C. Bernhard - Max-Planck-Institute for Solid State Research, Stuttgart

C. Bernhard
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C. Bernhard
Max-Planck-Institute for Solid State Research, Stuttgart

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Physics - Superconductivity (28)
Physics - Strongly Correlated Electrons (22)
Physics - Materials Science (18)
Quantum Physics (4)
Physics - Soft Condensed Matter (1)
Physics - Other (1)
Physics - Mesoscopic Systems and Quantum Hall Effect (1)
Physics - Optics (1)
Physics - Instrumentation and Detectors (1)

Publications Authored By C. Bernhard

$\mathrm{La_{1.85}Sr_{0.15}CuO_4}$/$\mathrm{La_2CuO_4}$ (LSCO15/LCO) bilayers with a precisely controlled thickness of N unit cells (UCs) of the former and M UCs of the latter ([LSCO15\_N/LCO\_M]) were grown on (001)-oriented {\slao} (SLAO) substrates with pulsed laser deposition (PLD). Read More

Elucidating the details of the electron-phonon coupling in semiconductors and insulators is a topic of pivotal interest, as it governs the transport mechanisms and is responsible for various phenomena such as spectral-weight transfers to phonon sidebands and self-trapping. Here, we investigate the influence of the electron-phonon interaction on the excitonic peaks of rutile TiO$_2$, revealing a strong anisotropic polarization dependence with increasing temperature, namely an anomalous blueshift for light polarized along the a-axis and a conventional redshift for light polarized along the c-axis. By employing many-body perturbation theory, we identify two terms in the electron-phonon interaction Hamiltonian that contribute to the anomalous blueshift of the a-axis exciton. Read More

We report local field strength enhancement of single-cycle terahertz (THz) pulses in an ultrafast time-resolved x-ray diffraction experiment. We show that patterning the sample with gold microstructures increases the THz field without changing the THz pulse shape or drastically affecting the quality of the x-ray diffraction pattern. We find a five-fold increase in THz-induced x-ray diffraction intensity change in the presence of microstructures on a SrTiO3 thin-film sample. Read More

With infrared (IR) ellipsometry and DC resistance measurements we investigated the photo-doping at the (001) and (110) surfaces of SrTiO$_3$ (STO) single crystals and at the corresponding interfaces of LaAlO$_3$/SrTiO$_3$ (LAO/STO) heterostructures. In the bare STO crystals we find that the photo-generated charge carriers, which accumulate near the (001) surface, have a similar depth profile and sheet carrier concentration as the confined electrons that were previously observed in LAO/STO (001) heterostructures. A large fraction of these photo-generated charge carriers persist at low temperature at the STO (001) surface even after the UV light has been switched off again. Read More

Using muon spin rotation and infrared spectroscopy we study the relation between magnetism and superconductivity in Ba$ _{1-x} $K$ _{x} $Fe$ _{2} $As$ _{2} $ single crystals from the underdoped to the slightly overdoped regime. We find that the Fe magnetic moment is only moderately suppressed in most of the underdoped region where it decreases more slowly than the N\'{e}el-temperature, $ T^{\mathrm{N}} $. This applies for both the total Fe moment obtained from muon spin rotation and for the itinerant component that is deduced from the spectral weight of the spin-density-wave pair breaking peak in the infrared response. Read More

With infrared and terahertz ellipsometry we investigated the anisotropy of the infrared active phonon modes in SrTiO3 (110) single crystals in the tetragonal state below the so-called antiferrodistortive transition at T* = 105 K. In particular, we show that the anisotropy of the oscillator strength of the so-called R-mode, which becomes weakly infrared active below T*, is a valuable indicator for the orientation of the structural domains. Our results reveal that a mono-domain state with the tetragonal axis (c-axis) parallel to the [001] direction can be created by applying a moderate uniaxial stress of about 2. Read More

With x-ray absorption spectroscopy and polarized neutron reflectometry we studied how the magnetic proximity effect at the interface between the cuprate high-$T_C$ superconductor $\mathrm{YBa_2Cu_3O_7}$ (YBCO) and the ferromagnet $\mathrm{La_{2/3}Ca_{1/3}MnO_3}$ (LCMO) is related to the electronic and magnetic properties of the LCMO layers. In particular, we explored how the magnitude of the ferromagnetic Cu moment on the YBCO side depends on the strength of the antiferromagnetic (AF) exchange coupling with the Mn moment on the LCMO side. We found that the Cu moment remains sizeable if the AF coupling with the Mn moments is strongly reduced or even entirely suppressed. Read More

