arXiv subjects

R. Werner

Publications and source records attributed to R. Werner.

12 recordsLinked to original sources

arXiv2013

Electron sampling depth and saturation effects in perovskite films investigated by soft x-ray absorption spectroscopy

Knowledge of the electron sampling depth and related saturation effects is important for quantitative analysis of X-ray absorption spectroscopy data, yet for oxides with the perovskite structure no quantitative values are so far available. Here we study absorption saturation in films of two of the most-studied perovskites, LCMO and YBCO, at the L2,3 edge of Mn and Cu, respectively. By measuring the electron-yield intensity as a function of photon incidence angle and film thickness, the sampling depth d, photon attenuation length, lambda, and the ratio lambda/d have been independently determined between 50 and 300 K. The extracted sampling depth dLCMO=3 nm for LCMO at high temperatures in its polaronic insulator state (150-300K) is not much larger than values reported for pure transition metals (d Co or Ni=2-2.5nm) at room temperature, but it is smaller than dYBCO=3.9 nm for metallic YBCO that is in turn smaller than the value reported for Fe3O4 (d=4.5 nm) The measured dLCMO increases to 4.5 nm when LCMO is in the metallic state at low temperatures. These results indicate that a universal rule of thumb for the sampling depth in oxides cannot be assumed, and that it can be measurably influenced by electronic phase transitions that derive from strong correlations.

cond-mat.mtrl-sci↗

arXiv2011

Edge superconductivity in Nb thin film microbridges revealed by integral and spatially resolved electric transport

The resistance $R$ vs perpendicular external magnetic field $H$ was measured for superconducting Nb thin--film microbridges with and without microholes [antidots (ADs)]. Well below the transition temperature, integral $R(H)$ measurements of the resistive transition to the normal state on the plain bridge show two distinct regions, which can be identified as bulk and edge superconductivity, respectively. The latter case appears when bulk superconductivity becomes suppressed at the upper critical field $H_{c2}$ and below the critical field of edge superconductivity $H_{c3}\approx 1.7\, H_{c2}$. The presence of additional edges in the AD bridge leads to a different shape of the $R(H)$ curves. We used low-temperature scanning laser microscopy (LTSLM) to visualize the current distribution in the plain and AD bridge upon sweeping $H$. While the plain bridge shows a dominant LTSLM signal at its edges for $H > H_{c2}$ the AD bridge also gives a signal from the inner parts of the bridge due to the additional edge states around the ADs. LTSLM reveals an asymmetry in the current distribution between left and right edges, which confirms theoretical predictions. Furthermore, the experimental results are in good agreement with our numerical simulations (based on the time-dependent Ginzburg--Landau model) yielding the spatial distribution of the order parameter and current density for different bias currents and $H$ values.

cond-mat.supr-con↗

arXiv2011

Crossover between different regimes of inhomogeneous superconductivity in planar superconductor-ferromagnet hybrids

We studied experimentally the effect of a stripe-like domain structure in a ferromagnetic BaFe_{12}O_{19} substrate on the magnetoresistance of a superconducting Pb microbridge. The system was designed in such a way that the bridge is oriented perpendicular to the domain walls. It is demonstrated that depending on the ratio between the amplitude of the nonuniform magnetic field B_0, induced by the ferromagnet, and the upper critical field H_{c2} of the superconducting material, the regions of the reverse-domain superconductivity in the H-T plane can be isolated or can overlap (H is the external magnetic field, T is temperature). The latter case corresponds to the condition B_0/H_{c2}<1 and results in the formation of superconductivity above the magnetic domains of both polarities. We discovered the regime of edge-assisted reverse-domain superconductivity, corresponding to localized superconductivity near the edges of the bridge above the compensated magnetic domains. Direct verification of the formation of inhomogeneous superconducting states and external-field-controlled switching between normal state and inhomogeneous superconductivity were obtained by low-temperature scanning laser microscopy.

cond-mat.supr-con↗

arXiv2011

Domain-wall and reverse-domain superconducting states of a Pb thin-film bridge on a ferromagnetic BaFe_{12}O_{19} single crystal

We report on imaging of the nonuniform superconducting states in a Pb thin film bridge on top of a ferromagnetic BaFe_{12}O_{19} single crystal with a single straight domain wall along the center of the bridge by low-temperature scanning laser microscopy. We have visualized domain wall superconductivity (DWS) close to the critical temperature of Pb, when the Pb film above the domain wall acts as a superconducting path for the current. The evolution of the DWS signal with temperature and the external-field-driven transition from DWS to reverse domain superconductivity was visualized.

