J. Pollanen - Academia.edu (original) (raw)
Papers by J. Pollanen
AIP Conference Proceedings, 2006
Porous aerogel is a source of elastic scattering in superfluid 3 He and modifies the properties o... more Porous aerogel is a source of elastic scattering in superfluid 3 He and modifies the properties of the superfluid, suppressing the transition temperature and order parameter. The specific heat jumps for the B-phase of superfluid 3 He in aerogel have been measured as a function of pressure and interpreted using the homogeneous and inhomogeneous isotropic scattering models. The specific heat jumps for other p-wave states are estimated for comparison.
AIP Conference Proceedings, 2006
We have designed an experiment to study the role of global anisotropic quasiparticle scattering o... more We have designed an experiment to study the role of global anisotropic quasiparticle scattering on the dirty aerogel superfluid 3 He system. We observe significant regions of two stable phases at temperatures below the superfluid transition at a pressure of 25 bar for a 98% aerogel.
Silica aerogel is a network of strands with a diameter of 3 nm and average separation xia 30 -- 1... more Silica aerogel is a network of strands with a diameter of 3 nm and average separation xia 30 -- 100 nm. Low-density aerogel can be used to introduce disorder in superfluid ^3He because the superfluid coherence length is of the same order as xia. We have developed novel sample growth and preparation techniques for producing aerogels for a variety of
ABSTRACT We have performed transverse acoustics experiments in superfluid ^3He-B, exploring the m... more ABSTRACT We have performed transverse acoustics experiments in superfluid ^3He-B, exploring the magnetic field splitting of the imaginary squashing mode (ISQ), a collective mode of the order parameter labelled by its total angular momentum J=2. We have compared theoretical calculationsootnotetextJ.A. Sauls and J.W. Serene, Phys. Rev. Lett. 49, 1183 (1982). of the Zeeman splitting, g2^-, and its dependence on the strength of f-wave pairing interactions, x3-1, with our recent experimental data, showing unexpected discrepancies. We suggest that the origin of these discrepancies can be traced to limits on the applicability of the theoretical calculations at high magnetic field and at frequencies some distance from the order parameter collective mode. We discuss the analysis done by Davis et al. in light of those limitations.ootnotetextJ.P. Davis et al., Phys. Rev. Lett. 100, 015301 (2008). This work was supported by the National Science Foundation DMR-1103625.
ABSTRACT It was proposed by Volovik that ^3He-A in aerogel will be a superfluid glass owing to th... more ABSTRACT It was proposed by Volovik that ^3He-A in aerogel will be a superfluid glass owing to the Larkin-Imry-Ma (LIM) effect where arbitrarily small amounts of disorder can disrupt long range directional coherence of a vector order parameter in a condensed system. Several reports of NMR experiments in ^3He-A have been interpreted as evidence for this phenomenon. However it is not trivial to distinguish the LIM effect induced from disorder on a microscopic scale from macroscopic non-uniformity or anisotropy in the aerogel sample. Order parameter disorder from these two possible mechanisms have very different distributions of order parameter orientations directly observable in the width of the NMR spectrum. If a complete LIM effect is operative there should be no contribution to the line width, contrary to previous reports. On warming from the low temperature isotropic state, we find NMR spectrum shifts characteristic of the dipole-locked axial state, i.e. no sign of a LIM superfluid glass. However, on cooling from the normal state this same phase is fully disordered in a LIM state. We will discuss the origin of the different order parameter structures in superfluid ^3He-A that result when prepared from the normal state, as compared with warming from the B-phase.
Physical Review B, 2007
In the Ginzburg-Landau theory of superfluid 3 He, the free energy is expressed as an expansion of... more In the Ginzburg-Landau theory of superfluid 3 He, the free energy is expressed as an expansion of invariants of a complex order parameter. Strong coupling effects, which increase with increasing pressure, are embodied in the set of coefficients of these order parameter invariants 1,2 . Experiments can be used to determine four independent combinations of the coefficients of the five fourth order invariants. This leaves the phenomenological description of the thermodynamics near Tc incomplete. Theoretical understanding of these coefficients is also quite limited. We analyze our measurements of the magnetic susceptibility and the NMR frequency shift in the B-phase which refine the four experimental inputs to the phenomenological theory. We propose a model based on existing experiments, combined with calculations by Sauls and Serene 3 of the pressure dependence of these coefficients, in order to determine all five fourth order terms. This model leads us to a better understanding of the thermodynamics of superfluid 3 He in its various states. We discuss the surface tension of bulk superfluid 3 He and predictions for novel states of the superfluid such as those that are stabilized by elastic scattering of quasiparticles from a highly porous silica aerogel.
