Condens. Matter, Volume 4, Issue 3 (September 2019) – 23 articles

Recently, Kayyalha et al. (Phys. Rev. Lett., 2019, 122, 047003) reported on the anomalous enhancement of the self-field critical currents (I_c(sf,T)) at low temperatures in Nb/BiSbTeSe₂-nanoribbon/Nb Josephson junctions. The enhancement was attributed to the low-energy Andreev-bound states arising from the winding of the electronic wave function around the circumference of the topological insulator BiSbTeSe₂ nanoribbon. It should be noted that identical enhancement in I_c(sf,T) and in the upper critical field (B_c2(T)) in approximately the same reduced temperatures, were reported by several research groups in atomically thin junctions based on a variety of Dirac-cone materials (DCM) earlier. The analysis shows that in all these S/DCM/S systems, the enhancement is due to a new superconducting band opening. Taking into account that several intrinsic superconductors also exhibit the effect of new superconducting band(s) opening when sample thickness becomes thinner than the out-of-plane coherence length (ξ_c(0)), we reaffirm our previous proposal that there is a new phenomenon of additional superconducting band(s) opening in atomically thin films. Full article

Tungsten oxides play a pivotal role in a variety of modern technologies, e.g., switchable glasses, wastewater treatment, and modern gas sensors. Metallic tungsten is used as armor material, for example in gas turbines as well as future fusion power devices. In the first case, oxides are desired as functional materials; while in the second case, oxides can lead to catastrophic failures, so avoiding the oxidation of tungsten is desired. In both cases, it is crucial to understand the reactivity of tungsten oxides with other chemicals. In this study, the different reactivities of tungsten oxides with the highly-oxophilic beryllium are studied and compared. Tungsten-(IV)-oxide and tungsten-(VI)-oxide layers are prepared on a tungsten substrate. In the next step, a thin film of beryllium is evaporated on the samples. In consecutive steps, the sample is heated in steps of 100 K from room temperature (r. t.) to 1273 K. The chemical composition is investigated after each experimental step by high-resolution X-ray photoelectron spectroscopy (XPS) for all involved core levels as well as the valence band. A model is developed to analyze the chemical reactions after each step. In this study, we find that tungsten trioxide was already reduced by beryllium at r. t. and started to react to form the ternary compounds BeWO₃ and BeWO₄ at temperatures starting from 673 K. However, tungsten dioxide is resistant to reduction at temperatures of up to 1173 K. In conclusion, we find WO₂ to be much more chemically resistant to the reduction agent Be than WO₃. Full article

In this brief report we discuss the action functional of a particle with damping, showing that it can be obtained from the dissipative equation of motion through a modification which makes the new dissipative equation invariant for time reversal symmetry. This action functional is exactly the effective action of Caldeira-Leggett model but, in our approach, it is derived without the assumption that the particle is weakly coupled to a bath of infinite harmonic oscillators. Full article

Motivated by the need to understand the role of internal interfaces in Li migration occurring in lithium-ion batteries, a first-principles study of a coincident site lattice grain boundary in LiFePO₄ cathode material and in its delithiated counterpart FPO₄ is performed. The structure of the investigated grain boundary is obtained, and the corresponding interface energy is calculated. Other properties, such as ionic charges, magnetic moments, excess free volume, and the lifetime of positrons trapped at the interfaces are determined and discussed. The results show that while the grain boundary in LiFePO₄ has desired structural and bonding characteristics, the analogous boundary in FePO₄ needs to be yet optimized to allow for an efficient Li diffusion study. Full article

Time-resolved optical conductivity is an often used tool to interrogate quantum materials driven out of equilibrium. Theoretically calculating this observable is a complex topic with several approaches discussed in the literature. Using a nonequilibrium Keldysh formalism and a functional derivative approach to the conductivity, we present a comparison of two particular approaches to the calculation of the optical conductivity and their distinguishing features, as applied to a pumped superconductor. The two methods are distinguished by the relative motion of the probe and gate times; either the probe or gate time is kept fixed while the other is swept. We find that both the methods result in same qualitative features of the time-resolved conductivity after pump is over. However, calculating the conductivity by keeping the gate fixed removes artifacts inherent to the other method. We provide software that, based on data for the first method, is able to construct the second approach. Full article

Collective phenomena existing universally in both biological systems and artificial active matter are increasingly attracting interest. The interactions can be grouped into active-active and active-passive ones, where the reports on the purely active system are still clearly dominating. Despite the growing interest, summarizing works for active-passive interactions in artificial active matter are still missing. For that reason, we start this review with a general introduction, followed by a short spotlight on theoretical works and then an extensive overview of experimental realizations. We classify the cases according to the active colloids’ mechanisms of motion and discuss the principles of the interactions. A few key applications of the active-passive interaction of current interest are also highlighted (such as cargo transport, flow field mapping, assembly of structures). We expect that this review will help the fundamental understanding and inspire further studies on active matter. Full article

