arXiv subjects

A. Brinkman

Publications and source records attributed to A. Brinkman.

At least 55 records · Page 3Linked to original sources

arXiv2010

Parallel electron-hole bilayer conductivity from electronic interface reconstruction

The perovskite SrTiO$_3$-LaAlO$_3$ structure has advanced to a model system to investigate the rich electronic phenomena arising at polar interfaces. Using first principles calculations and transport measurements we demonstrate that an additional SrTiO$_3$ capping layer prevents structural and chemical reconstruction at the LaAlO$_3$ surface and triggers the electronic reconstruction at a significantly lower LaAlO$_3$ film thickness than for the uncapped systems. Combined theoretical and experimental evidence (from magnetotransport and ultraviolet photoelectron spectroscopy) suggests two spatially separated sheets with electron and hole carriers, that are as close as 1 nm.

cond-mat.mtrl-sci↗

arXiv2009

Andreev spectra and subgap bound states in multiband superconductors

The theory of Andreev conductance is formulated for junctions involving normal metals (N) and multiband superconductors (S) and applied to the case of superconductors with nodeless extended $s_{\pm}$-wave order parameter symmetry, as possibly realized in the recently discovered ferro pnictides. We find qualitative differences from tunneling into s-wave or d-wave superconductors that may help to identify such a state. First, interband interference leads to a suppression of Andreev reflection in the case of a highly transparent N/S interface and to a current deficit in the tunneling regime. Second, surface bound states may appear, both at zero and at non-zero energies. These effects do not occur in multiband superconductors without interband sign reversal, though the interference can still strongly modify the conductance spectra.

cond-mat.supr-con↗

arXiv2009

In search for the superconducting spin-switch: Magnetization induced resistance switching effects in La$_{0.67}$Sr$_{0.33}$MnO$_3$/YBa$_2$Cu$_3$O$_{7-δ}$ bi- and trilayers

We have studied the influence of the magnetization on the superconducting transition temperature ($T_c$) in bi- and trilayers consisting of the half-metallic ferromagnet La$_{0.67}$Sr$_{0.33}$MnO$_3$ (LSMO) and the high-temperature superconductor YBa$_2$Cu$_3$O$_{7-δ}$ (YBCO). We have made use of tilted epitaxial growth in order to achieve contacts between the two materials that are partly in the crystallographic $ab$-plane of the YBCO. As a result of uniaxial magnetic anisotropy in the tilted structures, we observe sharp magnetization switching behavior. At temperatures close to $T_c$, the magnetization switching induces resistance jumps in trilayers, resulting in a magnetization dependence of $T_c$. In bilayers, this switching effect can be observed as well, provided that the interface to the ferromagnetic layer is considerably rough. Our results indicate that the switching behavior arises from magnetic stray fields from the ferromagnetic layers that penetrate into the superconductor. A simple model describes the observed behavior well. We find no evidence that the switching behavior is caused by a so-called superconducting spin-switch, nor by accumulation of spin-polarized electrons. Observation of magnetic coupling of the ferromagnetic layers, through the superconductor, supports the idea of field induced resistance switching.

cond-mat.supr-con↗

arXiv2008

Orbital reconstruction and two-dimensional electron gas at the LaAlO3/SrTiO3 interface

Conventional two-dimensional electron gases are realized by engineering the interfaces between semiconducting compounds. In 2004, Ohtomo and Hwang discovered that an electron gas can be also realized at the interface between large gap insulators made of transition metal oxides [1]. This finding has generated considerable efforts to clarify the underlying microscopic mechanism. Of particular interest is the LaAlO3/SrTiO3 system, because it features especially striking properties. High carrier mobility [1], electric field tuneable superconductivity [2] and magnetic effects [3], have been found. Here we show that an orbital reconstruction is underlying the generation of the electron gas at the LaAlO3/SrTiO3 n-type interface. Our results are based on extensive investigations of the electronic properties and of the orbital structure of the interface using X-ray Absorption Spectroscopy. In particular we find that the degeneracy of the Ti 3d states is fully removed, and that the Ti 3dxy levels become the first available states for conducting electrons.

cond-mat.mtrl-sci↗

arXiv2008

Structure-Property Relation of SrTiO3-LaAlO3 Interfaces

A large variety of transport properties have been observed at the interface between the insulating oxides SrTiO3 and LaAlO3 such as insulation, 2D interface metallicity, 3D bulk metallicity, Kondo scattering, magnetism and superconductivity. The relation between the structure and the properties of the SrTiO3-LaAlO3 interface can be explained in a meaningful way by taking into account the relative contribution of three structural aspects: oxygen vacancies, structural deformations (including cation disorder) and electronic interface reconstruction. The emerging phase diagram is much richer than for related bulk oxides due to the occurrence of interface electronic reconstruction. The observation of this interface phenomenon is a display of recent advances in thin film deposition and characterization techniques, and provides an extension to the range of exceptional electronic properties of complex oxides.

cond-mat.str-el↗

arXiv2008

Magnetoresistance oscillations and relaxation effects at the SrTiO3-LaAlO3 interface

