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The Raman spectrum of Ba2CuGe2O7, a tetragonal insulator which develops Dzyaloshinskii-Moriya helical magnetism below TN = 3 .2 K, has been detected at temperatures varying from 300 to 80 K in a single crystal, with the radiation polarized either in the ab plane or along the c axis of its tetragonal cell. Twenty-nine phonon lines out of the 35 allowed by the Raman selection rules for the present geometry were observed, and their vibrational frequencies were found in overall good agreement with those provided by shell-model calculations. Together with the previous report on the infrared-active phonons [A. Nucara et al., Phys. Rev. B 90, 014304 (2014)], the present study provides an exhaustive description, both experimental and theoretical, of the lattice dynamics in Ba2CuGe2O7. [ABSTRACT FROM AUTHOR]

The high Curie temperature multiferroic compound CuO has a quasidegenerate magnetic ground state that makes it prone to manipulation by the so-called "order-by-disorder" mechanism. First principle computations supplemented with Monte Carlo simulations and experiments show that isovalent doping allows us to stabilize the multiferroic phase in nonferroelectric regions of the pristine material phase diagram with experiments reaching a 250% widening of the ferroelectric temperature window with 5% of Zn doping. Our results allow us to validate the importance of a quasidegenerate ground state on promoting multiferroicity on CuO at high temperatures and open a path to the material engineering of multiferroic materials. In addition we present a complete explanation of the CuO phase diagram and a computation on the incommensurability in excellent agreement with experiment without free parameters. [ABSTRACT FROM AUTHOR]

The lattice dynamics of Ba2CuGe2O7, a compound which develops Dzyaloshinsky-Moriya helical magnetism below TN = 3.2 K, has been studied by measuring the infrared reflectivity of a single crystal with the radiation polarized both in the ab plane and along the c axis of its tetragonal cell, from 7 to 300 K. In this compound, where the unit cell has no inversion symmetry, 14 E phonon modes of the ab plane, out of the 18 predicted, and nine B2 modes of the c axis, out of the ten predicted, have been observed. They have been assigned to the atomic motions by a comparison with shell-model calculations, which provided vibrational frequencies in good agreement with the experiment, while most calculated intensities turned out to be much lower than the experimental values. This discrepancy has been tentatively explained by assuming strong electron-phonon interactions, a hypothesis supported by the failure of the f-sum rule if restricted to the phonon region. Indeed, we observe a remarkable increase in the oscillator strengths between 300 and 200 K for the phonons of the ab plane (between 300 and 100 K for the c axis), which implies an increase in the dielectric constant of Ba2CuGe2O7 below room temperature. [ABSTRACT FROM AUTHOR]

According to the Goldstone theorem the breaking of a continuous U(I) symmetry comes along with the existence of low-energy collective modes. In the context of superconductivity these excitations are related to the phase of the superconducting (SC) order parameter and for clean systems are optically inactive; that is, single-mode excitations do not directly couple to light. Here we show that for strongly disordered superconductors phase modes acquire a dipole moment and appear as a subgap spectral feature in the optical conductivity. This finding is obtained with both a gauge-invariant random-phase approximation scheme based on a fermionic Bogoliubov-de Gennes state and a prototypical bosonic model for disordered superconductors. In the strongly disordered regime, where the system displays an effective granularity of the SC properties, the optically active dipoles are linked to the isolated SC islands, offering a new perspective for realizing microwave optical devices. [ABSTRACT FROM AUTHOR]

Symmetry breaking across phase transitions often causes changes in selection rules and emergence of optical modes which can be detected via spectroscopic techniques or generated coherently in pump-probe experiments. In second-order or weakly first-order transitions, fluctuations of the ordering field are present above the ordering temperature, giving rise to intriguing precursor phenomena, such as critical opalescence. Here, we demonstrate that in magnetite (Fe3O4) light excitation couples to the critical fluctuations of the charge order and coherently generates structural modes of the ordered phase above the critical temperature of the Verwey transition. Our findings are obtained by detecting coherent oscillations of the optical constants through ultrafast broadband spectroscopy and analyzing their dependence on temperature. To unveil the coupling between the structural modes and the electronic excitations, at the origin of the Verwey transition, we combine our results from pump-probe experiments with spontaneous Raman scattering data and theoretical calculations of both the phonon dispersion curves and the optical constants. Our methodology represents an effective tool to study the real-time dynamics of critical fluctuations across phase transitions. [ABSTRACT FROM AUTHOR]