Bound electronic excitations play a major role in the electrodynamics of insulators and are typically described by the concept of Wannier-Mott and Frenkel excitons. The former represent hydrogenic electron-hole pairs delocalized over several unit cells of a crystal and they occur in materials with high dielectric constant; the latter correspond to a correlated electron-hole pair localized on a single lattice site and they mostly prevail in molecular solids. Between these two extremes, an intermediate type of excitons exists, typically referred to as charge-transfer excitons. Read More

With infrared ellipsometry we studied the response of the confined electrons in a $\gamma$-Al$_2$O$_3$/SrTiO$_3$ heterostructures in which they originate predominantly from oxygen vacancies. From the analysis of a so-called Berreman mode, that develops near the highest longitudinal optical phonon mode of SrTiO$_3$, we derive the sheet carrier density, ${N_{s}}$, the mobility, $\mu$, and also the depth profile of the carrier concentration. Notably, we find that ${N_{s}}$ and the shape of the depth profile are similar as in LaAlO$_3$/SrTiO$_3$ heterostructures for which the itinerant carriers are believed to arise from a polar discontinuity. Read More

We present a combined study with conventional far-infrared and time-domain terahertz ellipsometry of the temperature dependent optical response of SrTiO3 thin films (85 and 8.5 nm) that are grown by pulsed-laser deposition on LSAT substrates. We demonstrate that terahertz ellipsometry is very sensitive to the optical response of these thin films, in particular, to the soft mode of SrTiO3. Read More

In cuprates, a precursor state of superconductivity is speculated to exist above the critical temperature $\mathrm{T_C}$. Here we show via a combination of far-infrared ellipsometry and ultrafast broadband optical spectroscopy that signatures of such a state can be obtained via three independent observables in an underdoped sample of NdBa$_2$Cu$_3$O$_{6+\delta}$. The pseudogap correlations were disentangled from the response of laser-broken pairs by clocking their characteristic time-scales. Read More

With muon spin rotation ($ \mu $SR) we studied the transition between the orthorhombic antiferromagnetic (o-AF) and the tetragonal antiferromagnetic (t-AF) states of a weakly underdoped Ba$ _{1-x} $K$ _{x} $Fe$ _{2} $As$ _{2} $ single crystal. We observed some characteristic changes of the magnitude and the orientation of the magnetic field at the muon site which, due to the fairly high point symmetry of the latter, allow us to identify the magnetic structure of the t-AF state. It is the so-called, inhomogeneous double-$\mathbf{Q}$ magnetic structure with $ c $-axis oriented moments which has a vanishing magnetic moment on half of the Fe sites. Read More

With infrared spectroscopy we investigated the spin-reorientation transition from an orthorhombic antiferromagnetic (o-AF) to a tetragonal AF (t-AF) phase and the reentrance of the o-AF phase in the superconducting state of underdoped Ba$ _{1-x} $K$ _{x} $Fe$ _{2} $As$ _{2} $. In agreement with the predicted transition from a single-$\mathbf{Q}$ to a double-$\mathbf{Q}$ AF structure, we found that a distinct spin density wave (SDW) develops in the t-AF phase. The pair breaking peak of this SDW acquires much more low-energy spectral weight than the one in the o-AF state which indicates that it competes more strongly with superconductivity. Read More

We have studied the magnetic and superconducting properties of Ba(Fe$_{0.95}$Co$_{0.05}$)$_{2}$As$_{2}$ as a function of temperature and external magnetic field using neutron scattering and muon spin rotation. Read More

With x-ray absorption spectroscopy we investigated the orbital reconstruction and the induced ferromagnetic moment of the interfacial Cu atoms in YBa$_2$Cu$_3$O$_{7}$/La$_{2/3}$Ca$_{1/3}$MnO$_3$ (YBCO/LCMO) and La$_{2-x}$Sr$_{x}$CuO$_4$/La$_{2/3}$Ca$_{1/3}$MnO$_3$ (LSCO/LCMO) multilayers. We demonstrate that these electronic and magnetic proximity effects are coupled and are common to these cuprate/manganite multilayers. Moreover, we show that they are closely linked to a specific interface termination with a direct Cu-O-Mn bond. Read More

