arXiv subjects

Bernhard Keimer

Publications and source records attributed to Bernhard Keimer.

At least 55 records · Page 3Linked to original sources

arXiv2021

Non-collinear and strongly asymmetric polar moments at back-gated SrTiO3 interfaces

The highly mobile electrons at the interface of SrTiO3 with other oxide insulators, such as LaAlO3 or AlOx, are of great current interest. A vertical gate voltage allows controlling a metal/superconductor-to-insulator transition, as well as electrical modulation of the spin-orbit Rashba coupling for spin-charge conversion. These findings raise important questions about the origin of the confined electrons as well as the mechanisms that govern the interfacial electric field. Here we use infrared ellipsometry and confocal Raman spectroscopy to show that an anomalous polar moment is induced at the interface that is non-collinear, highly asymmetric and hysteretic with respect to the vertical gate electric field. Our data indicate that an important role is played by the electromigration of oxygen vacancies and their clustering at the antiferrodistortive domain boundaries of SrTiO3, which generates local electric and possibly also flexoelectric fields and subsequent polar moments with a large lateral component. Our results open new perspectives for the defect engineering of lateral devices with strongly enhanced and hysteretic local electric fields that can be manipulated with various other parameters, like strain, temperature, or photons.

cond-mat.mtrl-sci↗

arXiv2021

Hidden Charge Order in an Iron Oxide Square-Lattice Compound

Since the discovery of charge disproportionation in the FeO$_2$ square-lattice compound Sr$_3$Fe$_2$O$_7$ by Mössbauer spectroscopy more than fifty years ago, the spatial ordering pattern of the disproportionated charges has remained "hidden" to conventional diffraction probes, despite numerous x-ray and neutron scattering studies. We have used neutron Larmor diffraction and Fe K-edge resonant x-ray scattering to demonstrate checkerboard charge order in the FeO$_2$ planes that vanishes at a sharp second-order phase transition upon heating above 332 K. Stacking disorder of the checkerboard pattern due to frustrated interlayer interactions broadens the corresponding superstructure reflections and greatly reduces their amplitude, thus explaining the difficulty to detect them by conventional probes. We discuss implications of these findings for research on "hidden order" in other materials.

cond-mat.str-el↗

arXiv2021

In-plane Isotropy of the Low Energy Phonon Anomalies in YBa$_{2}$Cu$_{3}$O$_{6+x}$

We study the temperature dependence of the low energy phonons in the $(H, 0, L)$ reciprocal plane of the highly ordered ortho-II YBa$_2$Cu$_3$O$_{6.55}$ cuprate high temperature superconductor by means of high-resolution inelastic x-ray scattering. Anomalies associated with the emergence of long-range charge density wave (CDW) fluctuations are observed, and are qualitatively similar to those previously observed in the $(0, K, L)$ plane. This confirms the unconventional nature of this bi-dimensional CDW, which is not soft-phonon driven. With the support of first principles calculations, the symmetry of the anomalous phonon is identified and is found to match that of the charge modulation. This suggests in turn that these anomalies originate from a direct coupling between the phonons and the collective CDW excitations.

cond-mat.supr-con↗

arXiv2021

Discovery of Superconductivity in (Ba,K)SbO$_{3}$

Superconducting bismuthates (Ba,K)BiO$_{3}$ (BKBO) constitute an interesting class of superconductors in that superconductivity with a remarkably high $T_\mathrm{c}$ of 30 K arises in proximity to charge density wave (CDW) order. Prior understanding on the driving mechanism of the CDW and superconductivity emphasizes the role of either bismuth (negative $U$ model) or oxygen ions (ligand hole model). While holes in BKBO presumably reside on oxygen owing to their negative charge transfer energy, so far there has been no other comparative material studied. Here, we introduce (Ba,K)SbO$_{3}$ (BKSO) in which the Sb 5$s$ orbital energy is higher than that of the Bi 6$s$ orbitals enabling tuning of the charge transfer energy from negative to slightly positive. The parent compound BaSbO$_{3-δ}$ shows a larger CDW gap compared to the undoped bismuthate BaBiO$_{3}$. As the CDW order is suppressed via potassium substitution up to 65 %, superconductivity emerges, rising up to $T_\mathrm{c}$ = 15 K. This value is lower than the maximum $T_\mathrm{c}$ of BKBO, but higher by more than a factor of two at comparable potassium concentrations. The discovery of an enhanced CDW gap and superconductivity in BKSO indicates that the sign of the charge transfer energy may not be crucial, but instead strong metal-oxygen covalency plays the essential role in constituting a CDW and high-$T_\mathrm{c}$ superconductivity in the main-group perovskite oxides.

