We report the synthesis, structural and magnetothermal characterization down to sub-Kelvin temperatures of the polymeric complex {Ho(α-fur)3}n (α-fur = C4H3OCOO−). The α-furoate ligands bridge Ho(III) ions into one-dimensional zigzag chains running along the crystallographic c-axis. Two slightly distinct coordination environments, denoted Ho(A) and Ho(B), arise from positional disorder of one furoate ligand. Ab initio calculations were performed to estimate the zero-field splittings of both Ho at A and B sites. Magnetic susceptibility and heat capacity measurements show that at low temperatures, the compound behaves as an assembly of antiferromagnetically coupled S* = 1/2 Ising-like chains (J*/kB ≈ −0.4 K) of Ho ions with transverse anisotropy. The low-temperature susceptibility further reveals a small concentration of chain defects. Zero-field ac susceptibility measurements evidence slow relaxation of the magnetization assigned to defect-enabled single-chain magnet (SCM) dynamics in the two chain types, with activation energies EA,SCM = 2.9(3) K and EB,SCM = 4(3) K. Below 0.4 K, a crossover to a coherent quantum tunneling of the magnetization regime, assisted by hyperfine interactions, is observed. The relaxation behavior is discussed in comparison with the related compounds {Ln(α-fur)3}n (Ln = Dy, Tb) and {Ho2Ba(α-fur)8}n, highlighting the interplay between Kramers versus non-Kramers ion character, hyperfine interactions, and magnetic exchange topology in governing the low-temperature dynamics.
Vortex dynamics govern the dissipation and magnetoresistive properties of type-II superconductors. At large transport currents, the current-voltage (I–V) characteristics of a superconductor in the mixed state usually exhibit a nonlinear upturn followed by an abrupt jump occurring due to a flux-flow instability. However, other transition behaviors are also possible, including the formation of phase-slip lines and normal domains. Which mechanism dominates depends on the sample uniformity, its dimensions relative to the coherence length and penetration depth, and the rates of electron energy relaxation and heat removal. Here, based on the time-dependent Ginzburg–Landau equation, we present the results of numerical modeling of the I–V curves of superconducting films with various types of disorder. For a grain-boundary defect mesh, we find multiple voltage transitions overlaid with a nonlinear upturn of the I–V curves. For randomly arranged elongated defects, the I–V curves exhibit voltage steps, whereas for L-shaped defects oriented perpendicular to the transport current, the I–V curves show extended linear regimes separated by voltage transitions. We analyze the evolution of the order parameter along the I–V curves and discuss the experimental accessibility of the revealed dynamics regimes.
When metallic needle tips are illuminated with intense femtosecond laser pulses, the photoemitted electrons can be driven strongly in the optical field at the surface of the needle tip. In particular, electrons can elastically rescatter at the tip, leading to tell-tale features in the electron spectra, such as the plateau and the cut-off. In this work, we first review intense two-color fields driving the electron dynamics. We show how from the two-color phase-dependent spectra one can obtain the optimal phase, i.e., the phase as function of energy, for which the count rate is maximized. We demonstrate that rich information can be extracted from this optimal phase. This first, review part sets the stage for the second part of the paper, containing original data on carrier-envelope phase-dependent spectra. We show that with a similar analysis we can reveal similarly rich information from the optimal phase also in the CEP variation case. In particular, we show that the optimal phase is highly sensitive to the driving pulse duration. We expect this work to widen the tool park of strong-field physics, with direct ramifications to the fields of attosecond physics and future petahertz electronics.