We study the tuning curve of entangled photons generated by type-0 spontaneous parametric down-conversion in a periodically poled KTP crystal. We demonstrate the X-shaped spatiotemporal structure of the spectrum by means of measurements and numerical simulations. Experiments for different pump waists, crystal temperatures, and crystal lengths are in good agreement with numerical simulations. Read More

The insight due to John Bell that the joint behavior of individually measured entangled quantum systems cannot be explained by shared information remains a mystery to this day. We describe an experiment, and its analysis, displaying non-locality of entangled qutrit pairs. The non-locality of such systems, as compared to qubit pairs, is of particular interest since it potentially opens the door for tests of bipartite non-local behavior independent of probabilistic Bell inequalities, but of deterministic nature. Read More

With pulsed laser deposition we have grown c-axis oriented thin films of the nominal composition LaMnO$_{3}$ (LMO) on LSAT(001) substrates. We find that, depending on the oxygen background pressure during growth, the LMO films contain sizeable amounts of La and/or Mn vacancies that strongly influence their electronic and magnetic properties. Specifically, we show that the Mn/La ratio can be systematically varied from 0. Read More

We demonstrate the capability to discretize the frequency spectrum of broadband energy-time entangled photons by means of a spatial light modulator to encode qudits in various bases. Exemplarily, we implement three different discretization schemes, namely frequency bins, time bins and Schmidt modes. Entangled qudits up to dimension $d=4$ are then revealed by two-photon interference experiments with visibilities violating a $d$-dimensional Bell inequality. Read More

We studied the doping and temperature (T) dependence of the infrared (IR) response of Ba(Fe1-xCox)2As2 single crystals. We show that a weak band around 1000 cm-1, that was previously interpreted in terms of interaction of the charge carriers with magnetic excitations or of a pseudogap, is rather related to low-energy interband transitions. Specifically, we show that this band exhibits a similar doping and T-dependence as the hole pockets seen by angle resolved photoemission spectroscopy (ARPES). Read More

We have measured the temperature dependence of the direct band gap, $E_g$, in SrTi$^{16}$O$_3$ and BaTiO$_3$ and related materials with quantum-paraelectric and ferroelectric properties using optical spectroscopy. We show that $E_g$ exhibits an anomalous temperature dependence with pronounced changes in the vicinity of the ferroelectric transition that can be accounted for in terms of the Fr\"ohlich electron-phonon interaction with an optical phonon mode, the so-called soft mode. We demonstrate that these characteristic changes of $E_g$ can be readily detected even in very thin films of SrTi$^{16}$O$_3$ with a strain-induced ferroelectric order. Read More

Quantum entanglement between qudits - the d-dimensional version of qubits - is relevant for advanced quantum information processing and provides deeper insights in the nature of quantum correlations. Encoding qudits in the frequency modes of photon pairs produced by continuous parametric down-conversion enables access to high-dimensional states. By shaping the energy spectrum of entangled photons, we demonstrate the creation, characterization and manipulation of entangled qudits with dimension up to 4. Read More

With infrared ellipsometry, x-ray diffraction, and electric transport measurements we investigated the electric-field-effect on the confined electrons at the LaAlO3/SrTiO3 interface. We obtained evidence that the localization of the electrons at low temperature and negative gate voltage is induced, or at least strongly enhanced, by a pyroelectric phase transition in SrTiO3 which strongly reduces the lattice polarizability and the subsequent Coulomb screening. In particular, we show that the charge localisation and the polar order of SrTiO3 both develop below about 50 K and exhibit similar, unipolar hysteresis loops as a function of the gate voltage. Read More

The interplay among charge, spin and lattice degrees of freedom in solids gives rise to intriguing macroscopic quantum phenomena such as colossal magnetoresistance, multiferroicity and high-temperature superconductivity. Strong coupling or competition between various orders in these systems presents the key to manipulate their functional properties by means of external perturbations such as electric and magnetic fields or pressure. Ultrashort and intense optical pulses have emerged as an interesting tool to investigate elementary dynamics and control material properties by melting an existing order. Read More

Using muon spin rotation (muSR) we investigated the magnetic and superconducting properties of a series of Ba(Fe1-xCox)2As2 single crystals with 0 =< x =< 0.15. Our study details how the antiferromagnetic order is suppressed upon Co substitution and how it coexists with superconductivity. Read More