cond-mat.supr-con↗

arXiv2010

YBa2Cu3O7 grain boundary junctions and low-noise superconducting quantum interference devices patterned by a focused ion beam down to 80 nm linewidth

YBa$_2$Cu$_3$O$_7$ 24$^\circ$ (30$^\circ$) bicrystal grain boundary junctions (GBJs), shunted with 60\,nm (20\,nm) thick Au, were fabricated by focused ion beam milling with widths $80\,{\rm nm} \le w \le 7.8\,μ$m. At 4.2\,K we find critical current densities $j_c$ in the $10^5\,{\rm A/cm^2}$ range %\dkc{\#1} (without a clear dependence on $w$) and an increase in resistance times junction area $ρ$ with an approximate scaling $ρ\propto w^{1/2}$. For the narrowest GBJs $j_cρ\approx 100\,μ$V, which is promising for the realization of sensitive nanoSQUIDs for the detection of small spin systems. We demonstrate that our fabrication process allows the realization of sensitive nanoscale dc SQUIDs; for a SQUID with $w\approx 100$\,nm wide GBJs we find an rms magnetic flux noise spectral density of $S_Φ^{1/2}\approx 4\,μΦ_0/{\rm Hz}^{1/2}$ in the white noise limit. We also derive an expression for the spin sensitivity $S_μ^{1/2}$, which depends on $S_Φ^{1/2}$, on the location and orientation of the magnetic moment of a magnetic particle to be detected by the SQUID, and on the SQUID geometry. For the not optimized SQUIDs presented here, we estimate $S_μ^{1/2}=390\,μ_B/\sqrt{\rm{Hz}}$, which could be further improved by at least an order of magnitude.

cond-mat.supr-con↗

arXiv2010

YBa$_2$Cu$_3$O$_7$/La$_{0.7}$Ca$_{0.3}$MnO$_3$ bilayers: Interface coupling and electric transport properties

Heteroepitaxially grown bilayers of ferromagnetic La$_{0.7}$Ca$_{0.3}$MnO$_3$ (LCMO) on top of superconducting YBa$_2$Cu$_3$O$_7$ (YBCO) thin films were investigated by focusing on electric transport properties as well as on magnetism and orbital occupation at the interface. Transport measurements on YBCO single layers and on YBCO/LCMO bilayers, with different YBCO thickness $d_Y$ and constant LCMO thickness $d_L=50$\,nm, show a significant reduction of the superconducting transition temperature $T_c$ only for $d_Y<10$\,nm,with only a slightly stronger $T_c$ suppression in the bilayers, as compared to the single layers. X-ray magnetic circular dichroism (XMCD) measurements confirm recently published data of an induced magnetic moment on the interfacial Cu by the ferromagnetically ordered Mn ions, with antiparallel alignment between Cu and Mn moments. However, we observe a significantely larger Cu moment than previously reported, indicating stronger coupling between Cu and Mn at the interface. This in turn could result in an interface with lower transparency, and hence smaller spin diffusion length, that would explain our electric transport data, i.e.smaller $T_c$ suppression. Moreover, linear dichroism measurements did not show any evidence for orbital reconstruction at the interface, indicating that a large change in orbital occupancies through hybridization is not necessary to induce a measurable ferromagnetic moment on the Cu atoms.

cond-mat.supr-con↗

arXiv2009

Lowering of surface melting temperature in atomic clusters with a nearly closed shell structure

We investigate the interplay of particle number, N, and structural properties of selected clusters with N=12 up to N=562 by employing Gupta potentials parameterized for Aluminum and extensive Monte-Carlo simulations. Our analysis focuses on closed shell structures with extra atoms. The latter can put the cluster under a significant stress and we argue that typically such a strained system exhibits a reduced energy barrier for (surface) diffusion of cluster atoms. Consequently, also its surface melting temperature, T_S, is reduced, so that T_S separates from and actually falls well below the bulk value. The proposed mechanism may be responsible for the suppression of the surface melting temperature observed in a recent experiments.

cond-mat.mtrl-sci↗

arXiv2008

Transport, magnetic, and structural properties of La$_{0.7}$Ce$_{0.3}$MnO$_3$ thin films. Evidence for hole-doping

