Control Of Superconductivity Research Articles

Anomalous Charge State Evolution and Its Control of Superconductivity in M3Al2C (M= Mo, W).

SummaryThe charge states of elements dictate the behavior of electrons and phonons in a lattice, either directly or indirectly. Here, we report the discovery of an anomalous charge state evolution in the superconducting M3Al2C (M = Mo, W) system, where electron doping can be achieved through “oxidation.” Specifically, with the continuous removal of electron donor (Al) from the structure, we found an electron doping effect in the negatively charged transition metals. Over a certain threshold, the charge state of transition metals goes through a sudden reversion from negative to positive, which leads to a subsequent structure collapse. Concomitantly, the previous robust superconducting transition temperatures (Tcs) can be flexibly modulated. Detailed analysis reveals the origin of the superconductivity and the intimate relationship between the charge state and the electron-phonon coupling constant. The peculiar charge state in M3Al2C plays an important role in both its structure and superconductivity.

Control of superconductivity in MgB2 by ion bombardment

The superconducting properties of MgB2 can be modified via controlled introduction of lattice defects by ion bombardment. Here, magnetometry and Raman spectroscopy are used to study ∼160 nm-thick MgB2 films bombarded at room temperature with megaelectronvolt-energy He, Ar, or Xe ions. Ion bombardment leads to a monotonic reduction in the critical superconducting transition temperature, transition width, and critical current density. There is a clear collision cascade density effect, whereby radiation-induced changes, once normalized to the number of atomic displacements generated, are greater for heavier ions. The sharpening of the superconducting transition with increasing ion dose suggests improved homogeneity of irradiated films. The suppression of superconducting properties correlates with the growth of defect signatures revealed by Raman spectroscopy. Superconducting properties can be recovered by annealing at relatively low temperatures of 500 °C, even for films exposed to high doses of heavy ions. These results have direct implications for the performance of MgB2 in radiation environments and understanding its defect-mediated superconductivity.

Control of superconductivity by means of electric-field-induced strain in superconductor/piezoelectric hybrids

The controlled modification of superconductivity by any means, specifically in hybrid systems, has attracted much interest in the recent decades. Here, we present experimental data and phenomenological modeling on the control of TC of superconducting (SC) Nb thin films, with thickness 3 nm ≤ dNb≤50 nm, under the application of in-plane strain, S(Eex) induced by an external out-of-plane electric field, Eex to piezoelectric (PE) single crystals, namely, (1−x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-xPT), with x = 0.27 and 0.31. We report experimental modification of TC of Nb by Eex, accurately described by a phenomenological model that incorporates the constitutive relation S(Eex) of PMN-xPT. The systematic experimental-phenomenological modeling approach introduced here is generic and paves the way for an understanding of the underlying physical mechanisms in any SC/PE hybrid.

Control of Superconductivity with a Single Ferromagnetic Layer in Niobium/Erbium Bilayers

Superconducting spintronics in hybrid superconductor/ferromagnet (S-F) heterostructures provides an exciting potential new class of device. The prototypical super-spintronic device is the superconducting spin-valve, where the critical temperature, $T_c$, of the S-layer can be controlled by the relative orientation of two (or more) F-layers. Here, we show that such control is also possible in a simple S/F bilayer. Using field history to set the remanent magnetic state of a thin Er layer, we demonstrate for a Nb/Er bilayer a high level of control of both $T_c$ and the shape of the resistive transition, R(T), to zero resistance. We are able to model the origin of the remanent magnetization, treating it as an increase in the effective exchange field of the ferromagnet and link this, using conventional S-F theory, to the suppression of $T_c$. We observe stepped features in the R(T) which we argue is due to a fundamental interaction of superconductivity with inhomogeneous ferromagnetism, a phenomena currently lacking theoretical description.

Control of Superconductivity in a Hybrid Superconducting/Ferromagnetic Multilayer Using Nonequilibrium Tunneling Injection

Devices with multiple insulating ($I$), ferromagnetic ($F$), and superconducting ($S$) layers in various configurations have been studied for some time, both in basic research and to exploit quantum effects for improved electronics, and lately have enjoyed renewed interest. This systematic study addresses Cooper-pair tunneling out of equilibrium in a hybrid $S\phantom{\rule{0}{0ex}}I\phantom{\rule{0}{0ex}}S\phantom{\rule{0}{0ex}}F\phantom{\rule{0}{0ex}}I\phantom{\rule{0}{0ex}}F\phantom{\rule{0}{0ex}}S$ sandwich that acts as a transistor. The results of the analytical model agree well with experiment, providing an important tool for designing tomorrow's ultralow-power computing hardware.