The paper studies resonant optical phenomena in the dimer, which consists of spherical metallic nanoparticles. The relations for the diagonal components of the polarizability tensor, spectral figure of merit tensor, extinction cross-section, as well as for the frequencies of the transverse and longitudinal optical resonance are obtained. The qualitative similarity of the frequency dependences of the real and imaginary parts, as well as the modulus of the transverse and longitudinal polarizability, has been established, and the spectral shift of the extrema for the transverse component of the polarizability with respect to the longitudinal one has been determined. It has been shown that decreasing the gap between the nanoparticles in the dimer leads to increased scattering and decreased energy absorption associated with longitudinal electric fields, as well as increased splitting of the frequencies of the transverse and longitudinal optical resonances. The quantitative agreement between the theoretical and experimental results for dimers of spherical Au nanoparticles with respect to optical resonance splitting is established. In addition to the convergence of nanoparticles, the increase in the permittivity of the environment, and the use of metallic nanoparticles with a higher plasma frequency and a lower contribution of interband transitions to the permittivity to create the dimer also lead to the increase in resonance splitting. The size dependences of the diagonal components of the spectral figure of merit tensor were investigated. It is demonstrated that the behavior of the transverse and longitudinal spectral figure of merit is significantly different when the distance between the nanoparticles of moderate radius in the dimer is decreased. It was established that the longitudinal spectral figure of merit of the dimer of small nanoparticles, located at a sufficiently large distance from each other, has a sharp maximum. The reason for this maximum is the large change in the longitudinal optical resonance frequency with a small change in the permittivity of the surrounding dielectric and the decrease in scattering of the longitudinal electric fields. The feasibility of using the dimer of small and sufficiently distant nanoparticles as the sensitive element of a sensor based on longitudinal optical resonance is demonstrated.
We investigate the role of nanoscale surface geometry in the far-infrared absorption in metallic nanoparticles. In the quasistatic regime, electromagnetic dissipation can be expressed through a surface representation in which the absorbed power is determined by the spatial distribution of the electric field at the particle boundary. We show that regions of small local curvature strongly amplify both normal and tangential components of the electric field, leading to a geometric enhancement of absorption. For particles with minimal curvature radius rc, the electric dipole contribution scales approximately as αEeff∼αE(0)R/rc, where αE(0) is the Drude contribution and R is the particle radius. In addition, surface roughness induces an effective anisotropic conductivity in a thin near-surface layer, allowing tangential electric fields to drive surface currents. This provides an additional dissipation channel which can exceed the conventional magnetic dipole absorption in the far-infrared regime. The predicted enhancement is consistent with experimental observations of excess absorption in metallic nanoparticles. The results indicate that nanoscale surface curvature acts as a fundamental geometric parameter governing electromagnetic dissipation in metallic nanostructures.
First-principles density functional theory (DFT) calculations were performed to examine the structural, electronic, and bonding properties of Al/AlN 2D structures with cubic and hexagonal symmetries. The results indicate that cubic Al(001)/AlN(001) structures are stable at zero temperature for 1 to 3 layers of Al, while hexagonal Al(111)/AlN(001) structures are unstable for more than one Al layer. In cubic structures, the Al–Al bond lengths can decrease by up to 17%, and Al–N bonds shorten up to 7%, while N–N bonds increase up to 5% when transitioning from bulk to 2D structures. Hexagonal structures also show reduced bond lengths, especially for Al–N and N–N bonds for such transitions. Both types of slabs are metallic, with hexagonal structures exhibiting stronger metallicity. The charge transfer from Al to N reflects the ionic nature of the Al–N bonding in both structures.
In this study, the C–H ⋅⋅⋅ N hydrogen-bonded complexes formed by acetylene isotopologues (C2H2, C2D2, C2DH) and trimethylamine [N(CH3)3] were thoroughly investigated using cryosolvent infrared (IR) spectroscopy and anharmonic quantum-chemical calculations at the MP2/6-311++G(d,p) level. Experimental IR spectra recorded at 125 K in krypton, revealed a significant red shift Δν, and a sharp increase in intensity of the ν(CH) and ν(CD) stretching vibrations upon complex formation. For the first time, the study explains the observed difference between red shifts Δ(Δν) induced by the isotopic substitution (H → D) in acetylene in terms of the different changes in force constants ΔK. Furthermore, isotopic substitution was found to have a considerable effect on the value of the dipole moment derivative with respect to the normal coordinate ∂μ/∂Qk, which confirms that the changes in IR intensities are linked to changes in the complex’s electrical properties. The results obtained provide an important scientific contribution to understanding the mechanism of isotope effects in weakly hydrogen-bonded systems.