In several strongly correlated electron systems, the short range ordering of defects, charge and local lattice distortions are found to show complex inhomogeneous spatial distributions. There is growing evidence that such inhomogeneity plays a fundamental role in unique functionality of quantum complex materials. La_1.72Sr_0.28NiO₄ is a prototypical strongly correlated perovskite showing spin stripes order. In this work we present the spatial distribution of the spin order inhomogeneity by applying micro X-ray diffraction to La_1.72Sr_0.28NiO₄, mapping the spin-density-wave order below the 120 K onset temperature. We find that the spin-density-wave order shows the formation of nanoscale puddles with large spatial fluctuations. The nano-puddle density changes on the microscopic scale forming a multiscale phase separation extending from nanoscale to micron scale with scale-free distribution. Indeed spin-density-wave striped puddles are disconnected by spatial regions with negligible spin-density-wave order. The present work highlights the complex spatial nanoscale phase separation of spin stripes in nickelate perovskites and opens new perspectives of local spin order control by strain. Full article

We study numerically the problem of dynamical thermalization of interacting cold fermionic atoms placed in an isolated Sinai oscillator trap. This system is characterized by a quantum chaos regime for one-particle dynamics. We show that, for a many-body system of cold atoms, the interactions, with a strength above a certain quantum chaos border given by the Åberg criterion, lead to the Fermi–Dirac distribution and relaxation of many-body initial states to the thermalized state in the absence of any contact with a thermostate. We discuss the properties of this dynamical thermalization and its links with the Loschmidt–Boltzmann dispute. Full article

In our review, we focus on the quantum spin liquid (QSL), defining the thermodynamic, transport, and relaxation properties of geometrically frustrated magnet (insulators) represented by herbertsmithite ${\mathrm{ZnCu}}_3{\left(\mathrm{OH}\right)}_6{\mathrm{Cl}}_2$ . The review mostly deals with an historical perspective of our theoretical contributions on this subject, based on the theory of fermion condensation closely related to the emergence (due to geometrical frustration) of dispersionless parts in the fermionic quasiparticle spectrum, so-called flat bands. QSL is a quantum state of matter having neither magnetic order nor gapped excitations even at zero temperature. QSL along with heavy fermion metals can form a new state of matter induced by the topological fermion condensation quantum phase transition. The observation of QSL in actual materials such as herbertsmithite is of fundamental significance both theoretically and technologically, as it could open a path to the creation of topologically protected states for quantum information processing and quantum computation. It is therefore of great importance to establish the presence of a gapless QSL state in one of the most prospective materials, herbertsmithite. In this respect, the interpretation of current theoretical and experimental studies of herbertsmithite are controversial in their implications. Based on published experimental data augmented by our theoretical analysis, we present evidence for the the existence of a QSL in the geometrically frustrated insulator herbertsmithite ${\mathrm{ZnCu}}_3{\left(\mathrm{OH}\right)}_6{\mathrm{Cl}}_2$ , providing a strategy for unambiguous identification of such a state in other materials. To clarify the nature of QSL in herbertsmithite, we recommend measurements of heat transport, low-energy inelastic neutron scattering, and optical conductivity $\overline{\sigma }$ in ${\mathrm{ZnCu}}_3{\left(\mathrm{OH}\right)}_6{\mathrm{Cl}}_2$ crystals subject to an external magnetic field at low temperatures. Our analysis of the behavior of $\overline{\sigma }$ in herbertsmithite justifies this set of measurements, which can provide a conclusive experimental demonstration of the nature of its spinon-composed quantum spin liquid. Theoretical study of the optical conductivity of herbertsmithite allows us to expose the physical mechanisms responsible for its temperature and magnetic field dependence. We also suggest that artificially or spontaneously introducing inhomogeneity at nanoscale into ${\mathrm{ZnCu}}_3{\left(\mathrm{OH}\right)}_6{\mathrm{Cl}}_2$ can both stabilize its QSL and simplify its chemical preparation, and can provide for tests that elucidate the role of impurities. We make predictions of the results of specified measurements related to the dynamical, thermodynamic, and transport properties in the case of a gapless QSL. Full article

We report on the measurements of the remnant magnetization, and hence critical current, in a single crystal of YBa₂Cu₄O₈. A peak in the temperature dependence of the critical current is observed when the external magnetic field is tilted away from the a–b planes. The observed behavior is attributed to a thermally activated instability-driven vortex-lattice splitting or vortex chain formation. The nature of the peak and the possibility of a thermally-activated dimensional crossover have been discussed. Full article

