Browsing by Subject "Superconductivity"

Now showing 1 - 9 of 9

Direct measurement of dissipative forces in superconducting BSCCO
(2001-08) Judge, Elizabeth Eileen; Markert, John T.
Entanglement in superconducting heterostructures, and quantum circuit simulation hardware
(2021-04-27) Ostrove, Corey I; Reichl, L. E.; La Cour, Brian; Aaronson, Scott; Potter, Andrew; Lai, Keji
We begin this dissertation by studying noise correlations in superconducting heterostructures of various geometries. In recent years there has been a resurgence of interest in the nonlocal transport properties of superconducting heterostructures due to the possibility of their serving as a source of electronic entanglement in solid state quantum information processors. Devices designed for this purpose are called Cooper pair splitting devices. The utility of these devices as entanglement sources is known to have connections to the positivity of noise cross correlations in spatially separated leads. In Chapter 1 we outline the theoretical prerequisites for this work, outlining the scattering theory framework based on the Bogoliubov-de Gennes equations we adopt. Within this framework we apply a methodology first introduced by Demers and by Blonder, Tinkham and Klapwijk (BTK) in the early 1980s to find the scattering matrix for our superconducting structures. The current, local and nonlocal shot noise can all be expressed in terms of the underlying scattering processes. This framework allows us to investigate the behavior of the current and noise correlations in the structure as we change the geometry and other key system parameters such as the system size, superconducting phase difference and temperature. We also introduce the Andreev approximation, a commonly used approximation which simplifies the scattering theory for superconducting heterostructures. In Chapter 2, we study the local and nonlocal shot noise in a quasi-1D normal-superconducting-normal (NSN) geometry using material parameters relevant to high-T [subscript c] superconductivity. The scattering and shot noise distributions are studied in the short, intermediate and long system size limits, allowing us to examine the qualitative differences in these three parameter regimes. This allows us to, for example, identify the signatures of over-the-gap geometric resonances in the shot noise distributions that appear in the long system size limit. We also break the nonlocal shot noise distributions down further and study the individual contributions to the nonlocal shot due to particle-particle, hole-hole and particle-hole scattering processes. In Chapter 3, we extend our investigation of superconducting heterostructures to the more complicated NSNSN geometry. A novel feature introduced in the geometry is the presence of subgap quasibound states, which show up as resonances in the scattering matrix. We show that these quasibound states dramatically impact the nonlocal shot noise distributions in the system. At energies near the quasibound states the dominant transmission channel through the system is a process called particle-hole transmission, which results in sharp positive peaks in the nonlocal shot noise distribution of the system. The behavior of the nonlocal noise correlations as we change the size of the superconducting and normal regions is investigated and it is found that there is a "sweet spot'' with respect to the size of the superconducting regions that maximizes the positivity of the nonlocal noise distributions as well as a periodic-like behavior in the positivity of the noise distributions with respect to the normal region size. The results of the full scattering theory for the NSNSN geometry are compared to the results obtained using the Andreev approximation, where we find that the Andreev approximation breaks down at energies close to the quasibound state energies. In the second half of this dissertation we focus on work related to the development of a prototype special-purpose quantum circuit simulation device based on commercial off-the-shelf high-speed analog signal processing hardware. In Chapter 4 we introduce the embedding scheme used to represent quantum states and quantum gates in the frequency domain of a classical analog voltage signal. Experimental results are presented from an early two-qubit prototype device for the fidelity of the state generation and gate application circuits. In Chapter 5, a more in-depth investigation into the modeling of classical errors within our signal processing based simulation method is performed in terms of the effects this noise has on the results of the quantum computation being simulatied. It is shown, for example, that additive white gaussian noise (AWGN) in our system has the same effect as applying a depolarizing channel to the qubits in the simulation. We then perform a simulation of a simple quantum error correction (QEC) protocol using the device and show that, even in the presence of classical noise in the simulation hardware, an overall enhancement in the performance of gate operations as a result of applying QEC is observed.