We study the perpendicular transport characteristics of small superconductor/ferromagnetic insulator/superconductor (YBa$_2$Cu$_3$O$_{7-x}$/LaMnO$_{3+\delta}$/YBa$_2$Cu$_3$O$_{7-x}$) tunnel junctions. At a large bias voltage $V\sim 1$ V we observe a step-like onset of excess current that occurs below the superconducting transition temperature $TRead More

We present a low-energy muon-spin-rotation study of the magnetic and superconducting properties of YBa2Cu3O7/PrBa2Cu3O7 trilayer and bilayer heterostructures. By determining the magnetic-field profiles throughout these structures we show that a finite superfluid density can be induced in otherwise semiconducting PrBa2Cu3O7 layers when juxtaposed to YBa2Cu3O7 "electrodes" while the intrinsic antiferromagnetic order is unaffected. Read More

Using neutron reflectometry and resonant x-ray techniques we studied the magnetic proximity effect (MPE) in superlattices composed of superconducting YBa$_2$Cu$_3$O$_7$ (YBCO) and ferromagnetic-metallic (FM-M) La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LCMO) or ferromagnetic-insulating (FM-I) LaMnO$_{3+\delta}$ (LMO). Read More

The competition between collective quantum phases in materials with strongly correlated electrons depends sensitively on the dimensionality of the electron system, which is difficult to control by standard solid-state chemistry. We have fabricated superlattices of the paramagnetic metal LaNiO3 and the wide-gap insulator LaAlO3 with atomically precise layer sequences. Using optical ellipsometry and low-energy muon spin rotation, superlattices with LaNiO3 as thin as two unit cells are shown to undergo a sequence of collective metalinsulator and antiferromagnetic transitions as a function of decreasing temperature, whereas samples with thicker LaNiO3 layers remain metallic and paramagnetic at all temperatures. Read More

Heteroepitaxial superlattices of [YBa2Cu3O7(n)/ La0.67Ca0.33MnO3(m)]x, where n and m are the number of YBCO and LCMO monolayers and x the number of bilayer repetitions, have been grown with pulsed laser deposition on NdGaO3 (110) and Sr0. Read More

Far-infrared spectra of magneto-dielectric Dy3Fe5O12 garnet were studied between 13 and 100 cm-1 and at low temperatures between 5 and 80 K. A combination of transmission, reflectivity, and rotating analyzer ellipsometry was used to unambiguously identify the type of the dipole activity of the infrared modes. In addition to purely dielectric and magnetic modes, we observed several hybrid modes with a mixed magnetic and electric dipole activity. Read More

We show that a multilayer analysis of the infrared c-axis response of RBa2Cu3O7-d (R=Y, Gd, Eu) provides important new information about the anomalous normal state properties of underdoped cuprate high temperature superconductors. Besides competing correlations which give rise to a pseudogap that depletes the low-energy electronic states below T*>>Tc, it enables us to identify the onset of a precursor superconducting state below Tons>Tc. We map out the doping phase diagram of Tons which reaches a maximum of ~180 K at strong underdoping and present magnetic field dependent data which confirm our conclusions. Read More

Muon spin spectroscopy and in particular the avoided level crossing technique is introduced, with the aim of showing it as a very sensitive local probe for electron spin relaxation in organic semiconductors. Avoided level crossing data on TMS-pentacene at different temperatures are presented, and they are analysed to extract the electron spin relaxation rate, that is shown to increase on increasing the temperature from 0.02 MHz to 0. Read More

We measure the anisotropic mid-infrared response of electrons and phonons in bulk YBa2Cu3O7 after femtosecond photoexcitation. A line shape analysis of specific lattice modes reveals their transient occupation and coupling to the superconducting condensate. The apex oxygen vibration is strongly excited within 150 fs demonstrating that the lattice absorbs a major portion of the pump energy before the quasiparticles are thermalized. Read More

Electron spin relaxation rate (eSR) is investigated on several organic semiconductors of different morphologies and molecular structures, using avoided level crossing muon spectroscopy as a local spin probe. We find that two functionalized acenes (polycrystalline tri(isopropyl)silyl-pentacene and amorphous 5,6,11,12-tetraphenyltetracene) exhibit eSRs with an Arrhenius-like temperature dependence, each with two characteristic energy scales similar to those expected from vibrations. Polycrystalline tris(8-hydroxyquinolate)gallium shows a similar behavior. Read More