In the present study, we explore the phonon dynamics of barium bismuthate (BaBiO3) in a broad temperature range of 5–300 K using Raman spectroscopy. Thereby, the interaction between lattice vibrations, electronic behavior, and the structural phase transition of BaBiO3 is studied, issues of considerable interest due to the material’s relevance for high-temperature superconductivity. We notice an anomalous change in the phonon spectra induced by the structural phase transition from long-range monoclinic I2/m to P21/n symmetry at approximately 140 K. Local structural fluctuations within the I2/m phase have been identified, which could be linked to the P21/n phase. Additionally, strong polarization-dependent overtone bands were detected, which are linked to pre-resonant laser excitation conditions and pronounced electron–phonon coupling. The spectra further reveal a polarization-dependent electronic background around 2.3 eV (measured at 5 K), influenced by the restructuring of the electronic band structure. Our findings provide fresh perspectives on the electronic and vibrational properties of BaBiO3, highlighting the importance of in-depth spectroscopic studies for a thorough understanding of this material’s intricate physics. This work not only enhances the fundamental knowledge of perovskite oxides but also lays the groundwork for exploring their potential in future technological applications.
The intermediate state of type-I superconductors remains a fertile area of research because even in ultraclean materials, magnetization curves frequently exhibit hysteresis. In this work, we review magnetic irreversibility in type-I superconductors, emphasizing intrinsic mechanisms associated with the geometrical barrier and topological transitions in the intermediate state. These concepts are illustrated through a representative experimental study on a high-purity tantalum cylinder. Isothermal magnetization curves were measured at different temperatures and magnetic field orientations and analyzed using reduced representations normalized to the thermodynamic critical field. For magnetic fields applied parallel to the cylinder axis, irreversible behavior is observed above a reduced field h1, corresponding to the transition from the Meissner state to the intermediate state, whereas the response remains reversible in the perpendicular geometry. The results demonstrate that magnetic irreversibility in tantalum is governed by geometry- and topology-driven mechanisms and is largely independent of temperature when expressed in reduced variables, supporting a unified interpretation of magnetic irreversibility in type-I superconductors.
In the widely-used Kohn-Sham density functional theory for the manyelectron ground state, only the density functional for the exchange-correlation energy is approximated. While the exact density functional minimizes at the exact ground-state energy for any system, widely used approximations can at best describe normally-correlated ground states. A “proper” approximation for a given problem is defined as one that well describes relevant normally-correlated states. Strong correlation, present in many quantum materials, can arise due to abnormally high degeneracy of the Kohn-Sham occupied and unoccupied one-electron energies near the Fermi level. In such situations, the approximate functional may lower its energy by spontaneously breaking symmetries that the ground-state density shares with the many-electron Hamiltonian, reducing the degeneracies that lead to strong correlation. Here the Levy constrained search is invoked to argue that the total energy of a “proper” functional with maximal spontaneous symmetry breaking is bounded from below by the exact total energy, and that the bound may be close to the extent that the abnormal degeneracies are removed. This is a consequence of a variational principle, and it may not hold in non-variational but more accurate approaches to normal correlation such as coupled cluster theory. Since the correlation energy of any one-electron state is zero, a widely “proper” approximate functional should be nearly or exactly self-interaction free, and progress toward a proper self-interaction correction is reviewed here. The time-centered physical interpretation of symmetry breaking is also discussed.
We study nonlinear electrodynamics of magnetoplasma in a slab of compensated metal under a strong, slowly varying magnetic field. The problem is reduced to a nonlinear wave equation for the magnetic field, which turns into a linear system with the use of a hodograph transformation. We manage to construct exact solutions that describe nonlinear switching driven by a boundary field. The evolution differs qualitatively from the linear one giving rise to formation of nontrivial electromagnetic structures. For sufficiently large slab thickness, the switching process is accompanied by wavefront overturning resulting in emergence of shock waves. Our findings extend earlier studies of semi-infinite media and provide a new insight into nonlinear field dynamics. Furthermore, they offer a novel analytical framework for investigating nonlinear electromagnetic phenomena in confined metallic media.
The dynamics of two-dimensional topological magnetic solitons is discussed and compared with that of other topological solitons known in two-dimensional Josephson junctions. The origins of the soliton effective mass are analyzed for various magnetic models. For the general equations governing the coordinates of these magnetic solitons, with the inertial term added to the standard purely gyroscopic Thiele equation, canonical pairs of Hamiltonian variables are constructed, providing a straightforward way for the semiclassical quantization of this dynamics. One pair of these variables corresponds exactly to the guiding-center coordinates of the soliton; this motion is expected to be slow. For free soliton motion, the dynamics of the second pair resemble the standard Larmor precession of a charged particle. The effects of the mutual influence of these two degrees of freedom are estimated for a general form of the potential forces acting on the soliton. Within a simple approximation based on the Thiele equation, inertial effects in the soliton motion in long magnetic strips and the appearance of a Hofstadter butterfly-type fractal structure in the soliton spectrum of real spin-lattice systems are found.