Exotic excitonic phases and means to find them
(2024-05) Blinov, Igor V.; MacDonald, Allan H.; erez.berg@weizmann.ac.il; mikezaletel@berkeley.edu; Pablo Laguna
Often one phase accompanies another for example, crystalline arrangements of atoms may accompany ferromagnetic ordering of spins of electrons. Rarer, orders of different kind develop on top of the same system for example, ferromagnetism may coexist with exotic superconductivity. In this dissertation, we look upon 2 examples of such phases: one is the partial condensation of the intervalley excitons, when the phase can be viewed as a combination of metallic (coherent) and gapped (excitonic) orders. We argue that this state of matter may explain observed features of so-called PIP-phase adjacent to superconductivity, recently observed in a trilayer graphene [50, 51]. The main result is that for a regime with an annular Fermi surface a band model we used has an instability towards formation of an intervalley order at a finite pairing momentum. Mean-field critical temperature is calculated and the correction to the conductance. Critical temperature of the mixed phase is higher than that of the pure excitonic phase. An interesting result is that in the clean limit the correction behaves as a square of the scattering time, thus distinguishing it from the normal metallic phase. The sign of the correction is negative. It is possible to tune the parameters of the model such that the change of the conductance is within 50 %-100% observed in the experiment. In the second part, we looked at the another example of a mixed phase: a related phase with coexistence of superconductivity and interlayer coherence. It occurs in a system of 2 interacting 1s superconductors electrically isolated from each other. On the mean-field level, interlayer coherence and intralayer superconductivity is accompanied by the interlayer superconductivity. The presence of the interlayer coherence increases or decreases the superconducting gap dependent on the regime. We further showed that the phase is accompanied by a new physical effect: indirect Andreev reflection.
Low temperature scanning tunneling microscope study of low-dimensional superconductivity on metallic nanostructures
(2010-08) Kim, Jungdae; Shih, Chih-Kang; de Lozanne, Alex; Markert, John; Yao, Zhen; Shi, Li
Superconductivity is a remarkable quantum phenomenon in which a macroscopic number of electrons form a condensate of Cooper pairs that can be described by a single quantum wave function. According to the celebrated Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity, there is a minimum length scale (the coherence length) below which the condensate has a rigid quantum phase. The fate of superconductivity in a system with spatial dimensions smaller than [the coherence length] has been the subject of intense interest for decades and recent studies of superconductivity in ultra-thin epitaxial metal films have revealed some surprising behaviors in light of BCS theory. Notably, it was found that superconductivity remains robust in thin lead films with thicknesses orders of magnitude smaller than the coherence length (i.e. in the extreme two dimensional limit). Such studies raise the critical question: what happens to superconductivity as all dimensions are reduced toward the zero dimensional limit? By controlling the lateral size of ultra thin 2D islands, we systematically address this fundamental question with a detailed scanning tunneling microscopy/spectroscopy study. We show that as the lateral dimension is reduced, the strength of the superconducting order parameter is also reduced, at first slowly for dimensions larger than the bulk coherence length, and then dramatically at a critical length scale of ~ 40nm. We find this length scale corresponds to the lateral decay length of the order parameter in an island containing regions of different heights and different superconducting strength. Overall, our results suggest that fluctuation corrections to the BCS theory are important in our samples and may need to be systematically addressed by theory.
Low temperature scanning tunneling microscope study of metallic thin films and nanostructures on the semiconductor substrates
(2008-12) Qin, Shengyong, 1980-; Shih, Chih-kang
Many properties of the thin films are different from the bulk value and in many cases, depend dramatically on the film thickness. In the metallic ultra-thin films epitaxially grown on the semiconductor substrate, the conduction electrons are confined by the vacuum and metal-semiconductor interface. When the film thickness is comparable to the electron Fermi wavelength, this confinement will produce discrete energy levels known as quantum well states (QWS), which dramatically modify the electronic structures of the thin film and this is called quantum size effect (QSE). QSE will have a profound effect on a lot of physical properties of the thin films. Among various systems exhibiting QSE, Pb/Si (111) is the most widely studied one and exhibits the richest phenomena in QSE. In this study, a home made low temperature Scanning Tunneling Microscopy/Spectroscopy (LT-STM/S) was used to study the superconductivities of the Pb thin films. Quantum oscillations of the superconductivity have been observed for the films down to 4 monolayer and the oscillation amplitude increases as the film gets thinner. To resolve the discrepancies between the superconductivities measured with ex-situ transport and in-situ STS. We also studied the influence of Au overlay on the Pb thin films with LT-STM/S, and found out the deposition of Au on Pb dramatically roughened the Pb films. Finally, we successfully grew large scale near perfect 2ML Pb films. There are two types of films which exhibit different Moiré patterns. LT-STS studies revealed there is big difference in the superconductivity Tc of these two films, both of which decreased dramatically from that of the 4ML film.
