arXiv subjects

T. Loew

Publications and source records attributed to T. Loew.

At least 19 recordsLinked to original sources

arXiv2025

Resonant x-ray scattering study of charge-density wave correlations in YBa$_{2}$Cu$_{3}$O$_{6+x}$ under uniaxial stress

We report a comprehensive study of the uniaxial stress response of charge-density-wave (CDW) correlations in detwinned single crystals of the high temperature superconductor YBa$_2$Cu$_3$O$_{6+x}$ (YBCO$_{6+x}$) with $0.40 \leq x \leq 0.93$ (hole-doping levels $0.072 \leq p \leq 0.168$) by means of Cu $L_3$-edge resonant energy-integrated x-ray scattering (REXS). We show that the influence of uniaxial stress is strongly doping dependent: the quasi-two-dimensional CDW is enhanced by in-plane uniaxial stress in a wide hole doping range ($0.45 \leq x \leq 0.80$), but only barely affected in the most underdoped and optimally doped samples ($x = 0.40$ and 0.93), where the CDW correlation length is minimal. A stress-induced three-dimensionally long-range ordered (3D) CDW was observed only in YBCO$_{6.50}$ and YBCO$_{6.67}$. The temperature dependence of the 3D CDW clearly indicates a strong competition with superconductivity. Based on the systematic strain-, doping-, and temperature-dependent REXS measurements reported here, we discuss the relationship between charge order and superconductivity in YBCO$_{6+x}$ and other cuprates.

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arXiv2024

Signatures of two gaps in the spin susceptibility of a cuprate superconductor

A major obstacle to understanding high-Tc cuprates is that superconductivity precludes observing normal-state properties at low temperatures. One prime example is the normal-state spin susceptibility \c{hi}spin: although its decrease upon cooling far above Tc typifies pseudogap behavior, its behavior at low temperatures is generally unknown. Here, our measurements in high magnetic fields expose \c{hi}spin of YBa2Cu3Oy down to low temperatures. Even though superconductivity is suppressed by the field, we uncover two thermally-activated contributions alongside a residual \c{hi}spin(T=0) due to gapless excitations. We relate these two distinct gaps to short-range charge-density waves and to the formation of singlets as in certain quantum spin systems. Both phenomena thus contribute to the pseudogap at low temperature, supplementing short-lived antiferromagnetism that initiates pseudogap behavior at high temperatures. We therefore propose that the pseudogap ought to be regarded as a composite property and that, when not undergoing spin-stripe ordering, underdoped cuprates tend to form short-ranged spin singlets.

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arXiv2023

Plasmon dispersion in bilayer cuprate superconductors

The essential building blocks of cuprate superconductors are two-dimensional CuO$_2$ sheets interspersed with charge reservoir layers. In bilayer cuprates, two closely spaced CuO$_2$ sheets are separated by a larger distance from the subsequent pair in the next unit cell. In contrast to single-layer cuprates, prior theoretical work on bilayer systems has predicted two distinct acoustic plasmon bands for a given out-of-plane momentum transfer. Here we report random phase approximation (RPA) calculations for bilayer systems which corroborate the existence of two distinct plasmons bands. We find that the intensity of the lower-energy band is negligibly small, whereas the higher-energy band carries significant spectral weight. We also present resonant inelastic x-ray scattering (RIXS) experiments at the O $K$-edge on the bilayer cuprate Y$_{0.85}$Ca$_{0.15}$Ba$_2$Cu$_3$O$_7$ (Ca-YBCO), which show only one dispersive plasmon branch, in agreement with the RPA calculations. In addition, the RPA results indicate that the dispersion of the higher-energy plasmon band in Ca-YBCO is not strictly acoustic, but exhibits a substantial energy gap of approximately 250 meV at the two-dimensional Brillouin zone center.

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arXiv2021

Effect of pseudogap on electronic anisotropy in the strain dependence of the superconducting $T_c$ of underdoped YBa$_2$Cu$_3$O$_y$

For orthorhombic superconductors we define thermodynamic anisotropy $N \equiv d T_c/d \epsilon_{22} - dT_c/d \epsilon_{11}$ as the difference in how superconducting $T_c$ varies with strains $\epsilon_{ii}$, $i=(1, 2)$, along the in-plane directions. We study the hole doping ($p$) dependence of $N$ on detwinned single crystals of underdoped YBa$_2$Cu$_3$O$_y$ (YBCO) using ultrasound technique. While the structural orthorhombicity of YBCO reduces monotonically with decreasing doping over $0.065 <p<0.16$, we find that the thermodynamic anisotropy shows an intriguing enhancement at intermediate doping level, which is of electronic origin. Our theoretical analysis shows that the enhancement of the electronic anisotropy can be related to the pseudogap potential in the electronic specturm that itself increases when the Mott insulating state is approached. Our results imply that the pseudogap is controlled by a local energy scale that can be tuned by varying the nearest neighbor Cu-Cu bond length. Our work opens the possibility to strain engineer the pseudogap potential to enhance the superconducting \Tc.

