Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session G11: Focus Session: Superconductivity in the 2D Limit I |
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Sponsoring Units: DMP Chair: Ho Nyung Lee, Oak Ridge National Laboratory Room: 007B |
Tuesday, March 3, 2015 11:15AM - 11:27AM |
G11.00001: Superconductivity in Electric Double Layer Capacitor under Pressure Duncan McCann, Martin Misek, Konstantin Kamenev, Andrew Huxley Chemical doping generally provides the most common method for tuning into the superconducting state of a material yet can be difficult to control and also potentially introduces structural disorder complicating the underlying physics. Electric Double Layer devices however provide a means to electrostatically dope materials with high electric fields allowing continuous tuning of a 2D superconducting state thus avoiding such issues. One such device is the Electric Double Layer Capacitor which can detect the onset of superconductivity through AC magnetisation measurements. We make use of a similar device in an attempt to electrostatically dope and tune the superconductivity in the cuprate compound La$_{1.93}$Sr$_{0.07}$CuO$_{4}$ as well as investigating whether application of pressure improves its efficiency. [Preview Abstract] |
Tuesday, March 3, 2015 11:27AM - 11:39AM |
G11.00002: Doping evolution and polar surface reconstruction of the infinite-layer cuprate Sr$_{1-x}$La$_{x}$CuO$_{2}$ John Harter, Luigi Maritato, Darrell Schlom, Kyle Shen We use angle-resolved photoemission spectroscopy to study the doping evolution of infinite-layer Sr$_{1-x}$La$_{x}$CuO$_{2}$ thin films grown by molecular-beam epitaxy. At low doping, the material exhibits a dispersive lower Hubbard band typical of the superconducting cuprate parent compounds. Electron diffraction probes reveal a $p(2\times2)$ reconstruction of the surface. Using a number of simple assumptions, we develop a model of this reconstruction based on the polar nature of the infinite-layer material. As carriers are added to the system, a continuous evolution from Mott insulator to superconducting metal is observed as a coherent low-energy band develops with a concomitant remnant lower Hubbard band, gradually filling in the Mott gap. This two-component spectral function emphasizes the important role that strong local electron correlations play in the electronic structure of Sr$_{1-x}$La$_{x}$CuO$_{2}$ even at relatively high doping levels. Finally, we confirm the theoretical prediction of a thickness-controlled transition in ultrathin films of SrCuO$_{2}$ grown on nonpolar SrTiO$_{3}$, highlighting the diverse structural changes that can occur in polar complex oxide thin films. [Preview Abstract] |
Tuesday, March 3, 2015 11:39AM - 11:51AM |
G11.00003: Oxygen redistribution and induced high-Tc superconductivity at the CaCuO2/SrTiO3 interface. Claudia Cantoni, Daniele Di Castro, Carmela Aruta, Giuseppe Balestrino High-Tc cuprate superconductors (HTS) can be thought of as a sequence of natural interfaces between two blocks with different structure and functionality: an insulating block with infinite layer (IL) structure, containing the CuO2 planes, and a charge reservoir (CR) block, that, opportunely doped by chemical substitution or excess oxygen, provides charge carriers to the IL block, giving rise to superconductivity. The increased understanding of electronic phenomena in artificial interfaces between complex oxides, such as the LAO/STO interface, naturally suggests the opportunity to exploit similar interfaces as a charge reservoir to dope a cuprate IL. We have explored the system in which LAO is replaced by the insulating IL CaCuO2 (CCO/STO interface) and found a Tc of 50 K. This interface closely reproduces the IL/CR native interface of HTS and can be used to extract important information on the physical processes occurring in HTS. We present atomically resolved ABF-STEM and EELS measurements, which combined with XAS uncover the existence of interfacial apical O atoms. We will discuss their electronic signature for superconductivity. [Preview Abstract] |
