A long-standing controversial issue in the quest to understand the superconductivity in cuprates is the nature of the enigmatic pseudogap region of the phase diagram. Especially important is whether ...the pseudogap state is a distinct thermodynamic phase characterized by broken symmetries below the onset temperature T∗ . Here we report torque-magnetometry measurements of anisotropic susceptibility within the ab planes in orthorhombic YBa2 Cu3 Oy with exceptionally high precision. The in-plane anisotropy displays a significant increase with a distinct kink at the pseudogap onset temperature T∗ , showing a remarkable scaling behaviour with respect to T/T∗ in a wide doping range. Our systematic analysis reveals that the rotational symmetry breaking sets in at T∗ in the limit where the effect of orthorhombicity is eliminated. These results provide thermodynamic evidence that the pseudogap onset is associated with a second-order nematic phase transition, which differs from the recently reported charge-density-wave transition that accompanies translational symmetry breaking.
Nonlinear optical excitation of infrared active lattice vibrations has been shown to melt magnetic or orbital orders and to transform insulators into metals. In cuprates, this technique has been used ...to remove charge stripes and promote superconductivity, acting in a way opposite to static magnetic fields. Here, we show that excitation of large-amplitude apical oxygen distortions in the cuprate superconductor YBa2Cu3O6.5 promotes highly unconventional electronic properties. Below the superconducting transition temperature (Tc = 50 K) inter-bilayer coherence is transiently enhanced at the expense of intra-bilayer coupling. Strikingly, even above Tc a qualitatively similar effect is observed up to room temperature, with transient inter-bilayer coherence emerging from the incoherent ground state and similar transfer of spectral weight from high to low frequency. These observations are compatible with previous reports of an inhomogeneous normal state that retains important properties of a superconductor, in which light may be melting competing orders or dynamically synchronizing the interlayer phase. The transient redistribution of coherence discussed here could lead to new strategies to enhance superconductivity in steady state.
Terahertz-frequency optical pulses can resonantly drive selected vibrational modes in solids and deform their crystal structures. In complex oxides, this method has been used to melt electronic ...order, drive insulator-to-metal transitions and induce superconductivity. Strikingly, coherent interlayer transport strongly reminiscent of superconductivity can be transiently induced up to room temperature (300 kelvin) in YBa2Cu3O6+x (refs 9, 10). Here we report the crystal structure of this exotic non-equilibrium state, determined by femtosecond X-ray diffraction and ab initio density functional theory calculations. We find that nonlinear lattice excitation in normal-state YBa2Cu3O6+x at above the transition temperature of 52 kelvin causes a simultaneous increase and decrease in the Cu-O2 intra-bilayer and, respectively, inter-bilayer distances, accompanied by anisotropic changes in the in-plane O-Cu-O bond buckling. Density functional theory calculations indicate that these motions cause drastic changes in the electronic structure. Among these, the enhancement in the character of the in-plane electronic structure is likely to favour superconductivity.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Cuprates exhibit antiferromagnetic, charge density wave (CDW), and high-temperature superconducting ground states that can be tuned by means of doping and external magnetic fields. However, disorder ...generated by these tuning methods complicates the interpretation of such experiments. Here, we report a high-resolution inelastic x-ray scattering study of the high-temperature superconductor YBa
Cu
O
under uniaxial stress, and we show that a three-dimensional long-range-ordered CDW state can be induced through pressure along the
axis, in the absence of magnetic fields. A pronounced softening of an optical phonon mode is associated with the CDW transition. The amplitude of the CDW is suppressed below the superconducting transition temperature, indicating competition with superconductivity. The results provide insights into the normal-state properties of cuprates and illustrate the potential of uniaxial-pressure control of competing orders in quantum materials.
The electron-phonon interaction is a major factor influencing the competition between collective instabilities in correlated-electron materials, but its role in driving high-temperature ...superconductivity in the cuprates remains poorly understood. We have used high-resolution inelastic X-ray scattering to monitor low-energy phonons in YBa2Cu3O6.6 (superconducting transition temperature Tc = 61 K), which is close to a charge-density-wave (CDW) instability. Phonons in a narrow range of momentum space around the CDW ordering vector exhibit extremely large superconductivity-induced line-shape renormalizations. These results imply that the electron-phonon interaction has sufficient strength to generate various anomalies in electronic spectra, but does not contribute significantly to Cooper pairing. In addition, a quasi-elastic 'central peak' due to CDW nanodomains is observed in a wide temperature range above and below Tc, suggesting that the gradual onset of a spatially inhomogeneous CDW domain state with decreasing temperature is a generic feature of the underdoped cuprates.
