the TA phonon condensation will dominate and lead to the shear strain wave phase. We further reveal that when the wave vector of the particular phonon mode is smaller, the critical coupling strength will be weaker for the phonon condensation, which thus favors the condensation phase.

$2H$ phase transition-metal dichalcogenides (TMDCs) possess spin splitting bands induced by spin-orbit coupling at the $K$ valley. Distinguishing the dynamics of spin splitting bands is crucial to promoting the incorporation of TMDCs into optoelectronic and spintronic devices. Here we demonstrate the distinct ultrafast recombination behaviors of spin-orbital splitting bands in bulk $2H-\mathrm{MoT}{\mathrm{e}}_2$ by time- and angle-resolved photoemission measurements. The relaxation process of excited electrons with the same spin of the upper valence band includes a $K\text{−}\mathrm{\Sigma }$ conduction band scattering process with 0.44 ps relaxation time and a subsequent phonon-assisted recombination process with 8.47 ps relaxation time. However, the recombination time of holes in the lower valence band matches the decay time of excited electrons in the conduction band at the same momentum position. The valence band splitting gap was found to be constant under pump perturbations, showing the robustness of the spin-orbit coupling. Our results provide deep insight into excited carrier dynamics of $2H$ phase TMDCs.

We derive a mesoscopic theory of the Josephson junction from nonrelativistic scalar electrodynamics. Our theory reproduces the Josephson relations with the canonical current phase relation acquiring a weak second harmonic term, and it improves the standard lumped-element descriptions employed in circuit quantum electrodynamics by providing spatial resolution of the superconducting order parameter and electromagnetic field. By providing an ab initio derivation of the charge qubit Hamiltonian that relates traditionally free qubit parameters to geometric and material properties, we progress toward the quantum engineering of superconducting circuits at the subnanometer scale.

We study the complex band hybridization induced by nonreciprocal local resonances in photonic crystals. Composed of trimer unit cells, a two-dimensional magnetophotonic crystal with an analytically obtainable solution is considered. We find that a nonreciprocal spectral gap may appear without nonreciprocal transmission and that the imaginary parts of the complex wave vectors $\text{Im}\left(\mathbf{k}\right)$ may blow up at resonance to give extreme nonreciprocal transmission. We further show that, for a subwavelength lattice, the isolation ratio for the nonreciprocal transmission is determined solely by $\text{Im}\left(\mathbf{k}\right)$ instead of the extensively studied real part $\text{Re}\left(\mathbf{k}\right)$. Our finding contradicts the common belief that “spectral nonreciprocity $\left[\omega \right(\mathbf{k})\ne \omega (-\mathbf{k}\left)\right]$ always implies nonreciprocal transmission.”

The charge density wave (CDW) is a widely regarded emergent phenomenon in condensed matter physics. To establish a systematic understanding of the CDW, we develop a diagrammatic self-consistent field approach for a cubic Holstein model employing the fluctuation exchange approximation, and explore the emergence and transition of three-dimensional CDWs. A commensurate CDW (c-CDW) locked at $(\pi ,\pi ,\pi )$ is favored near half filling, and the transition temperature is predicted around half of the nearest-neighbor hopping. Large hole doping leads to a suppression of the CDW transition temperature and the emergence of incommensurate CDW (i-CDW), which is evidenced by a drifting of the ordering vector away from $(\pi ,\pi ,\pi )$ towards $(\pi ,\pi ,0)$. The phonon frequency significantly impacts the transition temperature and the phase boundary between c-CDW and i-CDW, and the optimal frequency for enlarging the CDW regime is also predicted near half of the nearest-neighbor hopping. These theoretical results provide a systematic understanding of CDW and a fresh perspective on emergent phenomena dominated by the electron-phonon interaction.

In-plane magnetization is induced by the Edelstein effect in a Rashba spin-orbit interaction system. However, out-of-plane magnetization is more useful for switching a ferromagnetic memory. We study analytically and numerically electric field induced magnetization in $p$-wave magnets with the aid of the Rashba interaction based on a simple two-band model. The out-of-plane magnetization is induced when the Néel vector of the $p$-wave magnet is along the $z$ direction. We also show that no magnetization is induced in the absence of the Rashba interaction. The electric field induced magnetization will be useful for future switching technologies of ferromagnetic memories based on the $p$-wave magnet.

