Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session N35: Focus Session: Search for New Superconductors II |
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Sponsoring Units: DMP Chair: Ivan Bozovic, Brookhaven National Laboratory Room: 343 |
Wednesday, March 20, 2013 11:15AM - 11:27AM |
N35.00001: Charge Density Wave Instability and Soft Phonon in $A$Pt$_3$P ($A$=Ca, Sr, and La) Chao Cao, Hui Chen, Xiaofeng Xu, Jianhui Dai The electronic and phonon properties of the platinum pnictide superconductors $A$Pt$_3$P ($A$=Ca, Sr, and La) were studied using first-principles calculations. The spin-orbit coupling effect is significant in LaPt$_3$P but negligible in CaPt$_3$P and SrPt$_3$P. Moreover, SrPt$_3$P has been demonstrated to exhibit an unexpected weak charge-density-wave (CDW) instability which is neither simply related to the Fermi-surface nesting nor to the momentum-dependent electron-phonon coupling alone. The instability is absent in CaPt$_3$P and can be quickly suppressed by the external pressure, accompanied with decreases in the phonon softening and BCS $T_c$. Our results suggest SrPt$_3$P as a rare example where superconductivity is enhanced by the CDW fluctuations. [Preview Abstract] |
Wednesday, March 20, 2013 11:27AM - 11:39AM |
N35.00002: Charge density wave fluctuations and possible heavy fermion behavior without magnetism in ThCr$_2$Si$_2$-type KNi$_2$S$_2$ and KNi$_2$Se$_2$ James Neilson, Anna Llobet, Jiajia Wen, Matthew Suchomel, Tyrel McQueen Materials with the ThCr$_2$Si$_2$-type structure host myriad examples of many-body physics, including high-temperature superconductivity and heavy fermion behavior. In these compounds, the emergence of the collective state frequently occurs near a magnetic instability, suggesting that magnetic fluctuations underlie the electronic phenomena. I will provide evidence for similar many-body physics in the structurally related, but non-magnetic compounds, KNi$_2$S$_2$ and KNi$_2$Se$_2$. From the analysis of synchrotron X-ray diffraction and neutron total scattering data, we observe spatially incoherent charge density wave fluctuations that disappear on cooling. Along with the implied and unusual increase in local symmetry, we find that there is negative thermal expansion and enhancement of the electronic band mass below $T\sim15$ K, with superconductivity emerging below 1 K. These findings demonstrate that collective electronic phenomena occurs in ThCr$_2$Si$_2$-type materials without direct proximity to localized magnetism. Furthermore, these results highlight the importance in understanding charge fluctuations and their hybridization in driving the emergence of coherent or many-body electronic states, akin to localized magnetism associated with heavy fermion behavior. [Preview Abstract] |
Wednesday, March 20, 2013 11:39AM - 11:51AM |
N35.00003: Charge-order fluctuations and electron-phonon coupling in organic superconductors Alberto Girlando, Matteo Masino, Natalia Drichko, Martin Dressel Organic superconductors (SC), like other new classes of SC, are characterized by important electronic correlations. Spin or charge-order (CO) fluctuations have been invoked as mediators in the pairing mechanism, in place of, or in addition to, the conventional phonon mediated pairing. In the phase diagram of BEDT-TTF based 1/4-filled layered charge transfer salts, a CO metal-insulator transition is close to the metal-SC transition, with CO fluctuations in the proximity of the instabilities. We present the characterization of CO fluctuations obtained through optical spectroscopy of SC and non-SC BEDT-TTF salts, with an estimate of the average charge on the molecules and the velocity of charge ``jump'' from one molecule to the other. It turns out that the latter is not connected to the SC. A correlation seems instead to occur between SC and the average charge on the molecules. We shall also discuss other possible signatures of the charge fluctuations in the optical spectra, as well as the connection between CO fluctuations and intra- and inter-molecular electron-phonon coupling. The relevance of these ideas to the recently discovered class of doped acene superconductors will be shortly discussed. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:03PM |
N35.00004: Ferroelectric Soft Phonons, Charge Density Wave Instability, and Strong Electron-Phonon Coupling in BiS$_2$-Layered Superconductors Taner Yildirim Very recently a new family of layered materials, containing BiS$_2$ planes was discovered to be superconducting at temperatures up to T$_c$=10 K, raising questions about the mechanism of superconductivity in these systems. Here, we present state-of-the-art first principles calculations that directly address this question and reveal several surprising findings [1]. The parent compound LaOBiS$_2$ possesses anharmonic ferroelectric soft phonons at the zone center with a rather large polarization of $\approx 10 \mu C/cm^2$, which is comparable to the well-known ferroelectric BiFeO$_3$. Upon electron doping, new unstable phonon branches appear along the entire line $Q=(q,q,0)$, causing Bi/S atoms to order in a one-dimensional charge density wave (CDW). We find that BiS$_2$ is a strong electron-phonon coupled superconductor in the vicinity of competing ferroelectric and CDW phases. Our results suggest new directions to tune the balance between these phases and increase T$_c$ in this new class of materials.\\[4pt] [1] T. Yildirim, arXiv:1210.2418 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 12:03PM - 12:15PM |
