Session T42: Superconductivity: Spin Properties

Universal d wave gap shape in the entire doping range of the cuprate High Temperature Superconductors

We performed detailed Angle Resolved Photoemission Spectroscopy (ARPES) measurements on Bi2212 high temperature superconductors over extensive doping range starting from non-superconducting insulating samples to highly doped superconducting ones. We observe that there are sharp quasiparticles all around the underlying Fermi surface in the superconducting state for all doping values. The superconducting gap anisotropy follows a simple d-wave form for all doping values. In non-superconducting samples although there are no sharp coherence peaks at any point on the underlying fermi surface, there is a clearly discernable \textit{low energy }gap. Quite remarkably, this low energy gap has the same d wave shape just like the superconducting one.   

Proposed New Mechanism for Nuclear Magnetic Relaxation (NMR) in Type-II Superconductors

By and large, nuclear magnetic resonance (NMR) data on cuprate superconductors in the vortex state has been interpreted as evidence for antiferromagnetism in the vortex cores. More specifically, the upturn in the spin-lattice relaxation rate divided by temperature, $\frac{1}{T_1 T}$, with decreasing temperature and the broadening of the line shape have been interpreted as such evidence. We find, however, that it is possible to account for these phenomena without invoking antiferromagnetism. The upturn in the spin-lattice relaxation rate may be explained as due to another relaxation process, namely one in which two spin-up quasiparticles are created or destroyed, in addition to the usual spin-flip scattering process. Furthermore, we show that the broadening of the line shape can be explained by the fact that the Knight shift in the vortex state is dependent on position inside the lattice.   

NMR evidence of a high field phase transition in $\kappa $-(ET)Cu(NCS)$_{2}$

The anisotropic electronic properties of the organic superconductor $\kappa $-(BEDT-TTF)$_{2}$Cu(NCS)$_{2}$ make it an excellent candidate for inhomogeneous superconductivity (FFLO state) when the magnetic field is applied in-plane, so as to avoid orbital suppression of the superconductivity. Results of various previous measurements have been interpreted as supporting the existence of an FFLO state for fields B$\sim $21-22T. In this work, $^{13}$C NMR measurements were used to probe the hyperfine fields in the range of B=14-27T and T=0.5-1.6K. A sharp peak in the relaxation rate 1/T1(B) at B$\sim $22T signals a phase transition associated with a turn-on of spin susceptibility, while evolution the spectrum with field indicate the system remains superconducting at higher fields. Additionally, only small variations of the hyperfine fields are observed for external fields B=14-20T, and we discuss this observation in relation to the superconducting order parameter symmetry.   

$\mu $SR search for loop currents in GdBCO

In the cuprate pseudogap phase, Varma [1] predicts loop currents above T$_{c}$. We search for magnetic fields created by such currents in GdBa$_{2}$Cu$_{3}$O$_{(7-\delta )}$ (GdBCO). Using Maximum Entropy (ME) [2] we analyze zero-field (ZF) muon-spin-resonance ($\mu $SR) data [3] of GdBCO. ME-$\mu $SR applied to ZF-GdBCO data yields a frequency distribution with T-dependent signals at 0-MHz [3] and 0.3-MHz (f1) and hints of 1.4-MHz (f*) signals. To cancel systematic f1 effects, we analyze DS(t,T) $\equiv $ S(t,T$>$T$_{c})$ - S(t,T'$<<$T$_{c})$ by ME analysis. We find, f1 disappears and weak f* signals are seen above T$_{c}$. Their existence is similar to anomalous polar-Kerr behavior observed in YBCO. [4] We attribute the f* signals at $\sim $1.3 MHz (95 Oe) for underdoped GdBCO and $\sim $1.5 MHz (110 Oe) for optimal doped GdBCO to fields created by loop currents. Our TF $\mu $SR GdBCO evidence indicates, the muon probes at muon-O sites near the BaO and CuO-chain insulating layers without disturbing potential loop currents in the CuO$_{2}$ planes.~Validating predicted loop currents [1] is essential for a better understanding of the pseudogap phase. [1] CM Varma, PRL 83 (1999) 3538. [2] C Boekema and MC Browne, MaxEnt 2008, AIP Conf Proc {\#}1073 p260; JC Lee \textit{et al,} J Appl Phys 95 (2004) 6906. [3] DW Cooke \textit{et al}, PRB37 (1988) 9401. [4] J Xia \textit{et al}, PRL 100 (2008) 127002   

A two gap preformed pair model for the underdoped cuprates: application to scanning tunneling and electronic Raman spectroscopies