Networks of superconducting nano-puddles in 1/8 doped YBa2Cu3O6.5 + y controlled by thermal manipulation

While it is known that the nature and the arrangement of defects in complex oxides have an impact on the material functionalities, little is known about control of superconductivity by oxygen interstitial organization in cuprates. Here we report direct compelling evidence for the control of Tc by manipulation of the superconducting granular networks of nanoscale puddles, made of ordered oxygen stripes, in a single crystal of YBa2Cu3O6.5 + y with average formal hole doping p close to 1/8. Upon thermal treatments we were able to switch from a first network of oxygen defect striped puddles with OVIII modulation (qOVIII(a*) = (h + 3/8, k, 0) and qOVIII(a*) = (h + 5/8, k, 0)) to a second network characterized by OXVI modulation (qOXVI(a*) = (h + 7/16, k, 0) and qox-VI(a*) = (h + 9/16, k, 0)) and finally to a third network with puddles of OV periodicity (qOV(a*) = (4/10, 1, 0) and qOV(a*) = (6/10, 1, 0)). We map the microscopic spatial evolution of the out of plane OVIII, OXVI and OV puddle nano-size distribution via scanning micro-diffraction measurements. In particular, we calculated the number of oxygen chains (n) and the charge density (hole concentration p) inside each puddle, analyzing areas of 160 × 80 μm2, and recording 12 800 diffraction patterns to reconstruct each spatial map. The high spatial inhomogeneity shown by all the reconstructed spatial maps reflects the intrinsic granular structure that characterizes cuprates and iron chalcogenides, disclosing the presence of several complex networks of coexisting superconducting domains with different lattice modulations, charge densities and gaps as in the proposed multi-gap scenario called superstripes.

Optical manipulation of coherent phonons in superconducting YBa2Cu3O7−δ thin films

The coherent phonons of YBa2Cu3O7-delta are believed to be strongly coupled to its superconductivity. Controlling the phonons below its transition temperature, therefore, may serve as a promising scheme of the control of superconductivity. Here we demonstrate optical manipulation of the Ba-O and Cu-O vibrations in a thin-film YBa2Cu3O7-delta below its transition temperature using a pair of femtosecond laser pulses. The interpulse delay is tuned to integral and half-integral multiples of the oscillation period of a specific phonon mode (Ba-O or Cu-O vibration) to enhance and suppress its amplitude, respectively.

Anisotropic Superconductivity and Vortex Dynamics in Magnetically Coupled F/S and F/S/F Hybrids

Magnetically coupled superconductor–ferromagnet hybrids offer advanced routes for nanoscale control of superconductivity. Magnetotransport characteristics and scanning tunneling microscopy images of vortex structures in superconductor–ferromagnet hybrids reveal rich superconducting phase diagrams. Focusing on a particular combination of a ferromagnet with a well-ordered periodic magnetic domain structure with alternating out-of-plane component of magnetization, and a small coherence length superconductor, we find directed nucleation of superconductivity above the domain wall boundaries. We show that near the superconductor-normal state phase boundary the superconductivity is localized in narrow mesoscopic channels.

Synthesis and physical properties of FeSe1/2Te1/2 superconductor

One of the most important properties of very recently reported FeSe based superconductors is the robustness of their superconductivity under applied magnetic field. The synthesis and control of superconductivity in FeSe based compounds is rather a difficult task. Synthesis and physical property characterization for optimized superconductivity of FeSe1/2Te1/2 at 13 K is reported here. The compound crystallized in a tetragonal structure with lattice parameters a=3.8015(2) and c=6.0280(4) Å. Magnetization measurements indicated bulk superconductivity with lower critical field (Hc1) of around 180 Oe. By applying Ginzburg–Landau theory, the Hc2(0) value is estimated to be ∼1840 kOe for the 90% of resistive transition. A heat capacity measurement revealed bulk superconductivity by a hump at Tc near 13 K and an expected decrease in the same was observed under an applied magnetic field.

Uniaxial Pressure Control of Superconductivity via Stripe Instability in La1.64Eu0.2Sr0.16CuO4 Crystals

Uni-axial pressure dependence of the superconducting critical temperature (Tc) with a static stripe instability was examined in La1.64Eu0.2Sr0.16CuO4 single crystals. We find anisotropy in the dependence of Tc on uni-axial pressure applied along the tetragonal [001], [100], and [110] directions. The behavior of dTc/dP[001] and dTc/dP[100] is essentially the same as that in the dynamical stripe phase of Eu-free La1.84Sr0.16CuO4. In contrast, novel behavior observed in dTc/dP[110] in the present material is attributed to the weakening of static stripe order. Our result points toward a promising new way to control the superconducting state with uni-axial pressures via a coupling of the stripe instability to the underlying lattice symmetry.