W. P. Halperin

Publications (121)203.18 Total impact

• Article: Absence of static orbital current magnetism at the apical oxygen site in HgBa$_2$CuO$_{4+\delta}$ from NMR

  A. M. Mounce, Sangwon Oh, Jeongseop A. Lee, W. P. Halperin, A. P. Reyes, P. L. Kuhns, M. K. Chan, C. Dorow, L. Ji, D. Xia, X. Zhao, M. Greven
  [show abstract] [hide abstract]
  ABSTRACT: The simple structure of HgBa$_2$CuO$_{4+\delta}$ (Hg1201) is ideal among cuprates for study of the pseudogap phase as a broken symmetry state. We have performed $^{17}$O nuclear magnetic resonance (NMR) on an underdoped Hg1201 crystal with transition temperature of 74 K to look for circulating orbital currents proposed theoretically and inferred from neutron scattering. The narrow spectra preclude static local fields in the pseudogap phase at the apical site, suggesting that the moments observed with neutrons are fluctuating. The NMR frequency shifts are consistent with a dipolar field from the Cu$^{+2}$ site.
  04/2013;
• Article: Temperature Dependence of the Magnetic Penetration Depth and Nodal Gap Structure of UPt$_3$ from Small Angle Neutron Scattering

  W. J. Gannon, W. P. Halperin, C. Rastovski, K. J. Schlesinger, J. Hlevyack, C. Steiner, M. R. Eskildsen, J. Gavilano, U. Gasser, G. Nagy
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  ABSTRACT: We report measurements of the temperature dependence of the different components of the London magnetic penetration depth using small angle neutron scattering from the vortex lattice in a high quality crystal of \UP. Deconvolution of the contributions to the neutron scattering form factor from currents along the $a^*$ and $c$-axes gives direct information about the nodal structure of the order parameter. Our observations of linear temperature dependence at low temperatures of all components of the penetration depth support the assignment of E$_{2u}$ symmetry to the superconducting state of \UP.
  02/2013;
• Article: Weak-coupling superconductivity in NaFe$_{0.975}$Co$_{0.025}$As from NMR Knight shift and penetration depth

  Sangwon Oh, A. M. Mounce, J. S. Lee, W. P. Halperin, C. L. Zhang, S. Carr, Pengcheng Dai
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  ABSTRACT: Based on ARPES and STM work, Co doped NaFeAs has been reported to be a strongly coupled superconductor, similar to cuprates. However, a recent ARPES result, claims otherwise, with $2\Delta/k_BT_c$ slightly beyond the BCS limit, 3.53. We have performed $^{75}$As and $^{11}$Na nuclear magnetic resonance(NMR) on high-quality, single crystals of NaFe$_{0.975}$Co$_{0.025}$As. The temperature dependence of the $^{75}$As Knight shift, fitted to a Yosida function, suggests moderately weak-coupling superconductivity in a singlet state with $2\Delta/k_BT_c = 4.0\pm0.3$. The temperature dependence of the penetration depth obtained from $^{11}$Na NMR was found to be consistent with the Knight shift results and $\lambda_{ab}(0)$ was found to be $5,327 \pm 78 \AA$.
  02/2013;
• Article: Magnetization in the Superconducting State of UPt$_3$ from Polarized Neutron Diffraction

  W. J. Gannon, W. P. Halperin, C. Rastovski, M. R. Eskildsen, Pengcheng Dai, A. Stunault
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  ABSTRACT: The heavy fermion superconductor UPt$_3$ is thought to have odd-parity, a state for which the temperature dependence of the spin susceptibility is an important signature. In order to address conflicting reports from two different experiments, the NMR Knight shift and measurements of the anisotropy of the upper critical field, we have measured the bulk susceptibility in a high quality single crystal using polarized-neutron diffraction. A temperature independent susceptibility was observed for $H||a$ through the transitions between the normal state and the superconducting A-, B- and C-phases, consistent with odd-parity, spin-triplet superconductivity.
  08/2012;
• Article: Zeeman splitting and nonlinear field-dependence in superfluid 3He