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arXiv2020

Charge density waves in YBa$_2$Cu$_3$O$_{6.67}$ probed by resonant x-ray scattering under uniaxial compression

We report a comprehensive Cu L$_3$-edge resonant x-ray scattering study of two- and three-dimensional (2D and 3D) incommensurate charge correlations in single crystals of the underdoped high-temperature superconductor YBa$_2$Cu$_3$O$_{6.67}$ under uniaxial compression up to 1% along the two inequivalent Cu-O-Cu bond directions (a and b) in the CuO$_2$ planes. The pressure response of the 2D charge correlations is symmetric: pressure along a enhances correlations along b, and vice versa. Our results imply that the underlying order parameter is uniaxial. In contrast, 3D long-range charge order is only observed along b in response to compression along a. Spectroscopic resonant x-ray scattering measurements show that the 3D charge order resides exclusively in the CuO$_2$ planes and may thus be generic to the cuprates. We discuss implications of these results for models of electronic nematicity and for the interplay between charge order and superconductivity.

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arXiv2019

Pump frequency resonances for light-induced incipient superconductivity in YBa$_2$Cu$_3$O$_{6.5}$

Optical excitation in the cuprates has been shown to induce transient superconducting correlations above the thermodynamic transition temperature, $T_C$, as evidenced by the terahertz frequency optical properties in the non-equilibrium state. In YBa$_2$Cu$_3$O$_{6+x}$ 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, like La$_{2-x}$Ba$_x$CuO$_4$, 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 photo-susceptibility for this phenomenon. Here, we make use of a newly developed optical parametric amplifier, which generates widely tunable high intensity femtosecond pulses, to excite YBa$_2$Cu$_3$O$_{6.5}$ throughout the entire optical spectrum (3 - 750 THz). In the far-infrared region (3 - 25 THz), signatures of non-equilibrium superconductivity are induced only for excitation of the 16.4 THz and 19.2 THz vibrational modes that drive $c$-axis apical oxygen atomic positions. For higher driving frequencies (25 - 750 THz), a second resonance is observed around the charge transfer band edge at ~350 THz. These observations highlight the importance of coupling to the electronic structure of the CuO$_2$ planes, either mediated by a phonon or by charge transfer.

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arXiv2018

Light-Induced Metastable State in Charge-Ordered YBa$_2$Cu$_3$O$_{6+x}$

We report temporal changes in Raman-scattering spectra of detwinned YBa$_2$Cu$_3$O$_{6+x}$ single crystals under exposure to red laser light polarized along the crystallographic $a$-axis. A recent publication by Bakr et al. (Phys. Rev. B 88 214517 (2013)) identified new Raman-active modes that appear upon cooling below $T \sim 200$ K, and attributed these modes to charge ordering phenomena observed in x-ray scattering and nuclear magnetic resonance experiments on the same materials. Here we report that the intensities of these modes depend not only on temperature and oxygen content, but also on the cumulative photon dose absorbed by the YBa$_2$Cu$_3$O$_{6+x}$ samples. The light-induced changes in the Raman spectra exhibit a stretched-exponential time dependence, with a characteristic time that increases strongly upon cooling. They also depend strongly on doping and are most pronounced for $x \sim 0.6$. In fully illuminated samples, the mode intensities exhibit a monotonic temperature dependence indicative of a second-order phase transition. The findings indicate a metastable state generated by light-induced rearrangement of the oxygen dopants. We hypothesize that the new Raman phonons are associated with a three-dimensional charge-ordered state induced by light illumination, analogous to a state that was recently observed by resonant x-ray scattering in oxygen-disordered YBa$_2$Cu$_3$O$_{6+x}$ films.