We have measured the reflectivity in the ab plane and along the c axis of a single crystal of the Dzyaloshinsky-Moriya multiferroic Ba2CuGe2O7, from 20 to 300 K and from 700 to 24 000 cm-1. The resulting optical conductivity σ(ω) has been compared with the results of calculations of its electronic band structure based on density functional theory (DFT). A broad midinfrared band, which in the ab plane strongly hardens for decreasing temperature, is consistent with d-d transitions of the Cu atom. A further electronic absorption, whose edge falls in the near infrared and has a strong O-p character, unusually softens with decreasing temperature by more than 0.15 eV, in both polarizations. Calculations show that the behavior with temperature of those electronic bands can be related to distortions of the CuO4 tetrahedra. [ABSTRACT FROM AUTHOR]

We investigate order parameter fluctuations in the Hubbard model within a time-dependent Gutzwiller approach. While in the weak coupling limit we find that the amplitude fluctuations are short-lived due to a degeneracy with the energy of the edge of the quasiparticle continua (and in agreement with Hartree-Fock+RPA theory), these are shifted below the edge upon increasing the interaction. Our calculations therefore predict undamped amplitude (Higgs) oscillations of the order parameter in strongly coupled superconductors, cold atomic fermion condensates, and strongly interacting charge- and spin-density wave systems. We propose an experimental realization for the detection of the spin-type Higgs mode in undoped cuprates and related materials where, due to the Dzyaloshinsky-Moriya interaction, it can couple to an out-of-plane ferromagnetic excitation that is visible via the Faraday effect.

High-temperature solid-state reaction between orthorhombic AgF 2 and monoclinic CuF 2 (y=0.15, 0.3, 0.4, 0.5) in a fluorine atmosphere resulted in coexisting solid solutions of Cu-poor orthorhombic and Cu-rich monoclinic phases with stoichiometry Ag 1-x Cu x F 2 . Based on X-ray powder diffraction analyses, the mutual solubility in the orthorhombic phase (AgF 2  : Cu) appears to be at an upper limit of Cu concentration of 30 mol % (Ag 0.7 Cu 0.3 F 2 ), while the monoclinic phase (CuF 2  : Ag) can form a nearly stoichiometric Cu : Ag=1 : 1 solid solution (Cu 0.56 Ag 0.44 F 2 ), preserving the CuF 2 crystal structure. Experimental data and DFT calculations showed that AgF 2  : Cu and CuF 2  : Ag solid solutions deviate from the classical Vegard's law. Magnetic measurements of Ag 1-x Cu x F 2 showed that the Néel temperature (T N ) decreases with increasing Cu content in both phases. Likewise, theoretical DFT+U calculations for Ag 1-x Cu x F 2 showed that the progressive substitution of Ag by Cu decreases the magnetic interaction strength |J 2D | in both structures. Electrical conductivity measurements of Ag 0.85 Cu 0.15 F 2 showed a modest increase in specific ionic conductivity (3.71 ⋅ 10 -13 ±2.6 ⋅ 10 -15  S/cm) as compared to pure AgF 2 (1.85 ⋅ 10 -13± 1.2 ⋅ 10 -15  S/cm), indicating the formation of a vacancy- or F adatom-free metal difluoride sample., (© 2023 Wiley-VCH GmbH.)

The nature of superconductivity and its interplay with strong spin-orbit coupling at the KTaO 3 (111) interfaces remain a subject of debate. To address this problem, we grew epitaxial LaMnO 3 /KTaO 3 (111) heterostructures. We show that superconductivity is robust against the in-plane magnetic field, with the critical field of superconductivity reaching ∼25 T in optimally doped heterostructures. The superconducting order parameter is highly sensitive to the carrier density. We argue that spin-orbit coupling drives the formation of anomalous quasiparticles with vanishing magnetic moment, providing significant condensate immunity against magnetic fields beyond the Pauli paramagnetic limit. These results offer design opportunities for superconductors with extreme resilience against the applied magnetic fields.

Flat monolayers of silver(II) fluoride, which could be obtained by epitaxial deposition on an appropriate substrate, have been recently predicted to exhibit very strong antiferro-magnetic superexchange and to have large potential for ambient pressure superconductivity if doped to an optimal level. It was shown that AgF 2 could become a magnetic glue-based superconductor with a critical superconducting temperature approaching 200 K at optimum doping. In the current work we calculate the optimum doping to correspond to 14% of holes per formula unit, i.e. quite similar to that for oxocuprates(II). Furthermore, using DFT calculations we show that flat [AgF 2 ] single layers can indeed be doped to a controlled extent using a recently proposed "chemical capacitor" setup. Hole doping associated with the formation of Ag(III) proves to be difficult to achieve in the setup explored in this work as it falls at the verge of charge stability of fluoride anions and does not affect the d( x 2 - y 2 ) manifold. However, in the case of electron doping, manipulation of different factors - such as the number of dopant layers and the thickness of the separator - permits fine tuning of the doping level (and concomitantly T C ) all the way from the underdoped to overdoped regime (in a similar manner to chemical doping for the Nd 2 CuO 4 analogue).