N35.00005: Searching for High-T$_{\mathrm{c}}$ Superconductivity in Low-Z, Low-Ne Materials O-Paul Isikaku-Ironkwe, Timothy Haugan, Alex Animalu The discovery in 2001 of HTSC at 39K in MgB$_{2}$, a low-atomic number (Z) and low-valence electron count per atom (Ne) material, strongly suggested that similar materials may exist with comparable or even higher Tcs. Efforts to find MgB$_{2}$-like HTSC materials in binary and ternary systems have not been very successful. Using recently developed material specific formula for Tc, we have extended the computational and experimental search for potential low-Z, low-Ne HTSCs beyond the ternary structure into the 4 and 5-element systems. Exploring the family of low-Z, low-Ne materials represented by Z $=$ 1.333p $+$ 2 where p is an integer, we find in this broad spectrum, hundreds of potential HTSC materials. Here we present some of the combinatorial-computational datasets and preliminary experimental results using a novel non-DFT material-specific characterization dataset (MSCD) method. MSCD promises to accelerate the computational search for new materials, particularly superconductors. The computations and preliminary experimental results suggest that HTSC, comparable to the cuprates may exist in low-Z, low-Ne materials with 4 and 5-element systems. [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:27PM |
N35.00006: High-temperature surface superconductivity in rhombohedral graphite Tero Heikkil\"a, Nikolai Kopnin, Mari Ij\"as, Ari Harju, Grigori Volovik We show that rhombohedral graphite may support surface superconductivity with an unusual relation between the BCS coupling constant and the order parameter. This feature results from the properties of the states localized on the graphite surfaces. In a description including only the nearest neighbour coupling of the graphene layers, the surface states are topologically protected and have a flat band dispersion. We show that including higher order couplings destroys this flat band character and leads to a particle-hole symmetry breaking quadratic dispersion with a large effective mass. Employing this dispersion, we then show its effect on superconductivity and find two regimes of parameters, depending on the relation between the strength of the coupling constant and the details of the quadratic dispersion. For low coupling strengths, superconductivity is localized on the surfaces, but the order parameter is exponentially suppressed as in a conventional BCS superconductor, whereas for large coupling strengths we obtain surface superconductivity with a linear relation between the order parameter and the coupling constant. Our results offer an explanation for the recent findings of graphite superconductivity with an unusually high trans [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N35.00007: Phase separation instabilities and pairing in layered BSCCO-like lattice geometries Armen Kocharian, Kun Fang, Gayanath Fernando, Alexander Balatsky, Kalum Palandage The electron spontaneous phase separations accompanied by local inhomogeneities are evaluated by monitoring the charge and spin pairing gaps in the ground state and corresponding crossovers at finite temperatures in various cluster geometries and wide range of inter-site interaction $U$. The effects of the next nearest neighbor hopping on electron instabilities at level crossings in the vicinity of quantum critical points are considered. The calculated energy gap at one hole away from half filling displays universal features consistent with the lattice structure symmetry in non-bipartite geometries. The charge and spin collective excitations in layered pyramidal structures driven by out-of-plane variation of lattice parameters yield intriguing insights into the coherent and incoherent pairings and gap modulations in Bi-like based cuprates, iron pnictides, and other transition metal oxides layered structures. The phase diagrams resemble a number of inhomogeneous, coherent and incoherent nanoscale phases seen in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$. The found similarities and differences in the mechanisms of electron pairing, driven by attractive and repulsive electron interaction are analyzed. The phase separation instabilities in related intercalated layered geometries are discussed. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N35.00008: Tuning the charge-transfer energy in hole-doped cuprates Chuck-Hou Yee, Gabriel Kotliar Chemical substitution, combined with strain, allows the charge-transfer energy in hole-doped cuprates to be broadly tuned. We theoretically characterize the structural and electronic properties of the family of compounds R$_2$CuO$_2$S$_2$, constructed by sulfur replacement of the apical oxygens and rare earth substitutions in the parent cuprate La$_2$CuO$_4$. Additionally, the enthalpies of formation for possible synthesis pathways are determined. [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N35.00009: The effective half-filled band model is inappropriate for the dimerized 2D organic superconductors Niladri Gomes, R. Torsten Clay, Sumit Mazumdar The antiferromagnetism in $\kappa$-(ET)$_{2}$X can be understood within the effective $\frac{1}{2}$-filled band anisotropic triangular lattice Hubbard Hamiltonian for strong anisotropy. DMFT theories have claimed antiferromagnetic-to-superconductor transition within the same model, as the anistropy is reduced. In previous work we have shown the absence of superconductivity within the triangular lattice $\frac{1}{2}$-filled band Hubbard model for any Hubbard $U$ and any anisotropy. Other DMFT approaches theories have claimed superconductivity within the so-called Hubbard-Heisenberg model, which incorporates an additional antiferromagnetic spin-exchange over and above that due to the Hubbard $U$. Very recent work has also claimed a valence-bond solid (VBS) phase within the Hubbard-Heisenberg model, which would seemingly explain the observed VBS phase in EtMe$_{3}$P[Pd(dmit)$_{2}$]$_{2}$. We report exact calculations that show that neither the VBS nor the superconducting phase occur within the Hubbard-Heisenberg model, showing clearly that the effective $\frac{1}{2}$-filled band model is unsuitable for describing the complete phase space of the $\kappa$-(ET)$_{2}$X. Our work raises serious doubts about the DMFT theories of superconductivity of metal intercalated C$_{60}$ and picene. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N35.00010: A possible paired-electron liquid in a $\frac{1}{4}$-filled band model of $\kappa$-(ET)$_2$X R.T. Clay, N. Gomes, S. Mazumdar A minimal model for the $\kappa$-(ET) conducting layers is a $\frac{1}{2}$-filled anisotropic triangular lattice Hubbard model, where a dimer of molecules is replaced with a single effective site. This effective model can explain occurrence of an antiferromagnetic (AFM) phase, but recent results do not find superconductivity in the model. We have shown that in a $\frac{1}{4}$-filled system on a dimerized square lattice, the AFM phase gives way to a Paired Electron Crystal singlet-paired state in the presence of lattice frustration. Here we present results of calculations on the actual $\frac{1}{4}$-filled $\kappa$ lattice rather than the simplified square lattice. We find not only an AFM-to-singlet transition, but show that the singlet phase may be a Paired Electron Liquid state consisting of a superposition of nearest-neighbor singlets. We show that in the excitation spectrum the lowest singlet excited state occurs below the lowest triplet. This may indicate gapless singlet excitations and gapped spin excitations, which would explain the observed heat capacity versus thermoelectric behavior in $\kappa$-(ET)$_2$-Cu$_2$(CN)$_3$ and EtMe$_3$Sb[Pd(dmit)$_2$]$_2$. We further discuss superconducting pair correlation functions. [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N35.00011: Crystal structure, electronic properties, and superconductivity mechanism of La-Phenanthrene Shahab Naghavi, Michele Fabrizio, Tao Qin, Erio Tosatti Recently, polycyclic aromatic hydrocarbon (PAH) molecular solids: picene, coronene, dibenzopentacene, phenanthrene among them, have been reported to turn from insulating to metallic and superconducting upon intercalation of electron-donating atoms, such as K, Ba, La. Despite experimental uncertainties, understanding these novel light-element based superconductors is important since both electron phonon coupling and electron electron correlations seem important, as indicated by early theory work. Choosing La-Phenanthrene (La-PA) as our working case, we first search for the theoretical optimal crystal structure and electronic properties by first principles density functional calculations. We single out a stable insulating phase with $P1$ symmetry and, slightly higher in energy, a metastable metallic $P2_{1}$ phase--the same (higher) symmetry of pristine PA, also proposed for La-PA. A tight binding model representing the metallic La-PA electronic structure, its dominant electron phonon coupling with an intermolecular dimerizing mode, and an intramolecular Coulomb $U$ is formulated and discussed. In that model it can be argued that BCS pairing may be essentially unhindered by the Coulomb repulsion. Being symmetry-based, the mechanism could apply to other PAH superconductors as well. [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N35.00012: Anisotropic Migdal-Eliashberg theory using Wannier functions Elena Roxana Margine, Feliciano Giustino We combine the fully anisotropic Migdal-Eliashberg theory with electron-phonon interpolation based on maximally-localized Wannier functions, in order to perform reliable and highly accurate calculations of the anisotropic temperature-dependent superconducting gap and critical temperature of conventional superconductors. Compared with the widely used McMillan approximation, our methodology yields a more comprehensive and detailed description of superconducting properties, and is especially relevant for the study of layered or low-dimensional systems as well as systems with complex Fermi surfaces. In order to validate our method, implemented within the EPW package [1,2], we perform calculations on two prototypical superconductors, Pb and MgB$_{2}$, and obtain good agreement with previous studies [3]. [1] F. Giustino, M. L. Cohen, and S. G. Louie, Phys. Rev. B 76, 165108 (2007). [2] J. Noffsinger et. al., Computer Physics Communications 181, 2140 (2010). [3] E. R. Margine and F. Giustino, Phys. Rev. B (submitted). [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N35.00013: Search for chalcogenide based superconductors: Sulfur based solution growth Udhara Kaluarachchi, Sergey Bud'ko, Paul Canfield As part of our effort to develop tools for searching for new chalcogenide based superconductors we are expanding the range of S-based binary melts that we can use for solution growth of single crystals. As a recent example, we have been able to grow single crystals of Rh$_{17}$S$_{15}$ and separate them for excess binary melt via high temperature decanting. In addition to refining the details of the Rh-S binary phase diagram, microscopic, thermodynamic and transport measurement on Rh$_{17}$S$_{15}$ crystals confirm their T$_{c}$ $\sim$ 5.5 K as well as their remarkably large H$_{c2}$(T) behavior. The possible cause of the enhanced H$_{c2}$(T) will be discussed. As time allows we will also review other S-based growths and compounds. [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N35.00014: Density Functional Theory studies of Epitaxial Charge Transfer Salts Geoffrey A. Rojas, P. Ganesh, Simon Kelly, Bobby J. Sumpter, John A. Schlueter, Petro Maksymovich Some of the fulvalene-based charge transfer salts (CTS) become superconducting in bulk. The basic physics and ways to control it has been explored by changing the intermolecular spacing using both chemical substitution and pressure, but the fixed stoichiometry limit the occupation of the filled states to what is naturally available. Recent experiments suggest growth of 2D epitaxial layers of CTS allowing stoichiometric and geometric control of the electronic structure, thereby leading to engineered superconducting interfaces. In a combined experiment and theory study, we provide new insight to understand the interplay between structure, stoichiometry and electronic-structure of epitaxially grown (ET)2SF5CH2CF2SO3 salt on Ag(111) surface. Density functional theory studies show that the cohesive energy of the 2D salts are very high, in spite of strong bonding to the underlying Ag surface via Ag-S metallic bonds, and provide a rationale for off-stoichiometric growth with different electronic structures as seen in our experiments, such as 3:1 and a 1:1 cation:anion stoichiometry, necessary for a monolayer coverage~and different from the bulk 2:1 stoichiometry. We also explore the role of van der Waals interactions for structural stability. This research was conducted at the Center for Nanophase Materials Sciences, sponsored at ORNL by the Division of User Facilities, U.S. DOE. [Preview Abstract] |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N35.00015: Rational design of novel thallium halide high T$_c$ superconductors from first-principles Zhiping Yin, Gabriel Kotliar Searching for new high-temperature superconductors remains one of the most active research areas in condensed matter physics and material physics. In this talk I will show how we designed, from first principles calculations, a novel family of thallium halide-based compounds as candidates for new high-temperature superconductors. This family together with the celebrated (Ba,K)BiO$_3$ and electron-doped HfNCl families are the ``other high-temperature supercondutors,'' whose superconductivity is mediated by the recently proposed mechanism of non-local correlation-enhanced strong electron-phonon coupling (arXiv:1110.5751). Two prototype compounds namely CsTlF$_3$ and CsTlCl$_3$ are studied with various hole doping levels and volumes. The critical superconducting temperature T$_c$ are predicted to be about 30 K and 20 K with optimal hole doping and volume, respectively. Our procedure of designing this class of superconductors is quite general and can be used to search for other ``other high temperature superconductors.'' [Preview Abstract] |
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