Tuesday, March 3, 2015 11:51AM - 12:27PM |
G11.00004: Engineering LaAlO$_{3}$/SrTiO$_{3}$ quantum wells by selective orbital occupancy Invited Speaker: Gervasi Herranz The two-dimensional confinement of electrons in quantum wells (QWs) along the interface between two semiconductors such as Si or GaAs has been fundamental for technology as well for the development of new fundamental concepts. More recently, the discovery of 2D-QWs in oxides, with the flagship interface between LaAlO$_{3}$ and SrTiO$_{3}$, has been heralded as a milestone in the research of 2D electron systems. Unlike more conventional 2D-QWs based on III-V or II-VI semiconductors, carriers at LaAlO$_{3}$/SrTiO$_{3}$ populate narrow 3d-bands, where strong correlations underlie complex phases not present in conventional semiconductors. In LaAlO$_{3}$/SrTiO$_{3}$ not all the electrons reside in the same QW subband and, indeed, they behave differently depending on which orbital are they occupying. Here we demonstrate that the symmetry of the conduction band inside the QWs can be selected, disclosing unprecedented ways to tailor the electron properties. More specifically, we show that the spatial extension and anisotropy of the 2D-superconductivity and the Rashba spin-orbit field can be largely modulated by controlling the 2D-QWsubband filling. Our results indicate a route to manipulate selectively the electronic properties of QWs at the LaAlO$_{3}$/SrTiO$_{3}$ interface, opening new prospects to understand the link between orbital symmetry and 2D-superconductivity and to achieve enhanced properties suitable for spin-dependent transport. [Preview Abstract] |
Tuesday, March 3, 2015 12:27PM - 12:39PM |
G11.00005: Top gating control of superconductivity at the LaAlO3/SrTiO3 interfaces Alexis Jouan, Simon Hurand, Cheryl Feuillet-Palma, Gyanendra Singh, Jerome Lesueur, Nicolas Bergeal, Edouard Lesne, Nicolas Reyren Transition metal oxides display a great variety of quantum electronic behaviors. Epitaxial interfaces involving such materials give a unique opportunity to engineer artificial materials where new electronic orders take place. It has been shown that a superconducting two-dimensional electron gas could form at the interface of two insulators such as LaAlO3 and SrTiO3 [1], or LaTiO3 and SrTiO3 [2]. An important feature of these interfaces lies in the possibility to control their electronic properties, including superconductivity and spin-orbit coupling (SOC) with field effect [3-5]. However, experiments have been performed almost exclusively with a metallic gate on the back of the sample. In this presentation, we will report on the realization of a top-gated LaAlO3/SrTiO3 device whose physical properties, including superconductivity and SOC, can be tuned over a wide range of electrostatic doping. In particular, we will present a phase diagram of the interface and compare the effect of the top-gate and back-gate. Finally, we will discuss the field-effect modulation of the Rashba spin-splitting energy extracted from the analysis of magneto-transport measurements. Our result paves the way for the realization of mesoscopic devices where both superconductivity and SOC can be tuned locally. [Preview Abstract] |
Tuesday, March 3, 2015 12:39PM - 12:51PM |
G11.00006: Electric field modulation of superconductivity and kondo effect in LaAl$_{1-x}$Cr$_{x}$O$_{3}$/SrTiO$_{3}$ interfaces Gyanendra Singh, Alexis Jouan, Simon Hurand, Cheryl Palma, Pramod Kumar, Anjana Dogra, Ramesh Budhani, Jerome Lesueur, Nicolas Bergeal Two dimensional electron gas exhibit superconductivity and spin orbit coupling (SOC) at the interfaces of two insulators LaXO$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$ (where X $=$ Al, Ti and Ga) [1-3], whereas no conductivity is seen when X site is replaced by Cr and Mn [4]. We present low temperature measurements of LaAl$_{\mathrm{1-x}}$Cr$_{\mathrm{x}}$O$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$ interfaces for Cr doping of x $=$0, 0.1, 0.2. We show a sharp superconducting transition (T$_{\mathrm{c}})$ at 175 mK for undoped sample (x $=$ 0) which becomes broader for the Cr doping of x $=$ 0.1 with Tc of 100 mK. Further Cr doping to x $=$ 0.2 indicate no sign of superconductivity. We have analyzed the results with hall carrier density and SOC as a function of Cr doping. The temperature dependent sheet resistance below 50 K display an upturn for x $=$ 0 which becomes more pronounced with Cr doping. We discuss the result on the basis of kondo scattering which can be modulated by varying the carrier density. \\[4pt] [1] N. Reyren et al., Science 317, 1196 (2007).