We report the development of glucose oxidase pumps characterized by small lateral dimensions (≈200 $\mathsf{\mu }$ m). We studied the effects of the activity of the enzyme pump on silica particles (“tracers”) sedimented around the enzyme pump/patch. Once the activity of the pump was turned on (i.e., the glucose substrate was added to the solution), in-plane motion of the tracers away from the enzyme patch, as well as the emergence of an in-plane region around the patch which was depleted by tracers, was observed. The lateral extent of this depletion zone increased in time at a rate dependent both on the glucose concentration and on the areal density of the enzyme in the patch. We argue that, when the tracers were very near the wall, their motion and the emergence of the depletion zone were most likely the result of diffusiophoresis and drag by osmotic flows induced at the wall, rather than that of drag by a solutal buoyancy driven convective flow. We infer that, for the glucose oxidase enzymatic pumps, bulk (solutal buoyancy), as previously reported, as well as surface (osmotic) driven flows coexist and have to be explicitly accounted for. It seems plausible to assume that this is the case in general for enzyme pumps, and these complementary effects should be considered in the design of applications, e.g., stirring or sensing inside microfluidic systems, based on such pumps. Full article

A multi-particle extension of the Arnol’d cat Hamiltonian system is presented, which can serve as a fully dynamical model of decoherence. The behavior of the von Neumann entropy of the reduced density matrix is studied, in time and as a function of the physical parameters, with special regard to increasing the mass of the cat particle. Full article

A simple equation of state model for metals at high temperature and pressure is described. The model consists of zero-temperature isotherm, thermal ionic components, and thermal electronic components, and is applicable in compressed as well as expanded volume regions. The three components of the model, together with appropriate correction terms, are described in detail using Cu as a prototype example. Shock wave Hugoniot, critical point parameters, liquid–vapor phase diagram, isobaric expansion, etc., are evaluated and compared with experimental data for Cu. The semianalytical model is expected to be useful to prepare extended tables for use in hydrodynamics calculations in high-energy-density physics. Full article

ScMN₂-type (M = V, Nb, Ta) phases are layered materials that have been experimentally reported for M = Ta and Nb. They are narrow-bandgap semiconductors with potentially interesting thermoelectric properties. Point defects such as dopants and vacancies largely affect these properties, motivating the need to investigate these effects. In particular, asymmetric peak features in the density of states (DOS) close to the highest occupied state is expected to increase the Seebeck coefficient. Here, we used first principles calculations to study the effects of one vacancy or one C, O, or F dopant on the DOS of the ScMN₂ phases. We used density functional theory to calculate formation energy and the density of states when a point defect is introduced in the structures. In the DOS, asymmetric peak features close to the highest occupied state were found as a result of having a vacancy in all three phases. Furthermore, one C dopant in ScTaN₂, ScNbN₂, and ScVN₂ implies a shift of the highest occupied state into the valence band, while one O or F dopant causes a shift of the highest occupied state into the conduction band. Full article

Inelastic neutron scattering measurements have been performed on a superconducting single crystal FeTe ${}_{0.5}$ Se ${}_{0.5}$ to examine the $\mathbf{Q}$ -dependent enhancement of the dynamical structure factor, $S(\mathbf{Q},E)$ , from $\mathbf{Q}$ = (0, 0) to ( $\pi$ , $\pi$ ), including ( $\pi$ , 0) in the superconducting state. In most of iron-based superconductors, $S(\mathbf{Q},E)$ is enhanced at $\mathbf{Q}$ = ( $\pi$ , 0), where the “magnetic resonance mode” is commonly observed in the unfolded Brillouin zone. Constant-E cuts of $S(\mathbf{Q},E)$ suggest that the enhancement is not uniform in the magnetic excitation, and limited around $\mathbf{Q}$ = ( $\pi$ , 0). This result is consistent with the theoretical simulation of the magnetic resonance mode due to the Bardeen–Cooper–Schrieffer coherence factor with the sign-reversing order parameter of s ${}_{\pm }$ wave. Full article

Nuclear magnetic resonance (NMR) in cuprate research is a prominent bulk local probe of magnetic properties. NMR also, as was shown over the last years, actually provides a quantitative measure of local charges in the CuO${}_2$ plane. This has led to fundamental insights, e.g., that the maximum $T_c$ is determined by the sharing of the parent planar hole between Cu and O. Using bonding orbital hole contents on planar Cu and O measured by NMR, instead of the total doping x, the thus defined two-dimensional cuprate phase diagram reveals significant differences between the various cuprate materials. Even more importantly, the reflected differences in material chemistry appear to set a number of electronic properties as we discuss here, for undoped, underdoped and optimally doped cuprates. These relations should advise attempts at a theoretical understanding of cuprate physics as well as inspire material chemists towards new high-$T_c$ materials. Probing planar charges, NMR is also sensitive to charge variations or ordering phenomena in the CuO${}_2$ plane. Thereby, local charge order on planar O in optimally doped YBCO could recently be proven. Charge density variations seen by NMR in both planar bonding orbitals with amplitudes between 1% to 5% appear to be omnipresent in the doped CuO${}_2$ plane, i.e., not limited to underdoped cuprates and low temperatures. Full article