Nonequilibrium order parameter dynamics in spin and pseudospin ferromagnets
(2009-08) Garate, Ion; MacDonald, Allan H.
Research on spintronics has galvanized the design of new devices that exploit the electronic spin in order to augment the performance of current microelectronic technologies. The sucessful implementation of these devices is largely contingent on a quantitative understanding of nonequilibrium magnetism in conducting ferromagnets. This thesis is largely devoted to expanding the microscopic theory of magnetization relaxation and current-induced spin torques in transition metals ferromagnets as well as in (III,Mn)V dilute magnetic semiconductors. We start with two theoretical studies of the Gilbert damping in electric equilibrium, which treat disorder exactly and include atomic-scale spatial inhomogeneities of the exchange field. These studies enable us to critically review the accuracy of the conventional expressions used to evaluate the Gilbert damping in transition metals. We follow by generalizing the calculation of the Gilbert damping to current-carrying steady states. We find that the magnetization relaxation changes in presence of an electric current. We connect this change with the non-adiabatic spin transfer torque parameter, which is an elusive yet potentially important quantity of nonequilibrium magnetism. This connection culminates in a concise analytical expression that will lead to the first ab initio estimates of the non-adiabatic spin transfer torque in real materials. Subsequently we predict that in gyrotropic ferromagnets the magnetic anisotropy can be altered by a dc current. In these systems spin-orbit coupling, broken inversion symmetry and chirality conspire to yield current-induced spin torques even for uniform magnetic textures. We thus demonstrate that a transport current can switch the magnetization of strained (Ga,Mn)As. This thesis concludes with the transfer of some fundamental ideas from nonequilibrium magnetism into the realm of superconductors, which may be viewed as easy-plane ferromagnets in the particle-hole space. We emphasize on the analogies between nonequilibrium magnetism and superconductivity, which have thus far been studied as completely separate disciplines. Our approach foreshadows potentially new effects in superconductors.
Scanning tunneling microscope studies of 2D superconductor and 3D intrinsic topological insulator
(2015-05) Nam, Hyoungdo; Shih, Chih-Kang; de Lozanne, Alex; Markert, John T; Fiete, Gregory A; Shi, Li
Electrons show unusual and interesting behaviors both in low dimensions and on material surfaces, distinct from what they display in bulk materials. These intriguing properties have been studied in order to understand their origins. One area where this can be seen is in the case of superconductivity, where superconducting phase fluctuation in a thin superconductor is supposed to substantially suppress the superconductivity of the material as the film thickness decreases. To test this, we prepared epitaxially grown and globally flat lead (Pb) films; here, the thinnest film was 1.4 nm thick. Four different length scale measurements, ranging from the nm to the mm scale, gave consistent superconducting transition temperatures. Our results proved that the film of 1.4 nm still has strong superconducting phase stiffness; namely, the superfluid phase is rigid even in 1.4 nm thin superconductor film. Moreover, the parallel critical magnetic field is remarkably strong so that superconductivity is still observed in Zeeman fields, exceeding the Pauli limit. In addition, the surface of 3D topological insulator has a novel quantum state induced by strong spin-orbit interaction. A number of material studies were conducted to find a surface dominated conduction topological insulator that has a large energy gap and a single Dirac cone. Moreover, it is necessary for the material to be stable against aging unlike most 3D topological insulators, such as Bi₂Se₃. Here, Bi₂Te₂Se and BiSbTeSe₂ were studied in terms of their structures, electronic properties, and aging effects on them. Scanning tunneling microscopy analysis attested that Bi₂Te₂Se is an order alloy, which has a slight randomness of 15 %, whereas BiSbTeSe₂ is a random alloy. Scanning tunneling spectroscopy on BiSbTeSe₂ confirmed that the Dirac point tends to stay around the Fermi level under the strong band structure change, induced by random structure. The most surprising observation was that BiSbTeSe₂ showed remarkable stability despite the rich composition of selenium (Se). Even after aging for seven days, the Fermi level and the Dirac point remained at almost the same level in bulk band gap. Both observations are very important for applications to utilize the exotic topological surface state.