Cerium-doped manganite thin films were grown epitaxially by pulsed laser deposition at $720 ^\circ$C and oxygen pressure $p_{O_2}=1-25 $Pa and were subjected to different annealing steps. According to x-ray diffraction (XRD) data, the formation of CeO$_2$ as a secondary phase could be avoided for $p_{O_2}\ge 8 $Pa. However, transmission electron microscopy shows the presence of CeO$_2$ nanoclusters, even in those films which appear to be single phase in XRD. With O$_2$ annealing, the metal-to-insulator transition temperature increases, while the saturation magnetization decreases and stays well below the theoretical value for electron-doped La$_{0.7}$Ce$_{0.3}$MnO$_3$ with mixed Mn$^{3+}$/Mn$^{2+}$ valences. The same trend is observed with decreasing film thickness from 100 to 20 nm, indicating a higher oxygen content for thinner films. Hall measurements on a film which shows a metal-to-insulator transition clearly reveal holes as dominating charge carriers. Combining data from x-ray photoemission spectroscopy, for determination of the oxygen content, and x-ray absorption spectroscopy (XAS), for determination of the hole concentration and cation valences, we find that with increasing oxygen content the hole concentration increases and Mn valences are shifted from 2+ to 4+. The dominating Mn valences in the films are Mn$^{3+}$ and Mn$^{4+}$, and only a small amount of Mn$^{2+}$ ions can be observed by XAS. Mn$^{2+}$ and Ce$^{4+}$ XAS signals obtained in surface-sensitive total electron yield mode are strongly reduced in the bulk-sensitive fluorescence mode, which indicates hole-doping in the bulk for those films which do show a metal-to-insulator transition.

cond-mat.str-el↗

arXiv2005

Clean Positive Operator Valued Measures

In quantum mechanics the statistics of the outcomes of a measuring apparatus is described by a positive operator valued measure (POVM). A quantum channel transforms POVM's into POVM's, generally irreversibly, thus loosing some of the information retrieved from the measurement. This poses the problem of which POVM's are "undisturbed", namely they are not irreversibly connected to another POVM. We will call such POVM clean. In a sense, the clean POVM's would be "perfect", since they would not have any additional "extrinsical" noise. Quite unexpectedly, it turns out that such cleanness property is largely unrelated to the convex structure of POVM's, and there are clean POVM's that are not extremal and vice-versa. In this paper we solve the cleannes classification problem for number n of outcomes n<=d (d dimension of the Hilbert space), and we provide a a set of either necessary or sufficient conditions for n>d, along with an iff condition for the case of informationally complete POVM's for n=d^2.

quant-ph↗

arXiv1999

The microscopic spin-phonon coupling constants in CuGeO_3

Using RPA results, mean field theory, and refined data for the polarization vectors we determine the coupling constants of the four Peierls-active phonon modes to the spin chains of CuGeO_3. We then derive the values of the coupling of the spin system to the linear ionic displacements, the bond lengths and the angles between bonds. Our values are consistent with microscopic theories and various experimental results. We discuss the applicability of static approaches to the spin-phonon coupling. The c-axis anomaly of the thermal expansion is explained. We give the values of the coupling constants in an effective one-dimensional Hamiltonian.

cond-mat↗

arXiv1999

Optimal Manipulations with Qubits: Universal NOT Gate

It is not a problem to complement a classical bit, i.e. to change the value of a bit, a 0 to a 1 and vice versa. This is accomplished by a NOT gate. Complementing a qubit in an unknown state, however, is another matter. We show that this operation cannot be done perfectly. We define the Universal-NOT (U-NOT) gate which out of N identically prepared pure input qubits generates M output qubits in a state which is as close as possible to the perfect complement. This gate can be realized by classical estimation and subsequent re-preparation of complements of the estimated state. Its fidelity is therefore equal to the fidelity F= (N+1)/(N+2) of optimal estimation, and does not depend on the required number of outputs. We also show that when some additional a priori information about the state of input qubit is available, than the fidelity of the quantum NOT gate can be much better than the fidelity of estimation.

quant-ph↗

arXiv1998

Shell filling in non-linear magneto-tunneling spectroscopy of vertical quantum dots

We report on non-linear magneto-tunneling experiments carried out in single GaAs vertical quantum dots. We show that conduction at low voltage bias can be a spectroscopic tool for both the ground state and first excited states of few electrons on the dot. Increasing voltage a large resonant peak is observed and attributed to tunneling across the quasi-continuum of higher excited states.

cond-mat.mes-hall↗