  C. A. Collett, J. Pollanen, J. I. A. Li, W. J. Gannon, W. P. Halperin
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  ABSTRACT: We have studied the acoustic Faraday effect in superfluid 3He up to significantly larger magnetic fields than in previous experiments achieving rotations of the polarization of transverse sound as large as 1710^{\circ}. We report nonlinear field effects, and use the linear results to determine the Zeeman splitting of the imaginary squashing mode (ISQ) frequency in 3He-B.
  08/2012;
• Article: Nonlinear field-dependence and f-wave interactions in superfluid 3He

  C. A. Collett, J. Pollanen, J. I. A. Li, W. J. Gannon, W. P. Halperin
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  ABSTRACT: We present results of transverse acoustics studies in superfluid ^{3}He-B at fields up to 0.11 T. Using acoustic cavity interferometry, we observe the Acoustic Faraday Effect for a transverse sound wave propagating along the magnetic field, and we measure Faraday rotations of the polarization as large as 1710^{\circ}. We use these results to determine the Zeeman splitting of the Imaginary Squashing mode, an order parameter collective mode with total angular momentum J=2. We show that the pairing interaction in the f-wave channel is attractive at a pressure of P=6 bar. We also report nonlinear field dependence of the Faraday rotation at frequencies substantially above the mode frequency not accounted for in the theory of the transverse acoustic dispersion relation formulated for frequencies near the mode. Consequently, we have identified the region of validity of the theory allowing us to make corrections to the analysis of Faraday rotation experiments performed in earlier work.
  08/2012;
• Source

  Available from: arxiv.org

  Article: Mass Coupling and Q−1 of Impurity-Limited Normal 3He in a Torsion Pendulum

  R. G. Bennett, N. Zhelev, A. D. Fefferman, K. Y. Fang, J. Pollanen, P. Sharma, W. P. Halperin, J. M. Parpia
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  ABSTRACT: We present results of the Q −1 and period shift, ΔP, for 3He confined in a 98% nominal open aerogel on a torsion pendulum. The aerogel is compressed uniaxially by 10% along a direction aligned to the torsion pendulum axis and was grown within a 400μm tall pancake (after compression) similar to an Andronikashvili geometry. The result is a high Q pendulum able to resolve Q −1 and mass coupling of the impurity-limited 3He over the whole temperature range. After measuring the empty cell background, we filled the cell above the critical point and observe a temperature dependent period shift, ΔP, between 100mK and 3mK that is 2.9% of the period shift (after filling) at 100mK. The Q −1 due to the 3He decreases by an order of magnitude between 100mK and 3mK at a pressure of 0.14±0.03bar. We compare the observable quantities to the corresponding calculated Q −1 and period shift for bulk 3He. KeywordsImpurity limited transport–Normal 3He
  Journal of Low Temperature Physics 04/2012; 162(3):174-181. · 1.19 Impact Factor
• Source

  Available from: J. A. Sauls

  Article: New Chiral Phases of Superfluid 3He Stabilized by Anisotropic Silica Aerogel

  J. Pollanen, J. I. A. Li, C. A. Collett, W. J. Gannon, W. P. Halperin, J. A. Sauls
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  ABSTRACT: A rich variety of Fermi systems condense by forming bound pairs, including high temperature [1] and heavy fermion [2] superconductors, Sr2RuO4 [3], cold atomic gases [4], and superfluid 3He [5]. Some of these form exotic quantum states having non-zero orbital angular momentum. We have discovered, in the case of 3He, that anisotropic disorder, engineered from highly porous silica aerogel, stabilizes a chiral superfluid state that otherwise would not exist. Additionally, we find that the chiral axis of this state can be uniquely oriented with the application of a magnetic field perpendicular to the aerogel anisotropy axis. At suffciently low temperature we observe a sharp transition from a uniformly oriented chiral state to a disordered structure consistent with locally ordered domains, contrary to expectations for a superfluid glass phase [6].
  01/2012;
• Source

  Available from: ArXiv

  Article: Nuclear Magnetic Resonance Studies of Vortices in High Temperature Superconductors