In the era of molecular devices and nanotechnology, precise control over electron-transfer processes is strongly desired. However, redox reactions are usually characterized by reaction equilibrium constants strongly departing from unity. This leads to strong favoring of either reactants or products and does not permit subtle control of transferred charge (doping). Here we propose, based on theoretical studies for periodic systems, how charge transfer between reactants could be finely manipulated in the epitaxially grown system composed of extremely strong oxidizer, reducing agent, and an inert separator-the key factor of control., (© 2021 Wiley-VCH GmbH.)

AgF2 is a layered material and a correlated charge transfer insulator with an electronic structure very similar to the parent compounds of cuprate high-TC superconductors. It is also interesting as it is a powerful oxidizer. Here we present a first principles computation of its electronic properties in a slab geometry including its work function for the (010) surface (7.76 eV) which appears to be the highest among known materials with non-dipolar surfaces, and surpassing even that of fluorinated diamond (7.24 eV). We demonstrate that AgF2 will show a "broken-gap" type alignment becoming electron doped and promoting injection of holes in many wide band gap insulators if chemical reaction can be avoided. Novel junction devices involving p type and n type superconductors have been proposed. The issue of chemical reaction is discussed in connection with the possibility to create flat AgF2 monolayers achieving high-TC superconductivity. As a first step in this direction, we studied the stability and properties of an isolated AgF2 monolayer.

The parent compound of high-[Formula: see text] superconducting cuprates is a unique Mott insulator consisting of layers of spin-[Formula: see text] ions forming a square lattice and with a record high in-plane antiferromagnetic coupling. Compounds with similar characteristics have long been searched for without success. Here, we use a combination of experimental and theoretical tools to show that commercial [Formula: see text] is an excellent cuprate analog with remarkably similar electronic parameters to [Formula: see text] but larger buckling of planes. Two-magnon Raman scattering and inelastic neutron scattering reveal a superexchange constant reaching 70% of that of a typical cuprate. We argue that structures that reduce or eliminate the buckling of the [Formula: see text] planes could have an antiferromagnetic coupling that matches or surpasses the cuprates., Competing Interests: The authors declare no conflict of interest., (Copyright © 2019 the Author(s). Published by PNAS.)

Meta-GGA calculations of the ambient and high-pressure polymorphs of silver difluoride indicate that the compression-induced structural changes lead to a 3.5-fold increase in the strength of antiferromagnetic spin-spin interactions resulting in coupling constant values higher than those found for record-holding oxocuprates(ii).

Objective:  There is no standard protocol to guide the optimal time to resume anti-clotting agents after traumatic brain injury (TBI) in patients with a continued indication for anticoagulation/antiplatelet therapy (AAT). This study develops baseline data supporting a future prospective cohort study. We predict that there will be significantly decreased adverse events when AAT is started on or after Day 7., Methods:  A retrospective chart review of 256 patients was performed. Patients admitted to a level I trauma center in West Texas between January 1, 2009, and December 31, 2012, on anti-clotting agents (specifically acetylsalicylic acid, coumadin, and/or clopidogrel) and who suffered a TBI were included. Patient metrics included admission coagulation studies, type of TBI and treatment, and time to continuation of AAT. Outcomes were assessed using follow-up appointment data. The primary outcome was death (mortality). Secondary outcomes included myocardial infarction, stroke, re-bleed, venous thromboembolism, and pneumonia., Results: A total of 256 patients met the inclusion criteria. However, only 85 patients on AAT presented for the six-month follow-up. Time to AAT resumption varied from immediate to 31 days. Out of the 85 patients, 32 patients never resumed AAT, 32 patients were restarted on AAT medication in less than seven days, 10 patients restarted medication between seven and 14 days, and 11 patients restarted AAT in more than 14 days. Adverse events occurred most infrequently in the AAT group resuming therapy between seven and 14 days (10%). Adverse events were most prevalent in the AAT group that never resumed therapy (68.8%)., Conclusion:  While most studies suggest that the safest time for resuming AAT lies between three and 10 days, our study revealed that adverse events were minimized in patients on AAT between seven and 9.5 days., Competing Interests: The authors have declared that no competing interests exist.

