arXiv subjects

W. G. Moulton

Publications and source records attributed to W. G. Moulton.

15 recordsLinked to original sources

arXiv2014

Superconducting anisotropy in the electron-doped high-T$_{c}$ superconductors Pr$_{2-x}$Ce$_{x}$CuO$_{4-y}$

We report superconducting anisotropy measurements in the electron-doped high-$T_{c}$ superconductors (HTSCs) Pr$_{2-x}$Ce$_{x}$CuO$_{4-y}$ (PCCO, $x$ = 0.15 and 0.17) with applied magnetic field ($H_{0}$) up to 28 T. Our results show that the upper critical field [$H_{c2}(T)$] is highly anisotropic, and as temperature $T$ $\rightarrow$ 0 the value of it at $H_{0}$ $\parallel$ $c$ [$H_{c2,\parallel c}$(0)] is far less than the Pauli limit, which is very different from that at $H_{0}$ $\perp$ $c$. The $H_{c2}(0)$ character along with the evaluated zero $T$ coherence length $[ξ_{ab(c)}(0)]$ and penetration depth $[λ_{ab(c)}(0)]$ is compared with those of the hole-doped cuprate HTSCs and typical Fe-based superconductors. We find that the low temperature anisotropic character of PCCO is rather similar to that of hole-doped cuprate HTSCs, but apparently larger than that of typical Fe-based superconductors. This study also proves a new sensitive probe of detecting rich properties of unconventional superconductors with the use of the resonant frequency of a NMR probe circuit.

cond-mat.supr-con↗

arXiv2010

Distinct high-T transitions in underdoped Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$

In contrast to the simultaneous structural and magnetic first order phase transition $T_{0}$ previously reported, our detailed investigation on an underdoped Ba$_{0.84}$K$_{0.16}$Fe$_{2}$As$_{2}$ single crystal unambiguously revealed that the transitions are not concomitant. The tetragonal ($τ$: I4/mmm) - orthorhombic ($\vartheta$: Fmmm) structural transition occurs at $T_{S}\simeq$ 110 K, followed by an adjacent antiferromagnetic (AFM) transition at $T_{N}\simeq$ 102 K. Hysteresis and coexistence of the $τ$ and $\vartheta$ phases over a finite temperature range observed in our NMR experiments confirm the first order character of the structural transition and provide evidence that both $T_{S}$ and $T_{N}$ are strongly correlated. Our data also show that superconductivity (SC) develops in the $\vartheta$ phase below $T_{c}$ = 20 K and coexists with long range AFM. This new observation, $T_{S}\neq T_{N}$, firmly establishes another similarity between the hole-doped BaFe$_{2}$As$_{2}$ via K substitution and the electron-doped iron-arsenide superconductors.

cond-mat.supr-con↗

arXiv2005

Evidence for Skyrmion crystallization from NMR relaxation experiments

A resistively detected NMR technique was used to probe the two-dimensional electron gas in a GaAs/AlGaAs quantum well. The spin-lattice relaxation rate $(1/T_{1})$ was extracted at near complete filling of the first Landau level by electrons. The nuclear spin of $^{75}$As is found to relax much more efficiently with $T\to 0$ and when a well developed quantum Hall state with $R_{xx}\simeq 0$ occurs. The data show a remarkable correlation between the nuclear spin relaxation and localization. This suggests that the magnetic ground state near complete filling of the first Landau level may contain a lattice of topological spin texture, i.e. a Skyrmion crystal.

cond-mat.mes-hall↗

arXiv2003

Interplay between the magnetic fluctuations and superconductivity in the lanthanum cuprates

We report the analysis of the magnetic fluctuations in the superconducing La_{2-x}Sr_xCuO_4 and the related lanthanum cuprates having the different symmetry of the low temperature structure. The NMR and ESR investigations revealed the dynamical coexistence of the superconductivity and the antiferromagnetic correlations in the large part of superconductivity region of the phase diagram. We show that for all compounds, independent on their low temperature symmetry and on their superconducting properties, the enhancement of the spin stiffness near 1/8 doping takes place.

cond-mat.supr-con↗

arXiv2003

Antiferromagnetism in the Vortex Cores of YBa$_2$Cu$_3$O$_{7-δ}$

We report spatially resolved nuclear magnetic resonance measurements on a high temperature superconductor that indicate the presence of correlated antiferromagnetic fluctuations in the vortex core. The nuclear spin-lattice relaxation rate, $1/^{17}T_1$, of planar $^{17}O$, in near-optimally doped YB$_2$Cu$_3$O$_{7-δ}$ (YBCO), was measured. Outside of the core, $(^{17}T_1T)^{-1}$ is independent of temperature consistent with theoretical predictions for a {\it d}-wave superconductor. In the vortex core $(^{17}T_1T)^{-1}$ increases with decreasing temperature following an antiferromagnetic Curie-Weiss law.

