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Unconventional Superconductivity  [PDF]
M. R. Norman
Physics , 2013,
Abstract: A brief review of unconventional superconductivity is given, stretching from the halcyon days of helium-3 to the modern world of Majorana fermions. Along the way, we will encounter such strange beasts as heavy fermion superconductors, cuprates, and their iron-based cousins. Emphasis will be put on the fact that in almost all cases, an accepted microscopic theory has yet to emerge. This is attributed to the difficulty of constructing a theory of superconductivity outside the Migdal-Eliashberg framework.
Unconventional Superconductivity in Double Quantum Dots  [PDF]
Bj?rn Sothmann,Stephan Weiss,Michele Governale,Jürgen K?nig
Physics , 2014, DOI: 10.1103/PhysRevB.90.220501
Abstract: The formation of electron pairs is a prerequisite of superconductivity. The fermionic nature of electrons yields four classes of superconducting correlations with definite symmetry in spin, space and time. Here, we suggest double quantum dots coupled to conventional s-wave superconductors in the presence of inhomogeneous magnetic fields as a model system exhibiting unconventional pairing. Due to their small number of degrees of freedom, tunable by gate voltages, quantum-dot systems provide an ideal tool to gain fundamental insight in unconventional pairing. We propose two detection schemes for unconventional superconductivity, based on either Josephson or Andreev spectroscopy.
Unconventional Superconductivity in MgCNi3  [PDF]
Klaus Voelker,Manfred Sigrist
Physics , 2002,
Abstract: A multiband superconductor with a conventional phonon mechanism can develop an unconventional state with a nontrivial order parameter phase relation between the individual bands. We propose that such a state can explain recent experimental results on MgCNi3, which are suggestive of both s-wave pairing symmetry and of unconventional superconductivity. We show that such a state gives rise to Andreev bound states, and to spontaneous currents, at surfaces and around impurities. We also investigate possible phase transitions between states with different order parameter symmetry.
Superconductivity in LaFeAs$_{1-x}$P$_{x}$O: effect of chemical pressures and bond covalency  [PDF]
Cao Wang,Shuai Jiang,Qian Tao,Zhi Ren,Yuke Li,Linjun Li,Chunmu Feng,Jianhui Dai,Guanghan Cao,Zhu'an Xu
Physics , 2008, DOI: 10.1209/0295-5075/86/47002
Abstract: We report the realization of superconductivity by an isovalent doping with phosphorus in LaFeAsO. X-ray diffraction shows that, with the partial substitution of P for As, the Fe$_2$As$_2$ layers are squeezed while the La$_2$O$_2$ layers are stretched along the c-axis. Electrical resistance and magnetization measurements show emergence of bulk superconductivity at $\sim$10 K for the optimally-doped LaFeAs$_{1-x}$P$_{x}$O ($x=0.25\sim0.3$). The upper critical fields at zero temperature is estimated to be 27 T, much higher than that of the LaFePO superconductor. The occurrence of superconductivity is discussed in terms of chemical pressures and bond covalency.
Interface induced high temperature superconductivity in single unit-cell FeSe films on SrTiO3  [PDF]
Qing-Yan Wang,Zhi Li,Wen-Hao Zhang,Zuo-Cheng Zhang,Jin-Song Zhang,Wei Li,Hao Ding,Yun-Bo Ou,Peng Deng,Kai Chang,Jing Wen,Can-Li Song,Ke He,Jin-Feng Jia,Shuai-Hua Ji,Yayu Wang,Lili Wang,Xi Chen,Xucun Ma,Qi-Kun Xue
Physics , 2012, DOI: 10.1088/0256-307X/29/3/037402
Abstract: Searching for superconducting materials with high transition temperature (TC) is one of the most exciting and challenging fields in physics and materials science. Although superconductivity has been discovered for more than 100 years, the copper oxides are so far the only materials with TC above 77 K, the liquid nitrogen boiling point. Here we report an interface engineering method for dramatically raising the TC of superconducting films. We find that one unit-cell (UC) thick films of FeSe grown on SrTiO3 (STO) substrates by molecular beam epitaxy (MBE) show signatures of superconducting transition above 50 K by transport measurement. A superconducting gap as large as 20 meV of the 1 UC films observed by scanning tunneling microcopy (STM) suggests that the superconductivity could occur above 77 K. The occurrence of superconductivity is further supported by the presence of superconducting vortices under magnetic field. Our work not only demonstrates a powerful way for finding new superconductors and for raising TC, but also provides a well-defined platform for systematic study of the mechanism of unconventional superconductivity by using different superconducting materials and substrates.