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arXiv2017

Thermodynamic evidence for nematic phase transition at the onset of pseudogap in YBa$_2$Cu$_3$O$_y$

A central issue in the quest to understand the superconductivity in cuprates is the nature and origin of the pseudogap state, which harbours anomalous electronic states such as Fermi arc, charge density wave (CDW), and $d$-wave superconductivity. A fundamentally important, but long-standing controversial problem has been whether the pseudogap state is a distinct thermodynamic phase characterized by broken symmetries below the onset temperature $T^*$. Electronic nematicity, a fourfold ($C_4$) rotational symmetry breaking, has emerged as a key feature inside the pseudogap regime, but the presence or absence of a nematic phase transition and its relationship to the pseudogap remain unresolved. Here we report thermodynamic measurements of magnetic torque in the underdoped regime of orthorhombic YBa$_2$Cu$_3$O$_y$ with a field rotating in the CuO$_2$ plane, which allow us to quantify magnetic anisotropy with exceptionally high precision. Upon entering the pseudogap regime, the in-plane anisotropy of magnetic susceptibility increases after exhibiting a distinct kink at $T^*$. Our doping dependence analysis reveals that this anisotropy is preserved below $T^*$ even in the limit where the effect of orthorhombicity is eliminated. In addition, the excess in-plane anisotropy data show a remarkable scaling behaviour with respect to $T/T^*$ in a wide doping range. These results provide thermodynamic evidence that the pseudogap onset is associated with a second-order nematic phase transition, which is distinct from the CDW transition that accompanies translational symmetry breaking. This suggests that nematic fluctuations near the pseudogap phase boundary have a potential link to the strange metallic behaviour in the normal state, out of which high-$T_c$ superconductivity emerges.

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arXiv2017

High field charge order across the phase diagram of $YBa_2Cu_3O_y$

In hole-doped cuprates there is now compelling evidence that inside the pseudogap phase, charge order breaks translational symmetry leading to a reconstruction of the Fermi surface. In $YBa_2Cu_3O_y$ charge order emerges in two steps: a 2D order found at zero field and at high temperature inside the pseudogap phase, and a 3D order that is superimposed below the superconducting transition $T_c$ when superconductivity is weakened by a magnetic field. Several issues still need to be addressed such as the effect of disorder, the relationship between those charge orders and their respective impact on the Fermi surface. Here, we report high magnetic field sound velocity measurements of the 3D charge order in underdoped $YBa_2Cu_3O_y$ in a large doping range. We found that the 3D charge order exists over the same doping range as its 2D counterpart, indicating an intimate connection between the two distinct orders. Moreover, we argue that the Fermi surface is reconstructed above the onset temperature of 3D charge order.

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arXiv2017

Optically induced lattice deformations, electronic structure changes, and enhanced superconductivity in YBa2Cu3O6.48

Resonant optical excitation of apical oxygen vibrational modes in the normal state of underdoped YBa2Cu3O6+x induces a transient state with optical properties similar to those of the equilibrium superconducting state. Amongst these, a divergent imaginary conductivity and a plasma edge are transiently observed in the photo-stimulated state. Femtosecond hard x-ray diffraction experiments have been used in the past to identify the transient crystal structure in this non-equilibrium state. Here, we start from these crystallographic features and theoretically predict the corresponding electronic rearrangements that accompany these structural deformations. Using density functional theory, we predict enhanced hole-doping of the CuO2 planes. The empty chain Cu dy2-z2 orbital is calculated to strongly reduce in energy, which would increase c-axis transport and potentially enhance the interlayer Josephson coupling as observed in the THz-frequency response. From these calculations, we predict changes in the soft x-ray absorption spectra at the Cu L-edge. Femtosecond x-ray pulses from a free electron laser are used to probe these changes in absorption at two photon energies along this spectrum, and provide data consistent with these predictions.

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arXiv2016

Accessing the Full Superconducting Dome in Pristine YBa2Cu3O6+x Under Pressure

We attain the previously unaccessed full superconducting dome in a pristine high temperature cuprate superconductor by applying pressures up to 280 kbar to samples of near stoichiometric YBa2Cu3O7. The obtained superconducting phase boundary as a function of hole doping closely follows the superconducting dome in La2-xSrxCuO4. Measurements are now enabled to trace the evolution of various entangled phases and the Fermi surface from the underdoped to overdoped regime in a single high purity cuprate superconducting family of materials.

