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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 in 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 which is visible via the Faraday effect., Comment: 6 pages, 5 figures

High-temperature solid-state reaction between orthorhombic AgF2 and monoclinic CuF2 (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 Ag1-xCuxF2. Based on X-ray powder diffraction analyses, the mutual solubility in the orthorhombic phase (AgF2 doped with Cu) appears to be at an upper limit of Cu concentration of 30 mol % (Ag0.7Cu0.3F2), while the monoclinic phase (CuF2 doped with Ag) can form a nearly stoichiometric Cu : Ag = 1 : 1 solid solution (Cu0.56Ag0.44F2), preserving the CuF2 crystal structure. Experimental data and DFT calculations showed that AgF2 doped with Cu and CuF2 doped with Ag solid solutions deviate from the classical Vegards law. Magnetic measurements of Ag1-xCuxF2 showed that the Neel temperature (TN) decreases with increasing Cu content in both phases. Likewise, theoretical DFT+U calculations for Ag1-xCuxF2 showed that the progressive substitution of Ag by Cu decreases the magnetic interaction strength (J2D) in both structures. Electrical conductivity measurements of Ag0.85Cu0.15F2 showed a ca. 2-fold increase in specific ionic conductivity (3.71 x 10-13 plus/minus 2.6 x 10-15 S/cm) as compared to pure AgF2 (1.85 x 10-13 plus/minus 1.2 x 10-15 S/cm), indicating the formation of a vacancy- or F adatom-free metal difluoride sample., Comment: 9 pages, 4 figures, 1 Table, and electronic supplement of 14 pages

We investigate the physics of an adiabatic transition from a BCS superconductor to a Fermi liquid for an exponentially slow decreasing pairing interaction. In particular, we show that the metal keeps memory of the parent BCS state so it is possible to reverse the dynamics and go back to the original state similarly to a spin/photon echo experiment. Moreover, we study the evolution of the order parameter phase phi in transforming the BCS superconductor to a conventional metal. Since the global phase is the conjugate variable of the density we explicitly show how to use the dynamics of phi together with gauge invariance to build up the non-interacting chemical potential away from particle-hole symmetry. We further analyze the role of phi in restoring the gauge invariant current response when the non-interacting Fermi liquid is approached starting from a BCS superconductor in the presence of an external vector field., Comment: 13 pages, 12 figures

Using density functional theory, we study the lattice dynamical properties of magnetite (Fe$_3$O$_4$) in the high-temperature cubic and low-temperature monoclinic phases. The calculated phonon dispersion curves and phonon density of states are compared with the available experimental data obtained by inelastic neutron, inelastic x-ray, and nuclear inelastic scattering. We find a very good agreement between the theoretical and experimental results for the monoclinic $Cc$ structure revealing the strong coupling between charge-orbital (trimeron) order and specific phonon modes. For the cubic phase, clear discrepancies arise which, remarkably, can be understood assuming that the strong trimeron-phonon coupling can be extended above the Verwey transition, with lattice dynamics influenced by the short-range trimeron order instead of the average cubic structure. Our results establish the validity of trimerons (and trimeron-phonon coupling) in explaining the physics of magnetite much beyond their original formulation., Comment: 11 pages, 9 figures, 1 table

We investigate the quench dynamics of strongly coupled superconductors within the time-dependent Gutzwiller approximation from the BCS to the BEC regime and evaluate the out-of-equilibrium transient spectral density and optical conductivity relevant for pump probe experiments. Fourier transformation of the order parameter dynamics reveals a frequency $\Omega_J$ which, as in the BCS case, is controlled by the spectral gap. However, we find a crossover from the BCS dynamics to a new strong coupling regime where a characteristic frequency $\Omega_U$, associated to double occupancy fluctuations controls the order parameter dynamics. The change of regime occurs close to a dynamical phase transition. Both, $\Omega_J$ and $\Omega_U$ give rise to a complex structure of self-driven slow Rabi oscillations which are visible in the non-equilibrium optical conductivity where also side bands appear due to the modulation of the double occupancy by superconducting amplitude oscillations. Analogous results apply to CDW and SDW systems., Comment: 14 pages, 14 figures

From systematic analysis of the high pulsed magnetic field resistance data of La$_{2-x}$Sr$_x$CuO$_{4}$ thin films, we extract an experimental phase diagram for several doping values ranging from the very underdoped to the very overdoped regimes. Our analysis highlights a competition between charge density waves and superconductivity which is ubiquitous between $x=0.08$ and $x=0.19$ and produces the previously observed double step transition. When suppressed by a strong magnetic field, superconductivity is resilient for two specific doping ranges centered around respectively $x\approx 0.09$ and $x\approx 0.19$ and the characteristic temperature for the onset of the competing charge density wave phase is found to vanish above $x = 0.19$. At $x=1/8$ the two phases are found to coexist exactly at zero magnetic field., Comment: 13 figures. Changes from previous version are in red. A few clarifications and a discussion about the different materials were added together with additional references

Although charge density waves (CDWs) are omnipresent in cuprate high-temperature superconductors, they occur at significantly different wavevectors, confounding efforts to understand their formation mechanism. Here, we use resonant inelastic x-ray scattering to investigate the doping- and temperature-dependent CDW evolution in La2-xBaxCuO4 (x=0.115-0.155). We discovered that the CDW develops in two stages with decreasing temperature. A precursor CDW with quasi-commensurate wavevector emerges first at high-temperature. This doping-independent precursor CDW correlation originates from the CDW phase mode coupled with a phonon and "seeds" the low-temperature CDW with strongly doping dependent wavevector. Our observation reveals the precursor CDW and its phase mode as the building blocks of the highly intertwined electronic ground state in the cuprates., Comment: In press at Physical Review X; 11 pages, 6 figures

Solid 4He may acquire superfluid characteristics due to the frustration of the solid phase at grain boundaries. Here, we show that an analogous effect occurs in systems with competition among charge-density-waves (CDWs) and superconductivity in the presence of disorder, as cuprate or dichalcogenide superconductors. The CDWs breaks apart in domains with topologically protected filamentary superconductivity (FSC) at the interfaces. Transport experiments carried out in underdoped cuprates with the magnetic field acting as a control parameter are shown to be in excellent agreement with the theoretical expectation. At high temperature and low fields we find a transition from CDWs to fluctuating superconductivity, weakly affected by disorder, while at high field and low temperature the protected filamentary superconducting phase appears in close analogy with "glassy" supersolid phenomena in 4He.

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 (RIXS) to follow the evolution of charge correlations in the canonical stripe ordered cuprate La$_{1.875}$Ba$_{0.125}$CuO$_{4}$ (LBCO~$1/8$) 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., Comment: Accepted in Proceeding of the National Academy of Sciences; 8 pages, 4 figures

The Raman spectrum of Ba2CuGe2O7, a tetragonal insulator which develops Dzyaloshinsky- 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. 29 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.

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., Comment: 10 pages, 4 figures plus supplementary information

We investigate the disorder dependence of the static density, amplitude and current correlations within the attractive Hubbard model supplemented with on-site disorder. It is found that strong disorder favors a decoupling of density and amplitude correlations due to the formation of superconducting islands. This emergent granularity also induces an enhancement of the density correlations on the SC islands whereas amplitude fluctuations are most pronounced in the 'insulating' regions. While density and amplitude correlations are short-ranged at strong disorder we show that current correlations have a long-range tail due to the formation of percolative current paths in agreement with the constant behavior expected from the analysis of one-dimensional models., Comment: 17 pages, 19 figures; revised discussion and conclusions section; accepted for Physical Review B