The Challenge of Unconventional Superconductivity  [PDF]
M. R. Norman
Physics , 2011, DOI: 10.1126/science.1200181
Abstract: During the past few decades, several new classes of superconductors have been discovered. Most of these do not appear to be related to traditional superconductors. As a consequence, it is felt by many that for these materials, superconductivity arises from a different source than the electron-ion interactions that are at the heart of conventional superconductivity. Developing a rigorous theory for any of these classes of materials has proven to be a difficult challenge, and will continue to be one of the major problems in physics in the decades to come.
The chemical physics of unconventional superconductivity  [PDF]
S. Mazumdar,R. T. Clay
Physics , 2014, DOI: 10.1002/qua.24637
Abstract: Attempts to explain correlated-electron superconductivity have largely focused on the proximity of the superconducting state to antiferromagnetism. Yet, there exist many correlated-electron systems that exhibit insulator-superconducting transitions where the insulating state exhibits spatial broken symmetry different from antiferromagnetism. Here we focus on a subset of such compounds which are seemingly very different in which specific chemical stoichiometries play a distinct role, and small deviations from stoichiometry can destroy superconductivity. These superconducting materials share a unique carrier concentration, at which we show there is a stronger than usual tendency to form local spin-singlets. We posit that superconductivity is a consequence of these pseudomolecules becoming mobile as was suggested by Schafroth a few years prior to the advent of the BCS theory.
Isotropic Quantum Scattering and Unconventional Superconductivity  [PDF]
Tuson Park,V. A. Sidorov,F. Ronning,J. -X. Zhu,Y. Tokiwa,H. Lee,E. D. Bauer,R. Movshovich,J. L. Sarrao,J. D. Thompson
Physics , 2009,
Abstract: Superconductivity without phonons has been proposed for strongly correlated electron materials that are tuned close to a zero-temperature magnetic instability of itinerant charge carriers. Near this boundary, quantum fluctuations of magnetic degrees of freedom assume the role of phonons in conventional superconductors, creating an attractive interaction that glues electrons into superconducting pairs. Here we show that superconductivity can arise from a very different spectrum of fluctuations associated with a local or Kondo-breakdown quantum-critical point that is revealed in isotropic scattering of charge carriers and a sub-linear temperature-dependent electrical resistivity. At this critical point, accessed by applying pressure to the strongly correlated, local-moment antiferromagnet CeRhIn5, magnetic and charge fluctuations coexist and produce electronic scattering that is maximal at the optimal pressure for superconductivity. This previously unanticipated source of pairing glue opens possibilities for understanding and discovering new unconventional forms of superconductivity.
Unconventional Superconductivity in Heavy Fermion Systems  [PDF]
Y. Kitaoka,S. Kawasaki,T. Mito,Y. Kawasaki
Physics , 2004, DOI: 10.1143/JPSJ.74.186
Abstract: We review the studies on the emergent phases of superconductvity and magnetism in the $f$-electron derived heavy-fermion (HF) systems by means of the nuclear-quadrupole-resonance (NQR) under pressure. These studies have unraveled a rich variety of the phenomena in the ground state of HF systems. In this article, we highlight the novel phase diagrams of magnetism and unconventional superconductivity (SC) in CeCu$_2$Si$_2$, HF antiferromagnets CeRhIn$_5$, and CeIn$_3$. A new light is shed on the difference and common features on the interplay between magnetism and SC on the magnetic criticality.
Unconventional superconductivity on a topological insulator  [PDF]
Jacob Linder,Yukio Tanaka,Takehito Yokoyama,Asle Sudb?,Naoto Nagaosa
Physics , 2009, DOI: 10.1103/PhysRevLett.104.067001
Abstract: We study proximity-induced superconductivity on the surface of a topological insulator (TI), focusing on unconventional pairing. We find that the excitation spectrum becomes gapless for any spin-triplet pairing, such that both subgap bound states and Andreev reflection is strongly suppressed. For spin-singlet pairing, the zero-energy surface state in the $d_{xy}$-wave case becomes a Majorana fermion, in contrast to the situation realized in the topologically trivial high-$T_c$ cuprates. We also study the influence of a Zeeman field on the surface states. Both the magnitude and direction of this field is shown to strongly influence the transport properties, in contrast to the case without TI. We predict an experimental signature of the Majorana states via conductance spectroscopy.
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