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arXiv2016

Dynamical decoherence of the light induced interlayer coupling in YBa$_{2}$Cu$_{3}$O$_{6+δ}$

Optical excitation of apical oxygen vibrations in YBa$_{2}$Cu$_{3}$O$_{6+δ}$ has been shown to enhance its c-axis superconducting-phase rigidity, as evidenced by a transient blue shift of the equilibrium inter-bilayer Josephson plasma resonance. Surprisingly, a transient c-axis plasma mode could also be induced above T$_{c}$ by the same apical oxygen excitation, suggesting light activated superfluid tunneling throughout the pseudogap phase of YBa$_{2}$Cu$_{3}$O$_{6+δ}$. However, despite the similarities between the above T$_{c}$ transient plasma mode and the equilibrium Josephson plasmon, alternative explanations involving high mobility quasiparticle transport should be considered. Here, we report an extensive study of the relaxation of the light-induced plasmon into the equilibrium incoherent phase. These new experiments allow for a critical assessment of the nature of this mode. We determine that the transient plasma relaxes through a collapse of its coherence length rather than its carrier (or superfluid) density. These observations are not easily reconciled with quasiparticle interlayer transport, and rather support transient superfluid tunneling as the origin of the light-induced interlayer coupling in YBa$_{2}$Cu$_{3}$O$_{6+δ}$.

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arXiv2016

Cu-NMR study of oxygen disorder in ortho-II YBa2Cu3Oy

We show that 63Cu NMR spectra place strong constraints on both the nature and the concentration of oxygen defects in ortho-II YBa2Cu3Oy. Systematic deviation from ideal ortho-II order is revealed by the presence of inequivalent Cu sites in either full or empty chains. The results can be explained by two kinds of defects: oxygen clustering into additional chains, or fragments thereof, most likely present at all concentrations (6.4<y<6.6) and oxygen vacancies randomly distributed in the full chains for y<6.50 only. Furthermore, the remarkable reproducibility of the spectra in different samples with optimal ortho-II order (y=6.55) shows that chain-oxygen disorder, known to limit electronic coherence, is ineluctable because it is inherent to these compounds.

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arXiv2015

Nonlinear lattice dynamics as a basis for enhanced superconductivity in YBa2Cu3O6.5

THz-frequency optical pulses can resonantly drive selected vibrational modes in solids and deform their crystal structure. In complex oxides, this method has been used to melt electronic orders, drive insulator to metal transitions or induce superconductivity. Strikingly, coherent interlayer transport strongly reminiscent of superconductivity can be transiently induced up to room temperature in YBa2Cu3O6+x. By combining femtosecond X-ray diffraction and ab initio density functional theory calculations, we determine here the crystal structure of this exotic non-equilibrium state. We find that nonlinear lattice excitation in normal-state YBa2Cu3O6+x at 100 K causes a staggered dilation/contraction of the Cu-O2 intra/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 dramatic changes in the electronic structure. Amongst these, the enhancement in the dx2-y2 character of the in-plane electronic structure is likely to favor superconductivity.

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arXiv2015

Coherent Modulation of the YBa2Cu3O6+x Atomic Structure by Displacive Stimulated Ionic Raman Scattering

We discuss the mechanism of coherent phonon generation by Stimulated Ionic Raman Scattering, a process different from conventional excitation with near visible optical pulses. Ionic Raman scattering is driven by anharmonic coupling between a directly excited infrared-active phonon mode and other Raman modes. We experimentally study the response of YBa2Cu3O6+x to the resonant excitation of apical oxygen motions at 20 THz by mid-infrared pulses, which has been shown in the past to enhance the interlayer superconducting coupling. We find coherent oscillations of four totally symmetric (Ag) Raman modes and make a critical assessment of the role of these oscillatory motions in the enhancement of superconductivity.

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arXiv2015

Femtosecond x rays link melting of charge-density wave correlations and light-enhanced coherent transport in YBa2Cu3O6.6

We use femtosecond resonant soft x-ray diffraction to measure the optically stimulated ultrafast changes of charge density wave correlations in underdoped YBa2Cu3O6.6. We find that when coherent interlayer transport is enhanced by optical excitation of the apical oxygen distortions, at least 50% of the in-plane charge density wave order is melted. These results indicate that charge ordering and superconductivity may be competing up to the charge ordering transition temperature, with the latter becoming a hidden phase that is accessible only by nonlinear phonon excitation.

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arXiv2015

Collective nature of spin excitations in superconducting cuprates probed by resonant inelastic x-ray scattering

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 YBa2Cu3O6+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.

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arXiv2014

Optically enhanced coherent transport in YBa2Cu3O6.5 by ultrafast redistribution of interlayer coupling

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), interbilayer 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.

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