In this study, thermoelectric transport through a quantum structure embedded in a bipolar semiconductor is theoretically investigated using a quantum point contact (QPC) as an example. This study focuses on the conditions under which nonequilibrium charge carriers exist in the semiconductor. Two key factors are analyzed in detail: the anisotropy of the charge carrier distribution function near the quantum system and the emergence of quasi-Fermi levels in its immediate vicinity. The analysis was performed within the single-electron approximation using the Landauer–Büttiker formalism. It was found that the conductance of the QPC is affected solely by the anisotropy of the charge carrier distribution function. In contrast, both mechanisms of the nonequilibrium significantly influence the Seebeck coefficient of the quantum system with bipolar semiconductor leads.
The in-plane conductivity of YBa2Cu3–yAlyO7–x single crystals with a defined topology of planar defects has been investigated. It is shown that Al impurities act as effective scattering centers for normal-state carriers. The excess conductivity of the studied samples over a wide temperature range follows an exponential temperature dependence, while near Tc it is satisfactorily described by the Aslamazov–Larkin theoretical model. Partial substitution of Cu with Al leads to a significant expansion of the temperature interval in which the pseudogap anomaly exists in the ab plane. The fractal analysis of the “tweed” structures has been performed.
We analyze repeated rank-1 projections onto a single Bell state as an entanglement-preserving protocol for a noisy two-qubit register. The register interacts with a Markovian dissipative environment described by a Lindblad master equation, while each qubit also experiences classical frequency fluctuations with specified noise statistics. We focus on Ornstein–Uhlenbeck noise, using random telegraph noise and Poissonian jump noise as complementary models. Analytic asymptotic expressions are derived for the noise-averaged dynamics, and performance is evaluated using concurrence and a survival-gain metric comparing measurement-assisted evolution with free decay. We identify parameter regimes where repeated Bell-basis projections provide a robust advantage, as well as regimes where the benefit depends on noise statistics, revealing the role of non-Gaussian frequency fluctuations in entanglement protection.
We study the statistical properties of the discrete spectra of the Zakharov–Shabat eigenvalue problem corresponding to the uniform constant background perturbed by weak correlated noise. The properties of these spectra are important for the studies of the later stages of modulation instability and soliton turbulence in a wide class of systems, from optical fibers to Bose–Einstein condensates. We show that for correlated disorder, the distribution of discrete eigenvalues is anisotropic, favoring the creation of bound multi-soliton states (breathers), while in the limit of delta-correlated disorder, most solutions are asymptotically free.
We investigate the spectral properties of input–output Jacobians arising in deep neural network (DNN) autoencoders. These Jacobians are crucial in various applications of DNNs in solid-state physics, for instance as tools for detecting phase transitions. In this work, we study untrained DNNs with random, light-tailed initializations of weight matrices and bias vectors. We derive an explicit expression for the maximal singular value of the Jacobian for an arbitrary number of DNN layers, L. The maximal singular value is an important characteristic, as it determine the contraction properties and fixed points of autoencoders. At the same time, a number of phase transitions in solid-state physics can be described in terms of these fixed points. In the limit of large L, we obtain the equation governing the distribution of singular values of these Jacobians and provide its exact solution.
A key physical mechanism that is important when considering energy or semiconductor materials is diffusion. High diffusivity can limit the applicability of semiconductor materials in functional nanoelectronic devices, whereas it is a prerequisite for energy materials used in batteries and solid oxide fuel cells. Typically, diffusion is thermally activated and governed by Arrhenius behavior. In only a few systems, very high diffusivities have been observed with low activation energies of diffusion (less than 0.1 eV). This, in turn, enables atomic diffusion even at low temperatures. We discuss representative examples of such systems and the implications of diffusion at low temperatures. The focus is on low-temperature diffusion mechanisms across multiple material types (oxides and semiconductors), including materials for different applications such as fuel cells, batteries, nanoelectronics, and superconducting devices. This brief review concludes with future perspectives and consideration of recent advances.