cond-mat.supr-con↗

arXiv2003

Magnetic phase separation in La1-xSrxCoO3 by 59Co NMR

59Co NMR measurements on La1-xSrxCoO3 reported here establish unequivocally, for the first time, the coexistence of ferromagnetic regions, spin glass regions, and hole poor low spin regions at all x values from 0.1 to 0.5. A zero external field NMR spectrum, which is assigned to the ferromagnetic regions, has a spectral shape which is nearly x independent at 1.9 K, as are the relaxation times, T1 and T2. The integrated spectral area increases rapidly with x up to x=0.2 and then becomes almost constant for larger x. In a field of 9.97 T, a narrow NMR line is observed at 102 MHz, identical to that found in x=0 samples in previous work. The integrated intensity of this spectrum decreases rapidly with increasing x, and is ascribed to hole poor low spin regions. Beneath this spectrum, a third broad line, centered at 100 MHz, with shorter T1 and T2,. is assigned to a spin glass, intermediate spin phase.

cond-mat.str-el↗

arXiv2002

Delocalized Quasiparticles in the Vortex State of an Overdoped High-Tc Superconductor Probed by 63Cu NMR

We report the spin Knight shift (K_s) and the nuclear spin-lattice relaxation rate (1/T_1) in the vortex state as a function of magnetic field (H) up to 28 T in the high-T_c superconductor TlSr_{2}CaCu_2O_{6.8} (T_c=68 K). At low temperatures well below T_{c}, both K_s and 1/T_1 measured around the middle point between two nearest vortices (saddle point) increase substantially with increasing field, which indicate that the quasiparticle states with an ungapped spectrum are extended outside the vortex cores in a d-wave superconductor. The density of states (DOS) around the saddle point is found to be κN_0\sqrt{H/H_{c2}}, with κ=0.5\sim0.7 and N_0 being the normal-state DOS.

cond-mat.supr-con↗

arXiv2001

High Magnetic Field NMR Studies of LiVGe$_2$O$_6$, a quasi 1-D Spin $S = 1$ System

We report $^{7}$Li pulsed NMR measurements in polycrystalline and single crystal samples of the quasi one-dimensional S=1 antiferromagnet LiVGe$_2$O$_6$, whose AF transition temperature is $T_{\text{N}}\simeq 24.5$ K. The field ($B_0$) and temperature ($T$) ranges covered were 9-44.5 T and 1.7-300 K respectively. The measurements included NMR spectra, the spin-lattice relaxation rate ($T_1^{-1}$), and the spin-phase relaxation rate ($T_2^{-1}$), often as a function of the orientation of the field relative to the crystal axes. The spectra indicate an AF magnetic structure consistent with that obtained from neutron diffraction measurements, but with the moments aligned parallel to the c-axis. The spectra also provide the $T$-dependence of the AF order parameter and show that the transition is either second order or weakly first order. Both the spectra and the $T_1^{-1}$ data show that $B_0$ has at most a small effect on the alignment of the AF moment. There is no spin-flop transition up to 44.5 T. These features indicate a very large magnetic anisotropy energy in LiVGe$_2$O$_6$ with orbital degrees of freedom playing an important role. Below 8 K, $T_1^{-1}$ varies substantially with the orientation of $B_0$ in the plane perpendicular to the c-axis, suggesting a small energy gap for magnetic fluctuations that is very anisotropic.

cond-mat↗

arXiv2001

Spatially resolved electronic structure inside and outside the vortex core of a high temperature superconductor

One of the puzzling aspects of high temperature superconductors is the prevalence of magnetism in the normal state and the persistence of superconductivity in very high magnetic fields. Generally, superconductivity and magnetism are not compatible. But recent neutron scattering results indicate that antiferromagnetism can appear deep in the superconducting state in an applied magnetic field. Magnetic fields penetrate a superconductor in the form of quantized flux lines each one representing a vortex of supercurrents. Superconductivity is suppressed in the core of the vortex and it has been suggested that antiferromagnetism might develop there. To address this question it is important to perform electronic structural studies with spatial resolution. Here we report on implementation of a high field NMR imaging experiment that allows spatial resolution of the electronic behavior both inside and outside the vortex cores. Outside we find strong antiferromagnetic fluctuations, and localized inside there are electronic states rather different from those found in conventional superconductors.