We use Monte Carlo simulations to study the finite temperature behavior of vortices in the XY model for tangent vector order on curved backgrounds. Contrary to naive expectations, we show that the underlying geometry does not affect the proliferation of vortices with temperature respect to what is observed on a flat surface. Long-range order in these systems is analyzed by using two-point correlation functions. As expected, in the case of slightly curved substrates these correlations behave similarly to the plane. However, for high curvatures, the presence of geometry-induced unbounded vortices at low temperatures produces the rapid decay of correlations and an apparent lack of long-range order. Our results shed light on the finite-temperature physics of soft-matter systems and anisotropic magnets deposited on curved substrates.

Although all superconducting cuprates display charge-ordering tendencies, their low-temperature properties are distinct, impeding efforts to understand the phenomena within a single conceptual framework. While some systems exhibit stripes of charge and spin, with a locked periodicity, others host charge density waves (CDWs) without any obviously related spin order. Here we use resonant inelastic X-ray scattering to follow the evolution of charge correlations in the canonical stripe-ordered cuprate La 1.875 Ba 0.125 CuO 4 across its ordering transition. We find that high-temperature charge correlations are unlocked from the wavevector of the spin correlations, signaling analogies to CDW phases in various other cuprates. This indicates that stripe order at low temperatures is stabilized by the coupling of otherwise independent charge and spin density waves, with important implications for the relation between charge and spin correlations in the cuprates., Competing Interests: The authors declare no conflict of interest.

Transmetatarsal amputations have generally been accepted as a relatively more definitive amputation compared with other lesser ray resections. However, many investigators have reported a high occurrence of more proximal amputation after transmetatarsal amputation. A systematic review was performed to evaluate the occurrence of reamputation and reoperation after transmetatarsal amputations. A search of the Medline, CINAHL, and Cochrane Central databases yielded 159 abstracts. After review, 24 reports were included in the study. A total of 391 (26.9%) reoperations were identified after 1453 transmetatarsal amputations. Any level reamputation occurred in 152 (29.7%) of 365 transmetatarsal amputations and major amputation occurred in 380 (33.2%) of 1146 transmetatarsal amputations. Using a random effects model, the reoperation rate was estimated at 24.43% (95% confidence interval 11.64% to 37.21%), the reamputation rate was estimated at 28.37% (95% confidence interval 19.56% to 37.19%), and the major amputation rate was estimated at 30.16% (95% confidence interval 23.86% to 36.47%). These findings raise questions about the conventional wisdom of performing primary transmetatarsal amputation in lieu of other minor amputations, such as partial first ray amputation, and suggest that the choice between transmetatarsal amputation and other minor amputations might be a decision that depends on very patient-specific factors., (Copyright © 2016 American College of Foot and Ankle Surgeons. All rights reserved.)

We present a model for describing the combined presence of nematic and 'smectic' or stripe-like orders seen in recent scanning tunneling microscopy (STM) experiments on cuprates. The smectic order is treated as an electronic charge density wave with an associated Peierls distortion or a 'Pomeranchuk wave'. This primary order is restricted to nanoscale domains by disorder effects, while the secondary coupling to strain generates the nematic order with a considerably longer range. A variety of experimental results are shown to be consistent with our theoretical predictions.

Empirical evidence in heavy fermion, pnictide and other systems suggests that unconventional superconductivity appears associated to some form of real-space electronic order. For the cuprates, despite several proposals, the emergence of order in the phase diagram between the commensurate antiferromagnetic state and the superconducting state is not well understood. Here we show that in this regime doped holes assemble in 'electronic polymers'. Within a Monte Carlo study, we find that in clean systems by lowering the temperature the polymer melt condenses first in a smectic state and then in a Wigner crystal both with the addition of inversion symmetry breaking. Disorder blurs the positional order leaving a robust inversion symmetry breaking and a nematic order, accompanied by vector chiral spin order and with the persistence of a thermodynamic transition. Such electronic phases, whose properties are reminiscent of soft-matter physics, produce charge and spin responses in good accord with experiments.