The controversy over whether the pseudogap arises from precursor superconductivity or from another elusive order parameter has been intensified with the discovery of the so-called two gap behavior. This two gap physics is associated with different responses and inferred spectral gaps in the nodal and anti-nodal regions as observed in angle resolved photoemission (ARPES), scanning tunneling microscopy and related quasi-particle interference (QPI) patterns and in electronic Raman scattering. We have previously shown that two gap behavior in ARPES is compatible with a pre-formed pair picture based on BCS-BEC crossover. Here we address recent STM experiments at the level of QPI and using a gap inversion procedure introduced by the Princeton group. We also consider Raman experiments in a related formalism. The two gap physics results from the presence of both condensed and non-condensed pairs associated with a stronger than BCS attraction, and associated small coherence length. Our results, which compare a variety of different inferred gaps as a function of temperature for the most part compare favorably with existing experiments and make predictions for future experimental studies.   

Disorder-induced magnetic quantum phase transition in cuprate materials

High temperature superconductors exhibit an unusual interplay between superconductivity and magnetism. Inside the superconducting dome antiferromagnetic order can be induced by external magnetic fields or chemically by impurity substitution. It is well-known that, for example, Zn or Ni may induce static order, but neutrons have also provided detailed inelastic data of the spin dynamics in the disordered case. We model the magnetic fluctuations and show how local Coulomb correlations naturally give rise to a slowing down of the dynamics in a d-wave superconductor in the presence of nonmagnetic imputities. We also discuss the difference between magnetic and nonmagnetic impurities, and show the connection to the so-called stripe picture.   

Spin-glass behavior in Ni-doped high-$T_{C}$ cuprate La$_{1.85}$Sr$_{0.15}$CuO$_{4}$

We report the dc and ac magnetic properties of La$_{1.85}$Sr$_{0.15}$Cu$_{1-y}$Ni$_{y}$O$_{4}$ ($0\leq y\leq0.5$), investigated in the field up to 5 T and in the temperature range 2 K - 400 K. For y$\geq$0.06 the system exhibits irreversibility of dc susceptibility $\chi(T)$ below $T_ {irr}(y)$ and a cusp at $T_{g}(y)$ in $\chi(T)$ measured after zero-field cooling. The decay of remnant magnetization with time is described by a stretched-exponential function. In accordance with the scaling theory, all the $\chi(T)$ data for a given sample taken in the vicinity of $T_{g}$ at different fields collapse onto two separate curves when plotted as $q/|t|^{\beta} $ vs. $B^{2}/|t|^{\beta+\gamma}$, where $q$ is the spin-glass (SG) order parameter, $t$=$(T-T_{g})/T_{g}$, and $\beta$ and $\gamma$ are the critical exponents. The temperature of the peak in the real part of $\chi_{ac}$ vs. $T$ curve increases with increased frequency according to the critical slowing-down formula. The obtained parameters do not exclude the presence of spin clusters. Variation of $T_{g}$ with $y$ is linear for $y<0.25$ and $T_{g}$ extrapolates to 0 K for $y\rightarrow0$, what strongly suggests that the SG phase extends into the superconducting region of the phase diagram.   

Emergence of a Novel Frozen Magnetic State in Heavily Over-Doped La$_{2-x}$Sr$_{x}$CuO$_{4}$

It has been proposed that the termination of superconductivity in heavily overdoped copper oxides is caused by the emergence of competing ferromagnetism. We report on a direct examination of this scenario in overdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$ using the technique of muon spin relaxation. We detect the onset of static magnetic moments of electronic origin beyond the superconducting ``dome'', where antiferromagnetic correlations are absent. The magnetism does not exist in a commensurate long-range ordered state, but instead appears as a dilute concentration of static magnetic moments. The results are qualitatively consistent with electronic band calculations that predict weak ferromagnetic order about Sr-rich clusters.   

Detection of magnetism in heavily overdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$ single crystals

We have measured the DC magnetic susceptibility of a heavily overdoped La$_{1.67}$Sr$_{0.33}$CuO$_{4}$ single crystal. In contrast to lower-doped superconducting samples no magnetic anisotropy is observed. However, as previously seen in the normal state, there is a Curie-like term that appears in the susceptibility for strontium content in excess of x$\sim $0.19. This is true even for La$_{1.67}$Sr$_{0.33}$CuO$_{4}$, in which the electronic system is fairly well established to be a Fermi liquid. A tentative explanation of the DC magnetic susceptibility and complimentary MuSR measurements on the same La$_{1.67}$Sr$_{0.33}$CuO$_{4}$ sample is discussed in terms of recent predictions of ferromagnetism in the heavily overdoped regime.   