cond-mat.supr-con↗

arXiv2001

The Pseudogap in YBa$_2$Cu$_3$O$_{7-δ}$ from NMR in High Magnetic Fields

We report $^{17}$O(2,3) and $^{63}$Cu(2) spin-lattice relaxation rates and the $^{17}$O(2,3) spin-spin relaxation rate in different magnetic fields in YBa$_2$Cu$_3$O$_7$ near $T_c$. Together these measurements enable us to test the magnetic field dependence of the pseudogap effect on the spin susceptibility in different regions of the Brillouin zone using the known form factors for different nuclei as filters. Thus, we study the momentum dispersion of the pseudogap behavior. We find that near the antiferromagnetic wave vector the pseudogap is insensitive to magnetic fields up to 15 T. In the remaining region, away from the $(π,π)$ point, the pseudogap shows a magnetic field dependence at fields less then 10 T. The first result is indicative of the opening of a spin-pseudogap that suppresses antiferromagnetic correlations below a temperature $T^*$; whereas, the second result shows the effect of pairing fluctuations on the spin susceptibility as a precursory effect of superconductivity.

cond-mat.supr-con↗

arXiv2000

Glassy Spin Freezing and NMR Wipeout Effect in the High-T_c Superconductor La1.90Sr0.10CuO4: What is the Relationship With Stripes ?

We report on 139La and 63Ccu NMR/NQR measurements in the high-Tc superconductor La1.90Sr0.10CuO4 with Tc=26.5 K. Spin fluctuations probed by 139La spin-lattice relaxation (T1), continuously slow down on cooling through Tc. We argue that spin-freezing and superconductivity are bulk effects in this sample. Thus, both phenomena have to coexist microscopically. The distribution of 139La T1 values at low temperature reveals a wide spread of spin fluctuation frequencies in CuO2 planes. A simple estimate shows that Cu nuclei at sites where electronic fluctuations are the slowest are not observable because of too short relaxation times (wipeout effect). This means that the Cu NQR wipeout, observed in this sample, can be explained primarily by slow magnetic, rather than charge, fluctuations. This result does not rule out the connection between wipeout effect and charge stripe order [Hunt et al., Phys. Rev. Lett. 82, 4300 (1999)], but it indicates that the relationship between both phenomena is not straightforward. We argue that the wipeout fraction cannot define a proper order parameter for a stripe phase, and cannot be used alone as a criterion for its existence.

cond-mat.supr-con↗

arXiv2000

Superconducting Fluctuations and the Pseudogap in the Slightly-overdoped High-Tc Superconductor TlSr2CaCu2O6.8: High Magnetic Field NMR Studies

From measurements of the ^{63}Cu Knight shift (K) and the nuclear spin-lattice relaxation rate (1/T_{1}) under magnetic fields from zero up to 28 T in the slightly overdoped superconductor TlSr_{2}CaCu_{2}O_{6.8} (T_{c}=68 K), we find that the pseudogap behavior, {\em i.e.}, the reductions of 1/T_{1}T and K above T_{c} from the values expected from the normal state at high T, is strongly field dependent and follows a scaling relation. We show that this scaling is consistent with the effects of the Cooper pair density fluctuations. The present finding contrasts sharply with the pseudogap property reported previously in the underdoped regime where no field effect was seen up to 23.2 T. The implications are discussed.

cond-mat.supr-con↗

arXiv1999

Superconducting Fluctuation Effects on the Electron Spin Susceptibility in YBa_2Cu_3O_{6.95}

The electronic spin susceptibility of YBa_2Cu_3O_{6.95} has been measured with high precision up to 24 Tesla with 17O nuclear magnetic resonance. Its temperature dependence can be accounted for by superconducting fluctuations that result in a smooth crossover from the normal to the vortex liquid state. A magnetic field-temperature phase diagram for this crossover has been established having strong upward curvature.

cond-mat.supr-con↗

arXiv1999

Superconducting Fluctuation Effects on the Spin-Lattice Relaxation Rate in YBa_2Cu_3O_{6.95}

We report 63Cu(2) spin-lattice relaxation rate measurements of YBa_2Cu_3O_{6.95} in magnetic fields from 2.1 T to 27.3 T obtained from 17O(2,3) nuclear magnetic resonance spin-spin relaxation. For T < 120 K, the spin-lattice rate increases with increasing magnetic field. We identify this magnetic field dependence with the change in the low-energy spectral weight originating from d-wave pairing fluctuation corrections to the density of states.

cond-mat.supr-con↗

arXiv1998

Magnetic field independence of the spin gap in YBa_2Cu_3O_{7-delta}

We report, for magnetic fields of 0, 8.8, and 14.8 Tesla, measurements of the temperature dependent ^{63}Cu NMR spin lattice relaxation rate for near optimally doped YBa_2Cu_3O_{7-delta}, near and above T_c. In sharp contrast with previous work we find no magnetic field dependence. We discuss experimental issues arising in measurements of this required precision, and implications of the experiment regarding issues including the spin or pseudo gap.

cond-mat.str-el↗