We present low-temperature and high-field magnetotransport data on SrTiO3-LaAlO3 interfaces. The resistance shows hysteresis in magnetic field and a logarithmic relaxation as a function of time. Oscillations in the magnetoresistance are observed, showing a square root periodicity in the applied magnetic field, both in large-area unstructured samples as well as in a structured sample. An explanation in terms of a commensurability condition of edge states in a highly mobile two-dimensional electron gas between substrate step edges is suggested.

cond-mat.str-el↗

arXiv2007

Magnetic effects at the interface between nonmagnetic oxides

The electronic reconstruction at the interface between two insulating oxides can give rise to a highly-conductive interface. In analogy to this remarkable interface-induced conductivity we show how, additionally, magnetism can be induced at the interface between the otherwise nonmagnetic insulating perovskites SrTiO3 and LaAlO3. A large negative magnetoresistance of the interface is found, together with a logarithmic temperature dependence of the sheet resistance. At low temperatures, the sheet resistance reveals magnetic hysteresis. Magnetic ordering is a key issue in solid-state science and its underlying mechanisms are still the subject of intense research. In particular, the interplay between localized magnetic moments and the spin of itinerant conduction electrons in a solid gives rise to intriguing many-body effects such as Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions, the Kondo effect, and carrier-induced ferromagnetism in diluted magnetic semiconductors. The conducting oxide interface now provides a versatile system to induce and manipulate magnetic moments in otherwise nonmagnetic materials.

cond-mat.str-el↗

arXiv2006

Crossed Andreev reflection in diffusive contacts

Crossed Andreev reflection in multiterminal structures in the diffusive regime is addressed within the quasiclassical Keldysh-Usadel formalism. The elastic cotunneling and crossed Andreev reflection of quasiparticles give nonlocal currents and voltages (depending on the actual biasing of the devices) by virtue of the induced proximity effect in the normal metal electrodes. The magnitude of the nonlocal processes is found to scale with the square of the barrier transparency and to decay exponentially with interface spacing. Nonlocal cotunneling and crossed Andreev conductances are found to contribute equally to the nonlocal current, which is of relevance to the use of normal metal-superconducting heterostructures as sources of entanglement.

cond-mat.supr-con↗

arXiv2006

Study of Correlation Effects in the High Formal Oxidation State Compound Sr$_2$CoO$_4$

Two recent reports confirm that the newly synthesized Sr$_2$CoO$_4$ (formal oxidation state Co$^{4+}$) shows a high Curie temperature (~ 250 K), but they report different moments of 1.8 $μ_B$ and 1 $μ_B$ per Co. Using both commonly used functionals in the correlated band approach (LDA+U) as well as the local density approximation (LDA),the combined effects of correlation and hybridization with O 2p states are calculated and analyzed. Sr$_2$CoO$_2$ is already ferromagnetic within LDA (M=1.95 $μ_B$). Increasing U from zero, the two LDA+U schemes affect the moment oppositely out to a critical value $U_c$=2.5 eV, at which point they transform discontinuously from different states to the same large U state. Fixing U at $U_c$, fixed spin moment calculations show similar behavior out to a minimum at 1$μ_B$ (a half metallic state), beyond which the fully-localized-limit scheme jumps to a state with energy minimum very near 2$μ_B$ very close to the LDA moment). Although the energy minima occur very near integer values of the moment/Co (1$μ_B, 2$μ_B$), the strong 3d-2p mixing and resulting 3d orbital occupations seem to preclude any meaningful S=1/2 or S=1 assignment to the Co ion.

cond-mat.str-el↗

arXiv2006

How strongly correlated is MnSi?

We present an experimental study of the electronic structure of MnSi. Using X-ray Absorption Spectroscopy, X-ray photoemission and X-ray fluorescence we provide experimental evidence that MnSi has a mixed valence ground state. We show that self consistent LDA supercell calculations cannot replicate the XAS spectra of MnSi, while a good match is achieved within the atomic multiplet theory assuming a mixed valence ground state. We discuss the role of the electron-electron interactions in this compound and estimate that the valence fluctuations are suppressed by a factor of 2.5, which means that the Coulomb repulsion is not negligible.

cond-mat.str-el↗

arXiv2005

Strong magnetic pair breaking in Mn substituted MgB_2 single crystals

Magnetic ions (Mn) were substituted in MgB_2 single crystals resulting in a strong pair-breaking effect. The superconducting transition temperature, T_c, in Mg_{1-x}Mn_xB_2 has been found to be rapidly suppressed at an initial rate of 10 K/%Mn, leading to a complete suppression of superconductivity at about 2% Mn substitution. This reflects the strong coupling between the conduction electrons and the 3d local moments, predominantly of magnetic character, since the nonmagnetic ion substitutions, e.g. with Al or C, suppress T_c much less effectively (e.g. 0.5 K/%Al). The magnitude of the magnetic moment, derived from normal state susceptibility measurements, uniquely identifies the Mn ions to be divalent, and to be in the low-spin state (S = 1/2). This has been found also in X-ray absorption spectroscopy measurements. Isovalent Mn^{2+} substitution for Mg^{2+} mainly affects superconductivity through spin-flip scattering reducing T_c rapidly and lowering the upper critical field anisotropy H_{c2}^{ab}/H_{c2}^c at T = 0 from 6 to 3.3 (x = 0.88% Mn), while leaving the initial slope dH_{c2}/dT near T_c unchanged for both field orientations.