High-energy synchrotron X-ray Compton scattering imaging was applied to a commercial 18650-type cell, which is a cylindrical lithium-ion battery in wide current use. By measuring the Compton scattering X-ray energy spectrum non-destructively, the lithiation state in both fresh and aged cells was obtained from two different regions of the cell, one near the outer casing and the other near the center of the cell. Our technique has the advantage that it can reveal the lithiation state with a micron-scale spatial resolution even in large cells. The present method enables us to monitor the operation of large-scale cells and can thus accelerate the development of advanced lithium-ion batteries. Full article

We evaluated isolated transition metal atoms (Sc, Ti, V, Cr, Mn, and Ni) embedded in hexagonal-BN as novel single atom catalysts for CO oxidation. We predicted that embedded Ni atoms should have superior performance for this task. Ti, V, and Mn bind CO₂ too strongly and so the reaction will not proceed smoothly. We studied the detailed reaction processes for Sc, Cr, and Ni. The Langmuir–Hinshelwood (LH), Eley–Rideal (ER), and the new termolecular Eley–Rideal (TER) processes for CO oxidation were investigated. Sc was not effective. Cr primarily used the ER process, although the barrier was relatively large at 1.30 eV. Ni was the best of the group, with a 0.44 eV barrier for LH, and a 0.47 eV barrier for TER. Therefore, we predicted that the LH and TER processes could operate at relatively low temperatures between 300 and 500 K. Full article

This work motivates and applies operational methodology to simulation of quantum statistics of separable qubit X states. Three operational algorithms for evaluating separability probability distributions are put forward. Building on previous findings, the volume function characterizing the separability distribution is determined via quantum measurements of multi-qubit observables. Three measuring states, one for each algorithm are generated via (i) a multi-qubit channel map, (ii) a unitary operator generated by a Hamiltonian describing a non-uniform hypergraph configuration of interactions among 12 qubits, and (iii) a quantum walk CP map in a extended state space. Higher order CZ gates are the only tools of the algorithms hence the work associates itself computationally with the Instantaneous Quantum Polynomial-time Circuits (IQP), while wrt possible implementation the work relates to the Lechner-Hauke-Zoller (LHZ) architecture of higher order coupling. Finally some uncertainty aspects of the quantum measurement observables are discussed together with possible extensions to non-qubit separable bipartite systems. Full article

The stability of cubic HfV₂ $\left(Fd\overline{3}m\right)$ was investigated as a function of temperature as well as interstitially solved oxygen and hydrogen using density functional theory. Mechanical and energetic instability of pristine cubic HfV₂ is obtained in the ground state at 0 K, which is unexpected as it can readily be synthesized. Combined Debye–Grüneisen and electronic entropy calculations indicate that HfV₂ is stabilized with increasing temperature primarily as a result of lattice vibrations. In contrast, temperature-induced mechanical stabilization, considering the Born stability criteria, is achieved due to the electronic entropy. Interstitial incorporation of hydrogen and oxygen into the cubic structure contributes to the energetic and mechanical stabilization in the ground state for impurity concentrations as low as 1 at%, owing to strong ionic/covalent bond formation with the matrix atoms. Full article

We study the ground state of a one-dimensional channel with strong attractive electron–electron interactions at low temperatures. In spite of the fact that, at low temperatures, the ground state of one-dimensional attracting electrons is a state with a macroscopically large number of cooperons, the resulting superconducting phase has a number of significant differences. Namely, the order parameter (which should appear in the superconducting phase according to Landau’s phenomenological theory) turns out to be zero. However, elastic impurities implanted in a one-dimensional channel will not lead to dissipation of the supercurrent associated with the condensate movement as a whole. Full article

Airborne dust extracted from deep ice core perforations can provide chemical and mineralogical insight into the history of the climate and atmospheric conditions, with unrivalled temporal resolution, time span and richness of information. The availability of material for research and the natural complexity of the particulate, however, pose significant challenges to analytical methods. We present the developments undertaken to optimize the experimental techniques, materials and protocols for synchrotron radiation-based analysis, in particular for the acquisition of combined Synchrotron Radiation X-Ray Fluorescence and X-ray Absorption Spectroscopy data. Full article