cond-mat.supr-con↗

arXiv2021

Electronic Structure of the Bond Disproportionated Bismuthate Ag$_2$BiO$_3$

We present a comprehensive study on the silver bismuthate Ag$_2$BiO$_3$, synthesized under high-pressure high-temperature conditions, which has been the subject of recent theoretical work on topologically complex electronic states. We present X-ray photoelectron spectroscopy results showing two different bismuth states, and X-ray absorption spectroscopy results on the oxygen $K$-edge showing holes in the oxygen bands. These results support a bond disproportionated state with holes on the oxygen atoms for Ag$_2$BiO$_3$. We estimate a band gap of $\sim$1.25~eV for Ag$_2$BiO$_3$ from optical conductivity measurements, which matches the band gap in density functional calculations of the electronic band structure in the non-symmorphic space group $Pnn2$, which supports two inequivalent Bi sites. In our band structure calculations the disproportionated Ag$_2$BiO$_3$ is expected to host Weyl nodal chains, one of which is located $\sim$0.5~eV below the Fermi level. Furthermore, we highlight similarities between Ag$_2$BiO$_3$ and the well-known disproportionated bismuthate BaBiO$_3$, including breathing phonon modes with similar energy. In both compounds hybridization of Bi-$6s$ and O-$2p$ atomic orbitals is important in shaping the band structure, but in contrast to the Ba-$5p$ in BaBiO$_3$, the Ag-$4d$ bands in Ag$_2$BiO$_3$ extend up to the Fermi level.

cond-mat.mtrl-sci↗

arXiv2021

Unconventional quantum vortex matter state hosts quantum oscillations in the underdoped high-temperature cuprate superconductors

A central question in the underdoped cuprates pertains to the nature of the pseudogap ground state. A conventional metallic ground state of the pseudogap region has been argued to host quantum oscillations upon destruction of the superconducting order parameter by modest magnetic fields. Here we use low applied measurement currents and millikelvin temperatures on ultra-pure single crystals of underdoped YBa$_2$Cu$_3$O$_{6+x}$ to unearth an unconventional quantum vortex matter ground state characterized by vanishing electrical resistivity, magnetic hysteresis, and non-ohmic electrical transport characteristics beyond the highest laboratory accessible static fields. A new model of the pseudogap ground state is now required to explain quantum oscillations that are hosted by the bulk quantum vortex matter state without experiencing sizeable additional damping in the presence of a large maximum superconducting gap; possibilities include a pair density wave.

cond-mat.supr-con↗

arXiv2021

Charge ordering in superconducting copper oxides

Charge order has recently been identified as a leading competitor of high-temperature superconductivity in moderately doped cuprates. We provide a survey of universal and materials-specific aspects of this phenomenon, with emphasis on results obtained by scattering methods. In particular, we discuss the structure, periodicity, and stability range of the charge-ordered state, its response to various external perturbations, the influence of disorder, the coexistence and competition with superconductivity, as well as collective charge dynamics. In the context of this journal issue which honors Roger Cowley's legacy, we also discuss the connection of charge ordering with lattice vibrations and the central-peak phenomenon. We end the review with an outlook on research opportunities offered by new synthesis methods and experimental platforms, including cuprate thin films and superlattices.

cond-mat.str-el↗

arXiv2021

Homogeneous superconducting gap in DBCO synthesized by oxide molecular beam epitaxy

Much of what is known about high-temperature cuprate superconductors stems from studies based on two surface analytical tools, angle-resolved photoemission spectroscopy (ARPES) and spectroscopic imaging scanning tunneling microscopy (SI-STM). A question of general interest is whether and when the surface properties probed by ARPES and SI-STM are representative of the intrinsic properties of bulk materials. We find this question is prominent in thin films of a rarely studied cuprate DBCO. We synthesize DBCO films by oxide molecular beam epitaxy and study them by in situ ARPES and SI-STM. Both ARPES and SI-STM show that the surface DBCO layer is different from the bulk of the film. It is heavily underdoped, while the doping level in the bulk is close to optimal doping evidenced by bulk-sensitive mutual inductance measurements. ARPES shows the typical electronic structure of a heavily underdoped CuO2 plane and two sets of one-dimensional bands originating from the CuO chains with one of them gapped. SI-STM reveals two different energy scales in the local density of states, with one corresponding to the superconductivity and the other one to the pseudogap. While the pseudogap shows large variations over the length scale of a few nanometers, the superconducting gap is very homogeneous. This indicates that the pseudogap and superconductivity are of different origins.

cond-mat.supr-con↗

arXiv2021

Lattice dynamics and structural transition of the hyperhoneycomb iridate $β$-Li$_2$IrO$_3$ investigated by high-pressure Raman scattering