Superconductivity, correlation, and topology in atomic-thin quantum materials
(2022-08-08) Liu, Mengke; Shih, Chih-Kang; MacDonald, Allan H.; De Lozanne, Alejandro L.; Lai, Keji; Shi, Li
Atomic thin quantum materials that host exotic quantum phases such as unconventional superconductivity, correlated magnetic insulating, and quantum anomalous Hall insulating states have become a new research frontier due to their intriguing physical phenomena and potential applications that may revolutionize human life. Atomic-thin quantum materials reduce the dimensionality into two, which leads to many unique properties. For example, monolayer or bilayer thin films reduce interlayer coupling compared with their 3D bulk counterparts, thus disentangling the interlayer interaction from interlayer interaction; weakened Coulomb screening at the 2D limit enhances the electron correlations; increased phase fluctuations of the order parameter make it possible for studying quasi-long range order, and quantum confinement from the third dimension may introduce quantum size effect. Thanks to the advancement of material engineering techniques, synthesis or separation of atomic thin quantum materials have become possible and popular. Among these techniques, such as molecular beam epitaxy, physical/chemical vapor deposition, and mechanical exfoliationmolecular beam epitaxy is very powerful due to its precise control of layer thickness and cleanness across a macroscopic wafer scale. Combining molecular beam epitaxy with other in-situ characterization techniques such as scanning tunneling microscopy and double-coil mutual inductance system allows us to design, control, and characterize the atomic thin quantum materials in both microscopic and macroscopic length scales. Here in this dissertation, I first briefly introduce the background motivations of the atomic thin quantum materials in the first chapter. In the second chapter, I cover the research techniques that have been employed during my graduate studies such as molecular beam epitaxy, scanning tunneling microscopy, and double-coil mutual inductance system. The third chapter is devoted to superconductivity in the 2D limit, I use monolayer indium thin film as a platform to discuss how the geometric arrangement of a monolayer indium thin-film affects the superconductivity transition temperature and superfluid density. In the fourth chapter, I use monolayer 1T phase NbSe₂ as a material example to discuss the manifestation of strong electron correlation and how it leads to magnetic charge-transfer insulators in its charge density wave phase. I also discuss the interplay of local magnetic moments with metallic/superconducting states, which lead to Kondo resonances and Yu-Shiba-Rusinov-like bound states. The fifth chapter focuses on the concept of band topology and its accompanied surface states. I use intrinsic magnetic topological insulator MnBi₂Te₄ as a material platform to discuss the interplay of the Dirac mass gap with magnetism and its theoretical understanding. Finally, I make some concluding remarks, including so far confronted obstacles in these topics, and comment on the further steps to make for future advancement.
Visualization of Topological Boundary Modes Manifesting Topological Nodal-Point Superconductivity
(2021-12-19) Nayak, Abhay Kumar; Steinbok, Aviram; Roet, Yotam; Koo, Jahyun; Margalit, Gilad; Yan, Binghai; Oreg, Yuval; Avraham, Nurit; Beidenkopf, Haim
The extension of the topological classification of band insulators to topological semimetals gave way to the topology classes of Dirac, Weyl, and nodal line semimetals with their unique Fermi arc and drum head boundary modes. Similarly, there are several suggestions to employ the classification of topological superconductors for topological nodal superconductors with Majorana boundary modes. Here, we show that the surface 1H termination of the transition metal dichalcogenide compound 4Hb-TaS2, in which 1T-TaS2 and 1H-TaS2 layers are interleaved, has the phenomenology of a topological nodal point superconductor. We find in scanning tunneling spectroscopy a residual density of states within the superconducting gap. An exponentially decaying bound mode is imaged within the superconducting gap along the boundaries of the exposed 1H layer characteristic of a gapless Majorana edge mode. The anisotropic nature of the localization length of the edge mode aims towards topological nodal superconductivity. A zero-bias conductance peak is further imaged within fairly isotropic vortex cores. All our observations are accommodated by a theoretical model of a two-dimensional nodal Weyl-like superconducting state, which ensues from inter-orbital Cooper pairing. The observation of an intrinsic topological nodal superconductivity in a layered material will pave the way for further studies of Majorana edge modes and its applications in quantum information processing.