Circuit-based quantum devices rely on keeping electrons at millikelvin temperatures. Improved coherence and sensitivity as well as discovering new physical phenomena motivate pursuing ever lower electron temperatures, accessible using on-chip cooling techniques. Here we show that a two-dimensional electron gas (2DEG), with the sub-band populations manipulated using gate voltages, works as an on-chip cooler only limited by a fundamental phonon heat leak. The 2DEG can, for example, be realized in a silicon-based double-gate complementary metal oxide semiconductor (CMOS) transistor. A single-shot 2DEG cooler can reduce the electron temperature by a factor of 2 with a hold time up to a second, achieved by expanding the electron gas into an additional sub-band. Integrating an array of such coolers—using, e.g., CMOS fabrication techniques—to obtain continuous cooldown may allow reaching down to microkelvin device temperatures.

Since the discovery of polar topological textures, achieving their efficient control and manipulation has emerged as a significant challenge for their integration into nanoelectronic devices. In this study, we use second-principles molecular dynamic simulations to demonstrate the precise and reversible control of domain arrangements stabilizing diverse polarization textures through the application of various inhomogeneous and time-dependent electric fields. Furthermore, we conduct an in-depth study of ferroelectric domain motion under such fields, revealing features consistent with creep dynamics and establishing an upper limit for their propagation speed. Notably, our findings show that domain walls exhibit an asymmetric response delay, present at the onset of the dynamics but absent at its end. These findings provide valuable insights into the dynamical behavior of polar textures, which are relevant for the development of high-speed, low-power nanoelectronic applications.

In this paper, we demonstrate that the vacuum itself suffices to delocalize an Anderson insulator inside a cavity. By studying a disordered one-dimensional spinless fermion system coupled to a single-photon mode describing the vacuum fluctuation, we find that even though the cavity mode does not qualitatively change the localization behavior for a single-fermion system, it indeed leads to delocalization via a vacuum fluctuation-induced correlated hopping mechanism for systems with at least three fermions. A mobility edge separating the low-energy localized eigenstates and the high-energy delocalized eigenstates has been revealed. It is shown that such a one-dimensional three-fermion system with correlated hopping can be mapped to a single-particle system with hopping along the face diagonals in a three-dimensional lattice. The effect of the dissipation as well as a many-body generalization have also been discussed.

Magnetic vortices and skyrmions are typically characterized by distinct topological invariants. This paper presents a unified approach for the topological classification of these textures, encompassing isolated objects and configurations where skyrmions and vortices coexist. Using homotopy group analysis, we derive topological invariants that form the free Abelian group, $\mathbb{Z}\times \mathbb{Z}$. We provide an explicit method for calculating the corresponding integer indices in continuous and discrete systems. This unified classification framework extends beyond magnetism and is applicable to physical systems in general.

There exists evidence indicating that the noncollinear magnetic structure is closely associated with the low-thermal-expansion property of the Fe-Ni Invar alloy. Here, atomic spin dynamics and constrained density functional theory are employed to examine the effect of noncollinear magnetic structures, including the orientation disorder and the moment magnitude, on the equilibrium volume of $\mathrm{F}{\mathrm{e}}_{65}\mathrm{N}{\mathrm{i}}_{35}$ alloy. Results show that the emergence of noncollinear magnetic structures in small volume could be attributed to complex magnetic exchange interactions, especially competing negative exchange interactions from further neighbors. However, the noncollinear orientation disorder, with the moment magnitude remaining fixed, would instead enhance the equilibrium volume of Invar alloy. The longitudinal spin attenuation (i.e., the reduction of moment magnitude) induced by magnetic disorder is related to the magnetostriction of Invar alloy. This study highlights the interaction between longitudinal and transverse spin fluctuations in the noncollinear magnetic structure model.

Potassium sodium niobate (KNN) based lead-free piezoelectric ceramics have garnered significant attention as a new generation of environmentally friendly materials for energy storage and conversion. However, achieving the critical coexistence of different phases at room temperature for superior piezoelectric response typically requires complex multicomponent adjustments through a lengthy process of trial and error. The intricate elemental composition poses tremendous challenges for experimental design and high-throughput calculation. In this study, we adopt a stepwise machine learning (ML) method to develop models for KNN-based solid solutions with multiple element joint doping. On the one hand, a critical composition with multiple phase coexistence at room temperature can be obtained on the forecasted phase diagrams calibrated by the shift of polymorphic phase transition temperatures, experimentally, which exhibits rhombohedral-orthorhombic-tetragonal slushlike polar nanoregions on the atomic polarization mapping. On the other hand, optimal experimental piezoelectric properties for the critical composition further confirm the accuracy of the predicted piezoresponse in the ML model. This work offers a perspective for lead-free piezoelectric materials design that differs from the traditional artificial trial and error method and may provide insights for other materials fields.