\\[0pt] [2] J. Biscaras et al., Nature Communications 1,89 (2010).\\[0pt] [3] J. Biscaras et al., Phys. Rev. Lett. 108, 247004 (2012).\\[0pt] [4] R. Colby et al.,~Phys. Rev. B\textbf{88}, 155325 (2013).\\[0pt] [Preview Abstract] |
Tuesday, March 3, 2015 12:51PM - 1:03PM |
G11.00007: Enhanced transition temperature due to tetragonal domains in two-dimensional superconducting strontium titanate Hilary Noad, Katja Nowack, Eric Spanton, Hisashi Inoue, Minu Kim, Chris Bell, Yasuyuki Hikita, Harold Y. Hwang, Kathryn Moler Strontium titanate (SrTiO$_3$) is a key component in superconducting heterostructures such as LaAlO$_3$/SrTiO$_3$ and monolayer FeSe on SrTiO$_3$, yet superconductivity in bare SrTiO$_3$ is not fully understood. We used a scanning superconducting quantum interference device susceptometer to image the diamagnetic response as a function of temperature in samples of SrTiO$_3$ containing either a 5.5 nm or a 36.9 nm-thick slab of niobium-doped SrTiO$_3$. We find that stripe-like regions remain superconducting at higher temperatures than the rest of the sample. The shape and orientation of the features, as well as their behavior in a subsequent cooldown, are consistent with an origin in the low-temperature tetragonal domain structure of SrTiO$_3$. These results suggest a variety of mechanisms by which the transition temperature could be enhanced and may help constrain theories of superconductivity in SrTiO$_3$. [Preview Abstract] |
Tuesday, March 3, 2015 1:03PM - 1:15PM |
G11.00008: Enhancing Triplet Superconductivity by the Proximity to a Singlet Superconductor in Oxide Heterostructures Mats Horsdal, Timo Hyart, Giniyat Khaliullin, Bernd Rosenow We show, how in principle, a coherent coupling between two superconductors of opposite parity can be realised in a three-layer oxide heterostructure. Due to strong intraionic spin-orbit coupling in the middle layer singlet Cooper pairs are converted into triplet ones, and vice versa. The result is a large enhancement of the triplet order parameter that persist well beyond the native triplet critical temperature. [Preview Abstract] |
Tuesday, March 3, 2015 1:15PM - 1:27PM |
G11.00009: Hidden Order as a Source of Interface Superconductivity Andreas Moor, Anatoly Volkov, Konstantin Efetov We propose a new mechanism of the interfacial superconductivity observed in many heterostructures composed of different materials including high-temperature superconductors. Our proposal is based on the use of the Ginzburg--Landau equations applicable to a wide class of systems. The system under consideration is assumed to have, alongside the superconducting order parameter, also another competing order that might be a charge- or spin-density wave. At certain temperatures or doping level the superconducting state is not realized (thus, ``hidden''), while the amplitude of another order parameter corresponds to a minimum of the free energy. We also assume that at an interface or at a defect, the non-superconducting order parameter is suppressed (strongly or weakly), e.g., due to an enhanced impurity scattering. The local superconductivity is shown to emerge at the interface, and the spatial dependence of the corresponding order parameter is described by the Gross--Pitaevskii equation. The quantized values of the temperature and doping levels, at which $\Delta(x)$ arises, are determined by the ``energy'' levels of the linearized Gross--Pitaevskii equation, i.e., of the Schrodinger equation. Interestingly, the local superconductivity arises even at a small suppression of the rival order. [Preview Abstract] |
Tuesday, March 3, 2015 1:27PM - 1:39PM |
