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 Physics , 2013, DOI: 10.1103/PhysRevB.88.174513 Abstract: In this work, we generalize the theory of localized surface plasmons to the case of high-Tc cuprate superconductors, spatially confined in the form of small spherical particles. At variance from ordinary metals, cuprate superconductors are characterized by a low-energy bulk excitation known as the Josephson plasma wave (JPW), arising from interlayer tunneling of the condensate along the c-axis. The effect of the JPW is revealed in a characteristic spectrum of surface excitations, which we call Josephson surface plasmons. Our results, which apply to any material with a strongly anisotropic electromagnetic response, are worked out in detail for the case of multilayered superconductors supporting both low-frequency (acoustic) and transverse-optical JPW. Spatial confinement of the Josephson plasma waves may represent a new degree of freedom to engineer their frequencies and to explore the link between interlayer tunnelling and high-Tc superconductivity.
 Physics , 2014, DOI: 10.1016/j.jmmm.2014.06.056 Abstract: Ultrafast pump-probe spectroscopy is a powerful tool to study the nonequilibrium dynamics in high-Tc cuprate superconductors. The photo-induced quasiparticle (QP) dynamics revealed by pump-probe spectroscopy are sensitive to the near-Fermi level electronic structures. Here we review several selected examples to illustrate the enduring challenges including pairing glue, phase separation, and phase transitions in cuprate superconductors. We also present the data obtained on thin films of YBa2Cu3O7-{\delta} in connection to these issues.
 Physics , 2002, DOI: 10.1103/RevModPhys.75.473 Abstract: This paper reviews the most recent ARPES results on the cuprate superconductors and their insulating parent and sister compounds, with the purpose of providing an updated summary of the extensive literature in this field. The low energy excitations are discussed with emphasis on some of the most relevant issues, such as the Fermi surface and remnant Fermi surface, the superconducting gap, the pseudogap and d-wave-like dispersion, evidence of electronic inhomogeneity and nano-scale phase separation, the emergence of coherent quasiparticles through the superconducting transition, and many-body effects in the one-particle spectral function due to the interaction of the charge with magnetic and/or lattice degrees of freedom. The first part of the paper introduces photoemission spectroscopy in the context of strongly interacting systems, along with an update on the state-of-the-art instrumentation. The second part provides a brief overview of the scientific issues relevant to the investigation of the low energy electronic structure by ARPES. The rest of the paper is devoted to the review of experimental results from the cuprates and the discussion is organized along conceptual lines: normal-state electronic structure, interlayer interaction, superconducting gap, coherent superconducting peak, pseudogap, electron self energy and collective modes. Within each topic, ARPES data from the various copper oxides are presented.
 Physics , 1995, DOI: 10.1088/0953-8984/7/18/024 Abstract: We present a variational estimate for the binding energy of a Frenkel exciton in the insulating cuprate superconductors. Starting from the three band Hubbard model we perform a canonical transformation to O($t^2$), where $t$ is the bare nearest neighbour copper-oxygen hopping integral. An effective Hamiltonian is then derived to describe the hopping of the exciton through the copper oxide plane. The critical parameter in the model is the nearest neighbour copper-oxygen coulomb repulsion, $V$. It is found that a critical value of $V$ is needed to observe bound Frenkel excitons, and that these excitons have the same symmetry as the parent copper orbital, $d_{x^2-y^2}$. We determine the critical value of $V$ using a variational approach, and attempt to fit the parameters of the model to known experimental results.
 Zhi-Xun Shen Physics , 2003, Abstract: This paper summarizes experimental results presented at the international conference honoring Prof. C.N. Yang's 80th birthday. I show seven examples that illustrate how one can use angle-resolved photoemission spectroscopy to gain insights into the many-body physics responsible for the rich phase diagram of cuprate superconductors. I hope to give the reader a snapshot of the evolution of this experimental technique from a tool to study chemical bonds and band structure to an essential many-body spectroscopy for one of the most important physics problems of our time.
 Physics , 2006, Abstract: This treatise reviews latest results obtained from angle-resolved photoemission spectroscopy (ARPES) on cuprate superconductors, with a special focus on the electron-phonon interaction. What has emerged is rich information about the anomalous electron-phonon interaction well beyond the traditional views of the subject. It exhibits strong doping, momentum and phonon symmetry dependence, and shows complex interplay with the strong electron-electron interaction in these materials.