Nucleation and growth is the dominant relaxation mechanism driving first-order phase transitions. In two-dimensional flat systems, nucleation has been applied to a wide range of problems in physics, chemistry and biology. Here we study nucleation and growth of two-dimensional phases lying on curved surfaces and show that curvature modifies both critical sizes of nuclei and paths towards the equilibrium phase. In curved space, nucleation and growth becomes inherently inhomogeneous and critical nuclei form faster on regions of positive Gaussian curvature. Substrates of varying shape display complex energy landscapes with several geometry-induced local minima, where initially propagating nuclei become stabilized and trapped by the underlying curvature.

High-temperature superconductors exhibit a characteristic hourglass-shaped spectrum of magnetic fluctuations which most likely contribute to the pairing glue in the cuprates. Recent neutron scattering experiments in strongly underdoped compounds have revealed a significant low energy anisotropy of these fluctuations which we explain by a model in which topological defects of the antiferromagnet clump to producing domain wall segments with ferronematic order. This state does not invoke global charge order but breaks C4 rotational and inversion symmetry. The incommensurability of the low doping charge-disordered state is in good agreement with experiment and interpolates smoothly with the incommensurability of the stripe phase at higher doping. Within linear spin-wave theory the dynamic structure factor is in very good agreement with inelastic neutron scattering data and can account for the observed energy dependent anisotropy.

In several materials, unconventional superconductivity appears nearby a quantum phase transition where long-range magnetic order vanishes as a function of a control parameter like charge doping, pressure or magnetic field. The nature of the quantum phase transition is of key relevance, because continuous transitions are expected to favour superconductivity, due to strong fluctuations. Discontinuous transitions, on the other hand, are not expected to have a similar role. Here we determine the nature of the magnetic quantum phase transition, which occurs as a function of doping, in the iron-based superconductor LaFeAsO(1-x)F(x). We use constrained density functional calculations that provide ab initio coefficients for a Landau order parameter analysis. The outcome is intriguing, as this material turns out to be remarkably close to a quantum tricritical point, where the transition changes from continuous to discontinuous, and several susceptibilities diverge simultaneously. We discuss the consequences for superconductivity and the phase diagram.

Cupric oxide is multiferroic at unusually high temperatures. From density functional calculations we find that the low-T magnetic phase is paraelectric, and the higher-T one is ferroelectric with a size and direction of polarization in good agreement with experiments. By mapping the ab initio results on to an effective spin model, we show that the system has a manifold of almost degenerate ground states. In the high-T magnetic state noncollinearity and inversion symmetry breaking stabilize each other via the Dzyaloshinskii-Moriya interaction. This leads to an unconventional mechanism for multiferroicity, with the particular property that nonmagnetic impurities enhance the effect.

A supercooled liquid is said to have a kinetic spinodal if a temperature Tsp exists below which the liquid relaxation time exceeds the crystal nucleation time. We revisit classical nucleation theory taking into account the viscoelastic response of the liquid to the formation of crystal nuclei and find that the kinetic spinodal is strongly influenced by elastic effects. We introduce a dimensionless parameter lambda, which is essentially the ratio between the infinite frequency shear modulus and the enthalpy of fusion of the crystal. In systems where lambda is larger than a critical value lambda(c) the metastability limit is totally suppressed, independently of the surface tension. On the other hand, if lambda

We perform a systematic slave-boson mean-field analysis of the three-band model for cuprates with first-principle parameters. Contrary to widespread belief based on earlier mean-field computations low doping stripes have a linear density close to 1/2 added hole per lattice constant. We find a dimensional crossover from 1D to 2D at doping approximately 0.1 followed by a breaking of particle-hole symmetry around doping 1/8 as doping increases. Our results explain in a simple way the behavior of the chemical potential, the magnetic incommensurability, and transport experiments as a function of doping. Bond centered and site-centered stripes become degenerate for small overdoping.

This paper develops a metatheoretical framework for understanding evolutionary systems (systems that develop in ways that increase their own variety). The framework addresses shortcomings seen in other popular systems theories. It concerns both living and nonliving systems, and proposes a metahierarchy of hierarchical systems. Thus, it potentially addresses systems at all descriptive levels. We restrict our definition of system to that of a core system whose parts have a different ontological status than the system, and characterize the core system in terms of five global properties: minimal length interval, minimal time interval, system cycle, total receptive capacity, and system potential. We propose two principles through the interaction of which evolutionary systems develop. The Principle of Combinatorial Expansion describes how a core system realizes its developmental potential through a process of progressive differentiation of the single primal state up to a limit stage. The Principle of Generative Condensation describes how the components of the last stage of combinatorial expansion condense and become the environment for and components of new, enriched systems. The early evolution of the Universe after the "big bang" is discussed in light of these ideas as an example of the application of the framework.