  A. M. Mounce, S. Oh, W. P. Halperin
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  ABSTRACT: The distinct distribution of local magnetic fields due to superconducting vortices can be detected with nuclear magnetic resonance (NMR) and used to investigate vortices and related physical properties of extreme type II superconductivity. This review summarizes work on high temperature superconductors (HTS) including cuprates and pnictide materials. Recent experimental results are presented which reveal the nature of vortex matter and novel electronic states. For example, the NMR spectrum has been found to provide a sharp indication of the vortex melting transition. In the vortex solid a frequency dependent spin-lattice relaxation has been reported in cuprates, including YBa$_2$Cu$_3$O$_{7-x}$, Bi$_2$SrCa$_2$Cu$_2$O$_{8+\delta}$, and Tl$_2$Ba$_2$CuO$_{6+\delta}$. These results have initiated a new spectroscopy via Doppler shifted nodal quasiparticles for the investigation of vortices. At very high magnetic fields this approach is a promising method for the study of vortex core excitations. These measurements have been used to quantify an induced spin density wave near the vortex cores in Bi$_2$SrCa$_2$Cu$_2$O$_{8+\delta}$. Although the cuprates have a different superconducting order parameter than the iron arsenide superconductors there are, nonetheless, some striking similarities between them regarding vortex dynamics and frequency dependent relaxation.
  12/2011;
• Source

  Available from: ArXiv

  Article: Modification of the 3He phase diagram by anisotropic disorder.

  R G Bennett, N Zhelev, E N Smith, J Pollanen, W P Halperin, J M Parpia
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  ABSTRACT: Motivated by the recent prediction that uniaxially compressed aerogel can stabilize the anisotropic A phase over the isotropic B phase, we measure the pressure dependent superfluid fraction of (3)He entrained in 10% axially compressed, 98% porous aerogel. We observe that a broad region of the temperature-pressure phase diagram is occupied by the metastable A phase. The reappearance of the A phase on warming from the B phase, before superfluidity is extinguished at T(c), is in contrast to its absence in uncompressed aerogel. The phase diagram is modified from that of pure (3)He, with the disappearance of the polycritical point (PCP) and the appearance of a region of A phase extending below the PCP of bulk (3)He, even in zero applied magnetic field. The expected alignment of the A phase texture by compression is not observed.
  Physical Review Letters 12/2011; 107(23):235504. · 7.37 Impact Factor
• Source

  Available from: ArXiv

  Article: Identification of superfluid phases of 3He in uniformly isotropic 98.2% aerogel.

  J Pollanen, J I A Li, C A Collett, W J Gannon, W P Halperin
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  ABSTRACT: Superfluid ^{3}He confined to high porosity silica aerogel is the paradigm system for understanding impurity effects in unconventional superconductors. However, a crucial first step has been elusive: exact identification of the microscopic states of the superfluid in the presence of quenched disorder. Using a new class of highly uniform aerogel materials, we report pulsed nuclear magnetic resonance experiments that demonstrate definitively that the two observed superfluid states in aerogel are impure versions of the isotropic and axial p-wave states. The theoretically predicted destruction of long-range orbital order (Larkin-Imry-Ma effect) in the impure axial state is not observed.
  Physical Review Letters 11/2011; 107(19):195301. · 7.37 Impact Factor
• Source

  Available from: Sangwon Oh

  Article: Magnetic field dependence of spin-lattice relaxation in the s_ {±} state of Ba_ {0.67} K_ {0.33} Fe_ {2} As_ {2}

  Sangwon Oh, A. M. Mounce, W. P. Halperin, C. L. Zhang, Pengcheng Dai, A. P. Reyes, P. L. Kuhns
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  ABSTRACT: The spatially averaged density of states 〈N(0)〉 of an unconventional d-wave superconductor is magnetic field dependent, proportional to H1/2, owing to the Doppler shift of quasiparticle excitations in a background of vortex supercurrents [ G. E. Volovik J. Phys. C 21 L221 (1988); JETP Lett. 58 469 (1993)]. This phenomenon, called the Volovik effect, has been predicted to exist for a sign-changing s± state [ Y. Bang Phys. Rev. Lett. 104 217001 (2010)], although it is absent in a single-band s-wave superconductor. Consequently, we expect there to be Doppler contributions to the NMR spin-lattice relaxation rate, 1/T1∝〈N(0)2〉, for an s± state which will depend on the magnetic field. We have measured the 75As 1/T1 in a high-quality single crystal of Ba0.67K0.33Fe2As2 over a wide range of field up to 28 T. Our spatially resolved measurements show that indeed there are Doppler contributions to 1/T1 which increase closer to the vortex core, with a spatial average proportional to H2, inconsistent with recent theory [ Y. Bang Phys. Rev. B 85 104524 (2012)].
  Phys. Rev. B. 09/2011; 85(17).
• Article: Spin-density wave near the vortex cores in the high-temperature superconductor Bi2Sr2CaCu2O8+y.