 Physics , 2015, DOI: 10.1103/PhysRevB.91.134501 Abstract: We explore possible signatures for observing Majorana Fermions in the tunneling spectroscopy of high-Tc cuprate superconductors. We find that as long as the Rashba spin orbit interaction is in presence either through proximity effect due to an electrode made by heavy metal or by the intrinsic nature of cuprates, in addition to the Heisenberg spin exchange interaction, the Dzyaloshinskii-Moriya and spin dipole-dipole interactions are induced. As a result, $p$-wave superconductivity is induced with the gap function $d$-vector being not aligned with the internal magnetic field of the spin-orbit interaction. Most importantly, the ground state goes through transitions into gapless phases with split nodal points. The split nodal structure always results in Majorana modes for any interfaces that are not exactly in (100) or (010) directions. Hence for general interfaces, existence of Majorana bound edge states is a robust feature. Our results indicate that these Majorana modes would result in a small plateau in tunneling spectrum near zero bias peak and in 4 $\pi$ periodicity in typical SIS$'$ junctions . As a result, it is easy for a $\pi$-ring in tricrystal experiments to hold Majorana Ferimions and exhibit periods of two flux quanta in external magnetic fields. These phenomena may have been already observed in experiments and their connections to experimental results are discussed.
 Physics , 2004, DOI: 10.1142/S0217979205028426 Abstract: We present scanning tunneling spectroscopic and high-field thermodynamic studies of hole- and electron-doped (p- and n-type) cuprate superconductors. Our experimental results are consistent with the notion that the ground state of cuprates is in proximity to a quantum critical point (QCP) that separates a pure superconducting (SC) phase from a phase comprised of coexisting SC and a competing order, and the competing order is likely a spin-density wave (SDW). The effect of applied magnetic field, tunneling current, and disorder on the revelation of competing orders and on the low-energy excitations of the cuprates is discussed.
 Physics , 2011, DOI: 10.1143/JPSJ.80.114715 Abstract: Central issues in the electronic structure of underdoped cuprate superconductors are to clarify the shape of the Fermi surfaces and the origin of the pseudogap. On the basis of the model proposed by Kamimura and Suwa, which bears important features originating from the interplay of Jahn-Teller physics and Mott physics, the feature of Fermi surfaces in underdoped cuprates is the presence of Fermi pockets constructed from doped holes under the coexistence of a metallic state and a local antiferromagnetic order. Below $T_{\rm c}$, the holes on Fermi pockets form Cooper pairs with d-wave symmetry in the nodal region. In the antinodal region, there are no Fermi surfaces. In this study we calculate the energy distribution curves (EDCs) of angle-resolved photoemission spectroscopy (ARPES) below $T_{\rm c}$. It is shown that the feature of ARPES profiles of underdoped cuprates consists of a coherent peak in the nodal region and real transitions of photoexcited electrons from occupied states below the Fermi level to a free-electron state above the vacuum level in the antinodal region, where the latter transitions form a broad hump. From this feature, the origin of the two distinct gaps observed by ARPES is elucidated without introducing the concept of the pseudogap. Finally, a remark is made on the phase diagram of underdoped cuprates.
 Physics , 2008, DOI: 10.1103/PhysRevLett.101.117006 Abstract: We have performed high resolution angle resolved photoemission (ARPES) studies on electron doped cuprate superconductors Sm2-xCexCuO4 (x=0.10, 0.15, 0.18), Nd2-xCexCuO4 (x=0.15) and Eu2-xCexCuO4 (x=0.15). Imaginary parts of the electron removal self energy show step-like features due to an electron-bosonic mode coupling. The step-like feature is seen along both nodal and anti-nodal directions but at energies of 50 and 70 meV, respectively, independent of the doping and rare earth element. Such energy scales can be understood as being due to preferential coupling to half- and full-breathing mode phonons, revealing the phononic origin of the kink structures. Estimated electron-phonon coupling constant lambda from the self energy is roughly independent of the doping and momentum. The isotropic nature of lambda is discussed in comparison with the hole doped case where a strong anisotropy exists.
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