We report a polarized Raman scattering study of the lattice dynamics of $β$-Li$_2$IrO$_3$ under hydrostatic pressures up to 7.62 GPa. At ambient pressure, $β$-Li$_2$IrO$_3$ exhibits the hyperhoneycomb crystal structure and a magnetically ordered state of spin-orbit entangled Jeff = 1/2 moments that is strongly influenced by bond-directional (Kitaev) exchange interactions. At a critical pressure of ~ 4.1 GPa, the phonon spectrum changes abruptly consistent with the reported structural transition into a monoclinic, dimerized phase. A comparison to the phonon spectra obtained from density functional calculations shows reasonable overall agreement. The calculations also indicate that the high-pressure phase is a nonmagnetic insulator driven by the formation of Ir-Ir dimer bonds. Our results thus indicate a strong sensitivity of the electronic properties of $β$-Li$_2$IrO$_3$ to the pressure-induced structural transition.

cond-mat.str-el↗

arXiv2021

Suppression of superconductivity by charge density wave order in YBa$_2$Cu$_3$O$_{6.67}$

Hole-doped cuprate superconductors show a ubiquitous tendency towards charge order. Although onset of superconductivity is known to suppress charge order, there has not so far been a decisive demonstration of the reverse process, namely, the effect of charge order on superconductivity. To gain such information, we report here the dependence of the critical temperature $T_{\mathrm{c}}$ of YBa$_2$Cu$_3$O$_{6.67}$ on in-plane uniaxial stress up to 2 GPa. At a compression of about 1 GPa along the $a$ axis, 3D-correlated charge density wave (3D CDW) order appears. We find that $T_{\mathrm{c}}$ decreases steeply as the applied stress crosses 1 GPa, showing that the appearance of 3D CDW order strongly suppresses superconductivity. Through the elastocaloric effect we resolve the heat capacity anomaly at $T_{\mathrm{c}}$, and find that it does not change drastically as the 3D CDW onsets, which shows that the condensation energy of the 3D CDW is considerably less than that of the superconductivity.

cond-mat.supr-con↗

arXiv2020

Paramagnons and high-temperature superconductivity in mercury-based cuprates

We present a comparative study of magnetic excitations in the first two Ruddlesden-Popper members of the Hg-family of high-temperature superconducting cuprates, which are chemically nearly identical and have the highest critical temperature ($T_\mathrm{c}$) among all cuprate families. Our inelastic photon scattering experiments reveal that the two compounds' paramagnon spectra are nearly identical apart from an energy scale factor of $\sim130\%$ that matches the ratio of $T_\mathrm{c}$'s, as expected in magnetic Cooper pairing theories. By relating our observations to other cuprates, we infer that the strength of magnetic interactions determines how high $T_\mathrm{c}$ can reach. Our finding can be viewed as a magnetic analogue of the isotope effect, thus firmly supporting models of magnetically mediated high-temperature superconductivity.

cond-mat.supr-con↗

arXiv2020

Imprint of charge and oxygen orders on Dy ions in DyBa$_2$Cu$_3$O$_{6+x}$ thin films probed by resonant x-ray scattering

We used resonant elastic x-ray scattering at the Cu $L_3$ and Dy $M_5$ edges to investigate charge order in thin films of underdoped DyBa$_2$Cu$_3$O$_{6+x}$ (DyBCO) epitaxially grown on NdGaO$_3$ (110) substrates. The films show an orthorhombic crystal structure with short-range ortho-II oxygen order in the charge-reservoir layers. At the Dy $M_5$ edge we observe diffraction peaks with the same planar wavevectors as those of the two-dimensional charge density wave in the CuO$_2$ planes and of the ortho-II oxygen order, indicating the formation of induced ordered states on the rare-earth sublattice. The intensity of the resonant diffraction peaks exhibits a non-monotonic dependence on an external magnetic field. Model calculations on the modulation of the crystalline electric field at the Dy sites by charge and oxygen order capture the salient features of the magnetic field, temperature, and photon energy dependence of the scattering intensity.

cond-mat.supr-con↗

arXiv2020

Hard antinodal gap revealed by quantum oscillations in the pseudogap regime of underdoped high-$T_{\rm c}$ superconductors

An understanding of the missing antinodal electronic excitations in the pseudogap state is essential for uncovering the physics of the underdoped cuprate high temperature superconductors. The majority of high temperature experiments performed thus far, however, have been unable to discern whether the antinodal states are rendered unobservable due to their damping, or whether they vanish due to their gapping. Here we distinguish between these two scenarios by using quantum oscillations to examine whether the small Fermi surface pocket, found to occupy only 2% of the Brillouin zone in the underdoped cuprates, exists in isolation against a majority of completely gapped density of states spanning the antinodes, or whether it is thermodynamically coupled to a background of ungapped antinodal states. We find that quantum oscillations associated with the small Fermi surface pocket exhibit a signature sawtooth waveform characteristic of an isolated two-dimensional Fermi surface pocket. This finding reveals that the antinodal states are destroyed by a hard gap that extends over the majority of the Brillouin zone, placing strong constraints on a drastic underlying origin of quasiparticle disappearance over almost the entire Brillouin zone in the pseudogap regime.