  A M Mounce, S Oh, S Mukhopadhyay, W P Halperin, A P Reyes, P L Kuhns, K Fujita, M Ishikado, S Uchida
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  ABSTRACT: Competition with magnetism is at the heart of high-temperature superconductivity, most intensely felt near a vortex core. To investigate vortex magnetism we have developed a spatially resolved probe based upon NMR spin-lattice-relaxation spectroscopy. With this approach we have found a spin-density wave associated with the vortex core in Bi(2)Sr(2)CaCu(2)O(8+y), similar to checkerboard patterns in the local density of electronic states reported from scanning tunneling microscope experiments. We have determined both the spin-modulation amplitude and decay length from the vortex core in fields up to H=30  T.
  Physical Review Letters 02/2011; 106(5):057003. · 7.37 Impact Factor
• Source

  Available from: Sangwon Oh

  Article: $^{75}$As NMR of Ba(Fe$_{0.93}$Co$_{0.07}$)$_{2}$As$_{2}$ in High Magnetic Field

  Sangwon Oh, A. M. Mounce, S. Mukhopadhyay, W. P. Halperin, A. B. Vorontsov, S. L. Bud'ko, P. C. Canfield, Y. Furukawa, A. P. Reyes, P. L. Kuhns
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  ABSTRACT: The superconducting state of an optimally doped single crystal of Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$ was investigated by $^{75}$As NMR in high magnetic fields from 6.4 T to 28 T. It was found that the Knight shift is least affected by vortex supercurrents in high magnetic fields, $H>11$ T, revealing slow, possibly higher order than linear, increase with temperature at $T \lesssim 0.5 \, T_c$, with $T_c \approx 23 \, K$. This is consistent with the extended s-wave state with $A_{1g}$ symmetry but the precise details of the gap structure are harder to resolve. Measurements of the NMR spin-spin relaxation time, $T_2$, indicate a strong indirect exchange interaction at all temperatures. Below the superconducting transition temperature vortex dynamics lead to an anomalous dip in $T_2$ at the vortex freezing transition from which we obtain the vortex phase diagram up to $H = 28$ T.
  Phys. Rev. B. 01/2011; 83(21).
• Source

  Available from: ArXiv

  Article: Spin-Density Wave near the Vortex Cores of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$

  A. M. Mounce, S. Oh, S. Mukhopadhyay, W. P. Halperin, A. P. Reyes, P. L. Kuhns, K Fujita, M. Ishikado, S Uchida
  [show abstract] [hide abstract]
  ABSTRACT: Competition with magnetism is at the heart of high temperature superconductivity, most intensely felt near a vortex core. To investigate vortex magnetism we have developed a spatially resolved probe using nuclear magnetic resonance. Our spin-lattice-relaxation spectroscopy is spatially resolved both within a conduction plane as well as from one plane to another. With this approach we have found a spin-density wave associated with the vortex core in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$, which is expected from scanning tunneling microscope observations of "checkerboard" patterns in the local density of electronic states.[1] We determine both the spin-modulation amplitude and decay length from the vortex core in fields up to H=30 T. Comment: 7 pages, 5 figures
  11/2010;
• Source

  Available from: ArXiv

  Article: Mass coupling and $Q^{-1} of impurity-limited normal $^3$He in a torsion pendulum