G11.00010: A The effects of non-linear electron-phonon interactions on superconductivity and charge-density-wave correlations Shaozhi Li, Steve Johnston Linear treatments of the electron-phonon (e-ph) interaction, derived from Taylor expansions of the lattice potential, often predict large lattice distortions in the strong coupling limit; however, the prediction of large lattice displacements violates the assumptions underlying the linear model, indicating that the higher-order non-linear terms should also be included. In this talk, we examine non-linear e-ph interactions in a two-dimensional Holstein-like model using non-perturbative determinant quantum Monte Carlo. We show that even small non-linear interactions dramatically suppress charge-density-wave formation and s-wave superconductivity that are predicted by the linear e-ph model. These effects are attributed to a combined hardening of the phonon frequency and a renormalization of the effective linear coupling to weaker values. [Preview Abstract] |
Tuesday, March 3, 2015 1:39PM - 1:51PM |
G11.00011: Large upper critical field in ion-gated MoS2 superconductivity Yu Saito, Yoshimitsu Kohama, Yuichi Kasahara, Jianting Ye, Masashi Tokunaga, Yoshihiro Iwasa Molybdenum disulfide (MoS$_2$) is an archetypal two-dimensional (2D) materials beyond graphene, and are attracting significant attention due to its multiple functionalities, including field effect transistor, photoluminescence, and valleytronics. An additional function of MoS$_2$ is electric-field-induced superconductivity, realized by ionic gating. In this presentation, we report anisotropic superconducting upper critical fields {\it H}$_{c2}$ in electric-field-induced superconductivity on MoS$_2$, measured under high magnetic fields up to 55 T. We found that the in-plane {\it H}$_{c2}$ increased with squarer root near {\it T}$_c$, reflecting the purely two-dimensionality of electric-field-induced superconductivity, and possibly reached nearly 60 T at $T \to 0 K$. This value is around 5 times larger than the normal Pauli limit simply expected from the {\it T}$_c$ value. Such an enhancement is possibly caused by the strong out-of-plane Zeeman filed, which is the consequence of the inplane-broken inversion symmetry of MoS$_2$ monolayer coupled with the spin orbit interactions. [Preview Abstract] |
Tuesday, March 3, 2015 1:51PM - 2:03PM |
G11.00012: Superconducting phases of monolayer transition-metal dichalcogenides Evan Sosenko, Vivek Aji Layered group-VI dichalcogenides (e.g., MoS$_2$) are two dimensional materials that engender novel coupled spin and valley physics. Characterized by strong spin-orbit coupling and inversion symmetry breaking, they give rise to novel phenomena such as the spin Hall effect and valley Hall effect. In this talk, we focus on the intrinsic and substrate induced superconducting phases expected in this new class of materials. Generically, two types are expected: (1) Cooper pairing with finite center-of-mass momentum, and (2) zero momentum pairs analogous to the conventional BCS phase. We establish the conditions for the realization of each type. Time permitting, we will discuss the nature of the quasiparticles resulting from valley-discriminating pair-breaking processes. [Preview Abstract] |
Tuesday, March 3, 2015 2:03PM - 2:15PM |
G11.00013: Unconventional superconductivity in p-doped MoS$_2$ monolayer Yi-Ting Hsu, Abolhassan Vaezi, Eun-Ah Kim Recent observation of superconductivity in n-doped MoS$_2$ monolayer has generated much interest. Though precise superconducting nature of the system is still under theoretical debate, the observation motivates the study of superconductivity in this atomic layer system. In particular, the valley-spin locking of the p-doped MoS$_2$ monolayer suggests possibility of exotic type of superconductivity in the system. We use two-step perturbative renormalization group treatment to study superconducting instabilities in various channels and discuss the possibility of an exotic modulated superconductor. [Preview Abstract] |
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