cond-mat.supr-con↗

arXiv2005

Electron-hole coupling in high-Tc cuprate superconductors

From recent Hall effect measurements and angle-resolved photo-emission spectroscopy the interesting picture emerges of co-existing hole- and electron-like quasiparticle bands, both in electron- and hole-doped superconducting cuprates. We reflect on the idea that bosonic electron-hole pairs may be formed in the cuprates and on the possibility that these pairs undergo Bose-Einstein condensation. The relevance to high-Tc superconductivity in the cuprates will be discussed.

cond-mat.supr-con↗

arXiv2004

The proximity effect in normal metal - multiband superconductor hybrid structures

A theory of the proximity effect in normal metal - multiband superconductor hybrid structures is formulated within the quasiclassical Green's function formalism. It is shown that the existence of multiple superconducting bands manifests itself as the occurence of additional peaks in the density of states in the structure. The interplay between the proximity effect and the interband coupling influences the magnitudes of the gaps in a superconductor in a non-trivial way. The developed theory is applied to the calculation of supercurrent in multiband Superconductor - Normal metal - Superconductor Josephson junctions with low-transparent interfaces, and the results are compared with the predictions for multiband tunnel junctions.

cond-mat.supr-con↗

arXiv2003

Microscopic nonequilibrium theory of double-barrier Josephson junctions

We study nonequilibrium charge transport in a double-barrier Josephson junction, including nonstationary phenomena, using the time-dependent quasiclassical Keldysh Green's function formalism. We supplement the kinetic equations by appropriate time-dependent boundary conditions and solve the time-dependent problem in a number of regimes. From the solutions, current-voltage characteristics are derived. It is understood why the quasiparticle current can show excess current as well as deficit current and how the subgap conductance behaves as function of junction parameters. A time-dependent nonequilibrium contribution to the distribution function is found to cause a non-zero averaged supercurrent even in the presence of an applied voltage. Energy relaxation due to inelastic scattering in the interlayer has a prominent role in determining the transport properties of double-barrier junctions. Actual inelastic scattering parameters are derived from experiments. It is shown as an application of the microscopic model, how the nature of the intrinsic shunt in double-barrier junctions can be explained in terms of energy relaxation and the opening of Andreev channels.

cond-mat.supr-con↗

arXiv2002

Ultraviolet images of Seyfert galaxies from the Optical Monitor on XMM-Newton

The Optical Monitor telescope (Mason et al. 2001) on XMM-Newton provides an exciting multi-wavelength dimension to observations of Active Galactic Nuclei. Here we present ultraviolet images, taken with the OM UVW2 filter (140-270 nm), of various Seyfert galaxies, some of which have never been observed in this waveband before. The images show UV emission from both the active nucleus and the host galaxy. The distribution of UV emission in the galaxy shows where star formation is occurring, thus giving us clues as to the evolution of the host galaxy and perhaps its relationship to the Seyfert Nucleus.

astro-ph↗

arXiv2002

Multiband model for tunneling in MgB2 junctions

A theoretical model for quasiparticle and Josephson tunneling in multiband superconductors is developed and applied to MgB2-based junctions. The gap functions in different bands in MgB2 are obtained from an extended Eliashberg formalism, using the results of band structure calculations. The temperature and angle dependencies of MgB2 tunneling spectra and the Josephson critical current are calculated. The conditions for observing one or two gaps are given. We argue that the model may help to settle the current debate concerning two-band superconductivity in MgB2.

cond-mat.supr-con↗

arXiv2002

Multiband model for penetration depth in MgB2

The results of first principles calculations of the electronic structure and the electron-phonon interaction in MgB2 are used to study theoretically the temperature dependence and anisotropy of the magnetic field penetration depth. The effects of impurity scattering are essential for a proper description of the experimental results. We compare our results with experimental data and we argue that the two-band model describes the data rather well.

cond-mat.supr-con↗

arXiv2001

Superconducting Quantum Interference Device based on MgB2 nanobridges

The recently discovered superconductor MgB2, with a transition temperature of 39K, has significant potential for future electronics. An essential step is the achievement of Josephson circuits, of which the superconducting quantum interference device (SQUID) is the most important. Here, we report Josephson quantum interference in superconducting MgB2 thin films. Modulation voltages of up to 30 microvolt are observed in an all-MgB2 SQUID, based on focused ion beam patterned nanobridges. These bridges, with a length scale < 100 nm, have outstanding critical current densities of 7 x 10^6 A/cm2 at 4.2 K.

cond-mat.supr-con↗