We report muon spin rotation (muSR) and infrared (IR) spectroscopy experiments on underdoped BaFe1.89Co0.11As2 which show that bulk magnetism and superconductivity (SC) coexist and compete on the nanometer length scale. Read More

We report an inelastic-neutron-scattering and muon-spin-relaxation study of the effect of 2% spinless (Zn) impurities on the magnetic order and dynamics of YBa$_2$Cu$_3$O$_{6.6}$, an underdoped high-temperature superconductor that exhibits a prominent spin-pseudogap in its normal state. Zn substitution induces static magnetic order at low temperatures and triggers a large-scale spectral-weight redistribution from the magnetic resonant mode at 38 meV into uniaxial, incommensurate spin excitations with energies well below the spin-pseudogap. Read More

We report spectroscopic ellipsometry measurements of the anisotropy of the interband transitions parallel and perpendicular to the planes of (LaTiO3)n(LaAlO3)5 multilayers with n = 1-3. These provide direct information about the electronic structure of the two-dimensional (2D) 3d^1 state of the Ti ions. In combination with LDA+U calculations, we suggest that 2D confinement in the TiO2 slabs lifts the degeneracy of the t_{2g} states leaving only the planar d_xy orbitals occupied. Read More

We performed combined infrared reflection and ellipsometry measurements of the in-plane optical reponse of single crystals of the pnictide high temperature superconductor BaFe$_{1.87}$Co$_{0.13}$As$_{2}$ with $T_{c}$ = 24. Read More

With infrared ellipsometry and transport measurements we investigated the electrons at the interface between LaAlO3 and SrTiO3. We obtained a sheet carrier density of Ns~5-9x 10E13 cm^-2, an effective mass of m*~3m_e, and a strongly frequency dependent mobility. The latter are similar as in bulk SrTi1-xNbxO3 and therefore suggestive of polaronic correlations of the confined carriers. Read More

We report on results of our theoretical study of the c-axis infrared conductivity of bilayer high-Tc cuprate superconductors using a microscopic model involving the bilayer-split (bonding and antibonding) bands. An emphasis is on the gauge-invariance of the theory, which turns out to be essential for the physical understanding of the electrodynamics of these compounds. The description of the optical response involves local (intra-bilayer and inter-bilayer) current densities and local conductivities. Read More

We present a muon spin rotation (\muSR) study of the magnetic and superconducting properties of single crystals of electron-doped BaFe2-xCoxAs2 with x=0.08, 0.20, and 0. Read More

The recent observation of superconductivity with critical temperatures up to 55 K in the FeAs based pnictide compounds marks the first discovery of a non copper-oxide based layered high-Tc superconductor (HTSC) [1-3]. It has raised the suspicion that these new materials share a similar pairing mechanism to the cuprates, since both exhibit superconductivity following charge doping of a magnetic parent material. Here we present a muon spin rotation study on SmFeAsO1-xFx (x=0-0. Read More

We investigated the electronic structures of the 5$d$ Ruddlesden-Popper series Sr$_{n+1}$Ir$_{n}$O$_{3n+1}$ ($n$=1, 2, and $\infty$) using optical spectroscopy and first-principles calculations. As 5$d$ orbitals are spatially more extended than 3$d$ or 4$d$ orbitals, it has been widely accepted that correlation effects are minimal in 5$d$ compounds. However, we observed a bandwidth-controlled transition from a Mott insulator to a metal as we increased $n$. Read More

Electronic phases with symmetry properties matching those of conventional liquid crystals have recently been discovered in transport experiments on semiconductor heterostructures and metal oxides at milli-Kelvin temperatures. We report the spontaneous onset of a onedimensional, incommensurate modulation of the spin system in the high-temperature superconductor YBa2Cu3O6.45 upon cooling below ~150 K, while static magnetic order is absent above 2 K. Read More

We investigated with synchrotron x-ray diffraction and reflectometry the formation of structural domains in the near-surface region of single crystalline SrTiO3 (001) substrates with Y0.6Pr0.4Ba2Cu3O7 / La2/3Ca1/3MnO3 superlattices grown on top. Read More