The concept that superconductivity competes with other orders in cuprate superconductors has become increasingly apparent, but obtaining direct evidence with bulk-sensitive probes is challenging. We ...have used resonant soft x-ray scattering to identify two-dimensional charge fluctuations with an incommensurate periodicity of ∼3.2 lattice units in the copper-oxide planes of the superconductors (Y,Nd)Ba 2 Cu 3 O 6+x , with hole concentrations of 0.09 to 0.13 per planar Cu ion. The intensity and correlation length of the fluctuation signal increase strongly upon cooling down to the superconducting transition temperature (T c ); further cooling below T c abruptly reverses the divergence of the charge correlations. In combination with earlier observations of a large gap in the spin excitation spectrum, these data indicate an incipient charge density wave instability that competes with superconductivity.
We used resonant inelastic x-ray scattering (RIXS) with and without analysis of the scattered photon polarization, to study dispersive spin excitations in the high temperature superconductor ...YBa_{2}Cu_{3}O_{6+x} over a wide range of doping levels (0.1≤x≤1). The excitation profiles were carefully monitored as the incident photon energy was detuned from the resonant condition, and the spin excitation energy was found to be independent of detuning for all x. These findings demonstrate that the largest fraction of the spin-flip RIXS profiles in doped cuprates arises from magnetic collective modes, rather than from incoherent particle-hole excitations as recently suggested theoretically Benjamin et al. Phys. Rev. Lett. 112, 247002 (2014). Implications for the theoretical description of the electron system in the cuprates are discussed.
Introduction
Placebo research investigated the underlying mechanisms of placebo effects, but they are rarely used to optimize treatments. Ethical and legal concerns have been raised, but research ...demonstrated that placebo mechanisms can be used without patients’ deception: Experimental studies showed that half of drugs in treatment of attention-deficit/hyperactivity disorder (ADHD) combined with open-label placebos could be as effective as standard medication to reduce ADHD symptoms.
Objectives
To estimate the health economic advantages of harnessing the combination of open-label placebos with standard medication in ADHD.
Methods
For preliminary estimation of the mean treatment costs, the 12-months prevalence of ADHD in children and adolescents aged 5 to 14 years as well as the percentage of medication treatments were extracted from the literature. Mean treatment costs per patient and year were calculated for four treatment plans (different drugs and dosages) with both treatment with standard medication and half of drugs in combination with placebos.
Results
A 12-months prevalence of 4.3% equals around 260,000 children and adolescents with a compulsory health insurance in Germany. Of those, around 40-50% are equally treated with two standard drugs and two different dosages. Full standard drug treatments cost around 119 million EUR, and treatment with half of drugs in combination with placebos cost around 66 million EUR.
Conclusions
The combination of open-label placebos with half of standard medication could considerably reduce health costs. Reduction of side effects still must be considered. However, current studies are of experimental nature and lasted for no longer than two weeks.
Optical excitation in the cuprates has been shown to induce transient superconducting correlations above the thermodynamic transition temperatureTC, as evidenced by the terahertz-frequency optical ...properties in the nonequilibrium state. InYBa2Cu3O6+x, this phenomenon has so far been associated with the nonlinear excitation of certain lattice modes and the creation of new crystal structures. In other compounds, likeLa2−xBaxCuO4, similar effects were reported also for excitation at near-infrared frequencies, and were interpreted as a signature of the melting of competing orders. However, to date, it has not been possible to systematically tune the pump frequency widely in any one compound, to comprehensively compare the frequency-dependent photosusceptibility for this phenomenon. Here, we make use of a newly developed nonlinear optical device, which generates widely tunable high-intensity femtosecond pulses, to exciteYBa2Cu3O6.5throughout the entire optical spectrum (3–750 THz). In the far-infrared region (3–24 THz), signatures of nonequilibrium superconductivity are induced only for excitation of the 16.4- and 19.2-THz vibrational modes that drivec-axis apical oxygen atomic positions. For higher driving frequencies (25–750 THz), a second resonance is observed around the charge transfer band edge at approximately 350 THz. These findings highlight the importance of coupling to the electronic structure of theCuO2planes, mediated either by a phonon or by charge transfer.
We use resonant x-ray scattering to determine the momentum-dependent charge correlations in YBa(2)Cu(3) O(6.55) samples with highly ordered chain arrays of oxygen acceptors (ortho-II structure). The ...results reveal nearly critical, biaxial charge density wave (CDW) correlations at in-plane wave vectors (0.315, 0) and (0, 0.325). The corresponding scattering intensity exhibits a strong uniaxial anisotropy. The CDW amplitude and correlation length are enhanced as superconductivity is weakened by an external magnetic field. Analogous experiments are carried out on a YBa(2)Cu(3)O(6.6) crystal with a dilute concentration of spinless (Zn) impurities, which had earlier been shown to nucleate incommensurate magnetic order. Compared to pristine crystals with the same doping level, the CDW amplitude and correlation length are found to be strongly reduced. These results indicate a three-phase competition between spin-modulated, charge-modulated, and superconducting states in underdoped YBa(2)Cu(3)O(6+δ).
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