  R. G. Bennett, N. Zhelev, A. D. Fefferman, K. Y. Fang, J. Pollanen, P. Sharma, W. P. Halperin, J M Parpia
  [show abstract] [hide abstract]
  ABSTRACT: We present results of the $Q^{-1}$ and period shift, $\Delta P$, for $^3$He confined in a 98% nominal open aerogel on a torsion pendulum. The aerogel is compressed uniaxially by 10% along a direction aligned to the torsion pendulum axis and was grown within a 400 $\mu$m tall pancake (after compression) similar to an Andronikashvili geometry. The result is a high $Q$ pendulum able to resolve $Q^{-1}$ and mass coupling of the impurity-limited $^3$He over the whole temperature range. After measuring the empty cell background, we filled the cell above the critical point and observe a temperature dependent period shift, $\Delta P$, between 100 mK and 3 mK that is 2.9$%$ of the period shift (after filling) at 100 mK. The $Q^{-1}$ due to the $^3$He decreases by an order of magnitude between 100 mK and 3 mK at a pressure of $0.14\pm0.03$ bar. We compare the observable quantities to the corresponding calculated $Q^{-1}$ and period shift for bulk $^3$He. Comment: 8 pages, 3 figures
  10/2010;
• Source

  Available from: ArXiv

  Article: Charge Induced Vortex Lattice Instability

  A. M. Mounce, S. Oh, S. Mukhopadhyay, W. P. Halperin, A. P. Reyes, P. L. Kuhns, K Fujita, M. Ishikado, S Uchida
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  ABSTRACT: It has been predicted that superconducting vortices should be electrically charged and that this effect is particularly enhanced for, high temperature superconductors.\cite{kho95,bla96} Hall effect\cite{hag91} and nuclear magnetic resonance (NMR) experiments\cite{kum01} suggest the existence of vortex charging, but the effects are small and the interpretation controversial. Here we show that the Abrikosov vortex lattice, characteristic of the mixed state of superconductors, will become unstable at sufficiently high magnetic field if there is charge trapped on the vortex core. Our NMR measurements of the magnetic fields generated by vortices in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+y}$ single crystals\cite{che07} provide evidence for an electrostatically driven vortex lattice reconstruction with the magnitude of charge on each vortex pancake of $\mathbf{\sim 2}$x$\mathbf{10^{-3} e}$, depending on doping, in line with theoretical estimates.\cite{kho95,kna05} Comment: to appear in Nature Physics; 6 pages, 7 figures
  09/2010;
• Article: The transition between real and complex superconducting order parameter phases in UPt3.

  J D Strand, D J Bahr, D J Van Harlingen, J P Davis, W J Gannon, W P Halperin
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  ABSTRACT: Order parameter symmetry is one of the basic characteristics of a superconductor. The heavy fermion compound UPt3 provides a rich system for studying the competition between superconductivity and other forms of electronic order and exhibits two distinct superconducting phases that are characterized by different symmetries. We fabricated a series of Josephson tunnel junctions on the as-grown surfaces of UPt3 single crystals spanning the a-b plane. By measuring their critical current, we mapped out the magnitude of the superconducting order parameter as a function of the momentum-space direction and temperature. In the high-temperature phase, we observed a sharp node in the superconducting gap at 45 degrees with respect to the a axis; an out-of-phase component appeared in the low-temperature phase, creating a complex order parameter.
  Science 06/2010; 328(5984):1368-9. · 31.20 Impact Factor
• Article: Proceedings of the 2009 International Conference on Quantum Fluids and Solids (QFS2009) - Part I Preface

  Finn Rasmussen, J A Sauls, H Kojima, W P Halperin
  J Low Temp Phys. 01/2010; 158(1-2):1-2.
• Article: Evidence for complex superconducting order parameter symmetry in the low-temperature phase of UPt3 from Josephson interferometry.

  J D Strand, D J Van Harlingen, J B Kycia, W P Halperin
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  ABSTRACT: We present data on the modulation of the critical current with applied magnetic field in UPt3-Cu-Pb Josephson junctions and SQUIDs. The junctions were fabricated on polished surfaces of UPt3 single crystals. The shape of the resulting diffraction patterns provides phase-sensitive information on the superconducting order parameter. Our corner junction data show asymmetric patterns with respect to magnetic field, indicating a complex order parameter, and both our junction and SQUID measurements point to a phase shift of pi, supporting the E(2u) representation of the order parameter.
  Physical Review Letters 11/2009; 103(19):197002. · 7.37 Impact Factor