cond-mat.supr-con↗

arXiv2020

Phase-resolved Higgs response in superconducting cuprates

In high energy physics, the Higgs field couples to gauge bosons and fermions and gives mass to their elementary excitations. Experimentally, such couplings can be inferred from the decay product of the Higgs boson, i.e. the scalar (amplitude) excitation of the Higgs field. In superconductors, Cooper pairs bear a close analogy to the Higgs field. Interaction between the Cooper pairs and other degrees of freedom provides dissipation channel for the amplitude mode, which may reveal important information about the microscopic pairing mechanism. To this end, we investigate the Higgs (amplitude) mode of several cuprate thin films using phase-resolved terahertz third harmonic generation (THG). In addition to the heavily damped Higgs mode itself, we observe a universal jump in the phase of the driven Higgs oscillation as well as a non-vanishing THG above Tc. These findings indicate coupling of the Higgs mode to other collective modes and potentially a nonzero pairing amplitude above Tc.

cond-mat.supr-con↗

arXiv2019

Magnetic Scattering Chapter

The present chapter reviews current neutron and x-ray scattering techniques employed to elucidate the magnetic structures and spin dynamics of magnetic materials. Both techniques provide measurements as a function of the energy and the momentum transferred from the spin system to the probe particles, in terms of five-dimensional data sets as a function of various thermodynamic fields at the control of the experimenter. These scattering techniques yield fundamental information about the equal-time correlations such the magnetic configuration and symmetry, as well as the dynamics that determine the exchange interactions for prototypical systems that behave as linear, planar, or three-dimensional systems. Historically, neutron scattering has been the magnetic scattering technique of choice for such investigations, but the extraordinary advances in resonant x-ray scattering techniques have enable new types of magnetic scattering measurements. The type of information obtained with the two techniques is largely complementary and depends on the interests of the investigators. We discuss these possibilities and provide numerous examples of the techniques applied to different classes of magnetic systems.

cond-mat.mtrl-sci↗

arXiv2019

Selective formation of apical oxygen vacancies in La$_{2-x}$Sr$_{x}$CuO$_{4}$

The superconducting properties of high-\tc\ materials are functions of carriers concentration, which is controlled by the concentration of defects including heterovalent cations, interstitial oxygen ions, and oxygen vacancies. Here we combine low-temperature thermal treatment of La$_{2-x}$Sr$_{x}$CuO$_{4}$ epitaxial thin films and confocal Raman spectroscopy to control and investigate oxygen vacancies. We demonstrate that the apical site is the most favorable position to accommodate oxygen vacancies under low-temperature annealing conditions. Additionally we show that in high-quality films of overdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$, oxygen vacancies strongly deform the oxygen environment around the copper ions. This observation is consistent with previous defect-chemical studies, and calls for further investigation of the defect induced properties in the overdoped regime of the hole-doped lanthanum cuprates.

cond-mat.supr-con↗

arXiv2019

Tunable perpendicular exchange bias in oxide heterostructures

The exchange bias effect is an essential component of magnetic memory and spintronic devices. Whereas recent research has shown that anisotropies perpendicular to the device plane provide superior stability against thermal noise, it has proven remarkably difficult to realize perpendicular exchange bias in thin-film structures. Here we demonstrate a strong perpendicular exchange bias effect in heterostructures of the quasi-two-dimensional canted antiferromagnet La$_2$CuO$_4$ and ferromagnetic (La,Sr)MnO$_3$ synthesized by ozone-assisted molecular beam epitaxy. The magnitude of this effect can be controlled via the doping level of the cuprate layers. Canted antiferromagnetism of layered oxides is thus a new and potentially powerful source of uniaxial anisotropy in magnetic devices.

cond-mat.str-el↗

arXiv2019

Control of dopant crystallinity in electrochemically treated cuprate thin films

We present a methodology based on \textit{ex-situ} (post-growth) electrochemistry to control the oxygen concentration in thin films of the superconducting oxide La$_2$CuO$_{4+y}$ grown epitaxially on substrates of isostructural LaSrAlO$_4$. The superconducting transition temperature, which depends on the oxygen concentration, can be tuned by adjusting the pH level of the base solution used for the electrochemical reaction. As our main finding, we demonstrate that the dopant oxygens can either occupy the interstitial layer in an orientationally disordered state or organize into a crystalline phase via a mechanism in which dopant oxygens are inserted into the substrate, changing the lattice symmetry of both the substrate and the epitaxial film. We discuss this mechanism, and describe the resulting methodology as a platform to be explored in thin films of other transition metal oxides.

cond-mat.mtrl-sci↗