Relation between weak-wide pseudo gap effect and spin fluctuations in electron doped cuprate Pr$_{1-x}$LaCe$_{x}$CuO$_{4}$

Magnetic behavior of electron doped cuprate Pr$_{1-x}$LaCe$_{x}$CuO$_{4}$(PLCCO) is different from that of typical electron doped cuprates such as Nd$_{2-x}$Ce$_{x}$CuO$_{4}$(NCCO). Its long range anti-ferromagnetic (AFM) order disappears at a lower Ce concentration than NCCO, and spin fluctuations as well as superconductivity exist over a wide doping range. It is therefore interesting to study in detail electronic structure of PLCCO and its spin correlations. We performed angle resolved photoemission spectroscopy studies on PLCCO(x=0.09, 0.12, 0.15) to investigate the doping dependent electronic structures. Different from other electron doped cuprates, nodal bands cross the Fermi level, even for x=0.09 doped samples. That is, the hole pocket near ($\pi $/2,$\pi $/2) region survives for doping as low as x=0.09. Yet, the pseudo gap (PG) is well defined for x=0.09 and is gradually suppressed as the doping increases to x=0.15. This behavior can be attributed to coupling of electrons to a weaker AF phase fluctuation which implies a weak PG effect over a wider doping range. This is consistent with earlier neutron scattering result.   

Weak scattering sources in YBa$_{2}$Cu$_{3}$O$_{6.5}$

We have measured the microwave conductivity of ultra-high purity single crystal YBa$_{2}$Cu$_{3}$O$_{6.5}$ as a continuous function of frequency (0.5 to 20 GHz) and at low temperatures. The samples were prepared in the highly ordered ortho-II phase, in which the off-plane oxygen atoms are arranged into alternating full and empty chains. Measurements were taken in this ordered state and then, following heating and rapid quench cooling, again in a disordered state. Our results help to illuminate the roles of both cation impurities and of CuO chain oxygen disorder as sources of in-plane quasi-particle scattering in ultra-high purity YBCO.   

Evidence of the Ortho II band folding in of YBa2Cu3O7-$\delta $ underdoped surface

Fermi surface and low energy electronic structure of YBa2Cu3O7-$\delta $ (Y123) is investigated using angle-resolved photoemission spectroscopy (ARPES) on pulsed laser deposition (PLD) in situ grown films. By PLD, the Y123 surface is carefully controlled to produce underdoped surfaces with ordered oxygen vacancies within the CuO chains. The resulting Fermi surface displays a clear Ortho II band folding, showing how order within the CuO chains strongly affect the electronic properties of the superconducting CuO2 planes. Until now, the Ortho II band folding has not been detected by ARPES measurements. This present approach opens the door to systematic studies in the remaining question concerning the ambiguous nature of the pseudogap phase for high-temperature cuprates.   

Improved Ordering of the Ortho-II Phase of YBCO

The cuprate YaBa$_{2}$Cu$_{3}$O$_{6.5}$ is a widely studied superconductor. Cation defects have been decreased in our group by using higher purity sample reagents. We attempt to further improve sample quality by decreasing point defects and domain boundaries in the Cu-O chains of the highly ordered ortho-II phase using a unique low temperature (400 to 300 K) annealing procedure. The point defects may be vacancies in the full chains or oxygen atoms in the empty chains; domain boundaries may be defined by a break in the alternating full-empty chain ortho-II symmetry. DC resistivity measurements are made during annealing to allow for the ordering process to be monitored. Preliminary results will be presented.   

Hysteresis and memory in the magnetoresistance of underdoped La$_{2-x}$Sr$_{x}$CuO$_4$ thin films

We have studied the in-plane magnetoresistance (MR) in atomically smooth, MBE grown La$_{2-x}$Sr$_{x}$CuO$_4$ thin films with $x=0.03$, 0.05, 0.055, and 0.06. These dopings span the region from non-superconducting to superconducting samples. The MR was measured at temperatures $0.3$~K~$ \leq T \leq 20$~K and in magnetic fields $0 \leq $~B~$ \leq 9$~T, applied both parallel and perpendicular to the $c$-axis. At low $T$, the MR shows the emergence of a strong, positive contribution, which exhibits hysteresis and memory. At the same time, the zero-field resistances obtained after field cooling and zero-field cooling are found to be different. These glassy features, which seem to reflect the slow dynamics of holes, become more pronounced as $T$ is reduced. The evolution of this glassy, low-$T$ state with doping is discussed in detail.