Yoshiharu Krockenberger

A hidden pseudogap under the ‘dome’ of superconductivity in electron-doped high-temperature superconductors

The ground state of superconductors is characterized by the long-range order of condensed Cooper pairs: this is the only order present in conventional superconductors. The high-transition-temperature (high-Tc) superconductors, in contrast, exhibit more complex phase behaviour, which might indicate the presence of other competing ground states. For example, the pseudogap—a suppression of the accessible electronic states at the Fermi level in the normal state of high-Tc superconductors—has been interpreted as either a precursor to superconductivity or as tracer of a nearby ground state that can be separated from the superconducting state by a quantum critical point. Here we report the existence of a second order parameter hidden within the superconducting phase of the underdoped (electron-doped) high-Tc superconductor Pr2-xCexCuO4-y and the newly synthesized electron-doped material La2-xCexCuO4-y (ref. 8). The existence of a pseudogap when superconductivity is suppressed excludes precursor superconductivity as its origin. Our observation is consistent with the presence of a (quantum) phase transition at T = 0, which may be a key to understanding high-Tc superconductivity. This supports the picture that the physics of high-Tc superconductors is determined by the interplay between competing and coexisting ground states.

Growth of superconducting Sr2RuO4 thin films

Sr2RuO4 is a superconductor with a low transition temperature (Tc∼1 K), but has a unique character of p-wave symmetry of the order parameter, demanding epitaxial thin films for Josephson junction studies. We have grown c-axis oriented epitaxial films by a pulsed laser deposition technique on (001) surfaces of (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 substrates. Careful tuning of growth conditions yielded in a layer-by-layer growth of high crystallinity films. The films show a residual resistivity ratio (ρ300 K/ρ2 K) as large as 82 and a superconducting transition with a zero resistivity at 0.6 K.

Single-crystal cubic boron nitride thin films grown by ion-beam-assisted molecular beam epitaxy

We investigated the formation of cubic boron nitride (c-BN) thin films on diamond (001) and (111) substrates by ion-beam-assisted molecular beam epitaxy (MBE). The metastable c-BN (sp3-bonded BN) phase can be epitaxially grown as a result of the interplay between competitive phase formation and selective etching. We show that a proper adjustment of acceleration voltage for N2+ and Ar+ ions is a key to selectively discriminate non-sp3 BN phases. At low acceleration voltage values, the sp2-bonded BN is dominantly formed, while at high acceleration voltages, etching dominates irrespective of the bonding characteristics of BN.

Perovskite LaRhO3 as a p-type active layer in oxide photovoltaics

Perovskite-type transition-metal oxides have a wide variety of physical properties and triggered intensive research on functional devices in the form of heteroepitaxial junctions. However, there is a missing component that is a p-type conventional band semiconductor. LaRhO3 (LRO) is one of very few promising candidates having its bandgap between filled t2g and empty eg of Rh in low-spin state, but there has been no report on the synthesis of large-size single crystals or thin films. Here, we report on the junction properties of single-crystalline thin films of LRO grown on (110) oriented Nb-doped SrTiO3 substrates. The external quantum efficiency of the photo-electron conversion exceeds 1% in the visible-light region due to the wide depletion layer and long diffusion length of minority carriers in LRO. Clear indication of p-type band semiconducting character in a perovskite oxide of LRO will pave a way to explore oxide electronics of perovskite heterostructures.

Shubnikov-de Haas quantum oscillations reveal a reconstructed Fermi surface near optimal doping in a thin film of the cuprate superconductor Pr1.86Ce0.14CuO4±δ

Author(s): Breznay, NP; Hayes, IM; Ramshaw, BJ; McDonald, RD; Krockenberger, Y; Ikeda, A; Irie, H; Yamamoto, H; Analytis, JG | Abstract:

Transistors driven by superconductors

A hybrid transistor device has been made in which a superconductor forms a seamless interface with a semiconductor. The study of such interfaces could open the way to innovative applications in electronics. A hybrid transistor device has been made in which a superconductor forms a seamless interface with a semiconductor. The study of such interfaces could open the way to innovative applications in electronics.

Infinite-layer phase formation in the Ca1–xSrxCuO2 system by reactive molecular beam epitaxy

We synthesized thin films of the thermodynamically unstable infinite-layer compound Ca1–xSrxCuO2 by reactive molecular beam epitaxy and established thermodynamic boundary conditions that allow for the minimization of impurity phases and defects. In particular, the choice of radio-frequency oxygen plasma as an oxidizing agent as well as diverse substrate materials has been found to limit the stability region in a way that is comparable to the synthesis temperature. We employed scanning transmission electron microscopy to gain microscopic information and feedback on the formation behavior of the infinite layer phase. Moreover, we find that minute variations of the oxidizing power coerce strong responses, i.e., termination of the formation of the infinite layer phase.We synthesized thin films of the thermodynamically unstable infinite-layer compound Ca1–xSrxCuO2 by reactive molecular beam epitaxy and established thermodynamic boundary conditions that allow for the minimization of impurity phases and defects. In particular, the choice of radio-frequency oxygen plasma as an oxidizing agent as well as diverse substrate materials has been found to limit the stability region in a way that is comparable to the synthesis temperature. We employed scanning transmission electron microscopy to gain microscopic information and feedback on the formation behavior of the infinite layer phase. Moreover, we find that minute variations of the oxidizing power coerce strong responses, i.e., termination of the formation of the infinite layer phase.