The maximum deposition eigenchannel provides the largest possible power delivery to a target region inside a diffusive medium by optimizing the incident wavefront of a monochromatic beam. It originates from constructive interference of scattered waves, which is frequency sensitive. We investigate the spectral width of the maximum deposition eigenchannels over a range of target depths using numerical simulations of a 2D diffusive system. Compared to tight focusing into the system, power deposition to an extended region is more sensitive to frequency detuning. The spectral width of enhanced delivery to a large target displays a rather weak, nonmonotonic variation with target depth, in contrast to a sharp drop of focusing bandwidth with depth. While the maximum enhancement of power deposited within a diffusive system can exceed that of power transmitted through it, this comes at the cost of a narrower spectral width. We investigate the narrower deposition width in terms of the constructive interference of transmission eigenchannels within the target. We further observe that the spatial field distribution inside the target region decorrelates more slowly with spectral detuning than the power decay of the maximum deposition eigenchannel. Additionally, absorption increases the spectral width of deposition eigenchannels, but the depth dependence remains qualitatively identical to that without absorption. These findings hold for any diffusive waves, including electromagnetic waves, acoustic waves, pressure waves, mesoscopic electrons, and cold atoms.

Ultrafast magnetization dynamics driven by ultrashort pump lasers is typically explained by changes in the electronic populations and scattering pathways of excited conduction electrons. This conventional approach overlooks the fundamental role of quantum mechanical selection rules, governing transitions from the core states to the conduction band, that form the key method of the probing step in these experiments. By employing fully ab initio time-dependent density functional theory, we reveal that these selection rules profoundly influence the interpretation of ultrafast spin dynamics at specific probe energies. Our analysis for hcp Co and fcc Ni at the $M$ edge demonstrates that the transient dynamics, as revealed in pump-probe experiments, arises from a complex interplay of optical excitations of the $M$ shell. Taking into account the selection rules and conduction electron spin flips leads to highly energy-dependent dynamics. These findings address long-standing discrepancies in experimental transverse magneto-optical Kerr effect measurements and show that only through meticulous consideration of the matrix elements at the probe stage, can one ensure that the magnetization dynamics is revealed in its true nature, instead of being muddled by artifacts arising from the choice of probe energy.

We study the Dzyaloshinskii-Moriya interaction (DMI) appearing at the interface of a nonmagnetic/noncollinear-antiferromagnetic bilayer. The DMI is an antisymmetric exchange interaction between neighboring magnetic spins, arising in the absence of an inversion center between the spins, the explicit expression of which is dictated by system symmetry. We formulate the interfacial DMI for different crystalline orientations of the noncollinear antiferromagnet with a stacked-kagome lattice structure. From this formulation, we show that, when the kagome planes are perpendicular to the sample film plane, the DMI serves as a uniaxial magnetic anisotropy for the antiferromagnetic order parameter. Our findings reveal an important physical manifestation of a DMI, shedding light on microscopic mechanisms of the magnetic anisotropy in noncollinear antiferromagnets.

Iron-based superconductors exhibit multiple superconducting gaps opened in different energy bands. The debate surrounding their pairing mechanism stems from the diverse Fermi-surface topologies and orbital-dependent electronic correlations, raising questions about whether it belongs to weak-coupling scenarios or strong-coupling regimes based on short-range antiferromagnetic exchange interactions. In this paper, we present findings on angle-dependent modulations of superconductivity induced by local strains in the isolated topmost FeAs layer of ${\mathrm{KCa}}_2{\mathrm{Fe}}_4{\mathrm{As}}_4{\mathrm{F}}_2$. Leveraging scanning tunneling microscopy/spectroscopy, we observed significant variations in the superconducting gap depending on the orientation of the strain relative to the crystal lattice. This angle-dependent suppression of pairing strength in response to tiny tensile strains defies the explanation within weak-coupling scenarios. Instead, it can be qualitatively understood by considering the strain-induced symmetry-breaking distortion of the FeAs layer. These results provide compelling evidence that antiferromagnetic exchange interactions play a critical role in achieving high-temperature superconductivity in iron-based superconductors, challenging the prevailing assumptions about the nature of the pairing mechanism.

We investigate the phase diagram of the extended Hubbard model on a half-filled honeycomb lattice through determinant quantum Monte Carlo simulations. At zero temperature, the repulsive Hubbard model undergoes a transition to an antiferromagnetic state only for sufficiently strong on-site coupling $U$. However, the inclusion of a repulsive extended interaction $V$ between fermions in nearest-neighbor orbitals induces a charge-ordered phase. For attractive interactions, we also identify superconducting and phase-separated phases. We have determined a complete phase diagram with the aid of structure factors associated with various orderings, combined with measurements of double occupancy and the average sign of fermionic determinants. We have established that the semimetallic phase forms a zone from which ordered phases are excluded, preventing the stabilization of a superconducting phase with $d$-wave symmetry, i.e., only $s$-wave pairing is allowed.