Physical Review Letters, 2014
In recent work it was shown that new anisotropic p-wave states of superfluid 3 He can be stabiliz... more In recent work it was shown that new anisotropic p-wave states of superfluid 3 He can be stabilized within high porosity silica aerogel under uniform positive strain . In contrast, the equilibrium phase in an unstrained aerogel, is the isotropic superfluid B-phase [2]. Here we report that this phase stability depends on the sign of the strain. For negative strain of ∼20% achieved by compression, the B-phase can be made more stable than the anisotropic A-phase resulting in a tricritical point for A, B, and normal phases with a critical field of ∼ 100 mT. From pulsed NMR measurements we identify these phases and the orientation of the angular momentum.
Physical Review Letters, 2006
Precision measurements of collective mode frequencies in superfluid 3 He-B are sensitive to quasi... more Precision measurements of collective mode frequencies in superfluid 3 He-B are sensitive to quasiparticle and f -wave pairing interactions. Measurements were performed at various pressures using interference of transverse sound in an acoustic cavity. We fit the measured collective mode frequencies, which depend on the strength of f -wave pairing and the Fermi liquid parameter F s 2 , to theoretical predictions and discuss what implications these values have for observing new order parameter collective modes.
Physical Review Letters, 2006
High resolution measurements of the specific heat of liquid 3 He in the presence of a silver surf... more High resolution measurements of the specific heat of liquid 3 He in the presence of a silver surface have been performed at temperatures near the superfluid transition in the pressure range of 1 to 29 bar. The surface contribution to the heat capacity is identified with Andreev bound states of 3 He quasiparticles that have a range of half a coherence length.
Physical Review Letters, 2005
We have measured the 14 N(p, γ) 15 O excitation function for energies in the range E lab p = 155-... more We have measured the 14 N(p, γ) 15 O excitation function for energies in the range E lab p = 155-524 keV. Fits of these data using R-matrix theory yield a value for the S-factor at zero energy of 1.64 ± 0.07 (stat) ±0.15 (sys) keV·b, which is significantly smaller than the result of a previous direct measurement. The corresponding reduction in the stellar reaction rate for 14 N(p, γ) 15 O has a number of interesting consequences, including an impact on estimates for the age of the Galaxy derived from globular clusters.
Physical Review Letters, 2008
We present the first measurements of the attenuation of transverse sound in superfluid 3 He-B. We... more We present the first measurements of the attenuation of transverse sound in superfluid 3 He-B. We use fixed path length interferometry combined with the magneto-acoustic Faraday effect to vary the effective path length by a factor of two, resulting in absolute values of the attenuation. We find that attenuation is significantly larger than expected from the theoretical dispersion relation, in contrast to the phase velocity of transverse sound. We suggest that the anomalous attenuation can be explained by surface Andreev bound states.
Physical Review Letters, 2008
We have used the acoustic Faraday effect in superfluid 3 He to perform high resolution spectrosco... more We have used the acoustic Faraday effect in superfluid 3 He to perform high resolution spectroscopy of an excited state of the superfluid condensate, called the imaginary squashing mode. With acoustic cavity interferometry we measure the rotation of the plane of polarization of a transverse sound wave propagating in the direction of the magnetic field from which we determine the Zeeman energy of the mode. We interpret the Landé g factor, combined with the zero-field energies of this excited state, using the theory of Sauls and Serene, to calculate the strength of f-wave interactions in 3 He.
Physical Review Letters, 2011
Motivated by the recent prediction that uniaxially compressed aerogel can stabilize the anisotrop... more 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 Tc, 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 C, 2002
The 14N(p,gamma)15O reaction regulates the energy produced by the CN cycle in main-sequence stars... more The 14N(p,gamma)15O reaction regulates the energy produced by the CN cycle in main-sequence stars and in red giants. Recently, preliminary evidence was presented for a new resonance in this reaction, which would significantly increase the reaction rate for temperatures near 108 K. We have attempted to confirm this result and find no indication of a resonance near Elabp=127 keV. Our
Physical Review B, 2008
It has been suggested that anisotropic quasiparticle scattering will stabilize anisotropic phases... more It has been suggested that anisotropic quasiparticle scattering will stabilize anisotropic phases of superfluid 3 He contained within highly porous silica aerogel. For example, global anisotropy introduced via uniaxial compression of aerogel might stabilize the axial state, which is called the A-phase in bulk superfluid 3 He. Here we present measurements of the phase diagram of superfluid 3 He in a 98% porous silica aerogel using transverse acoustic impedance methods. We show that uniaxial compression of the aerogel by 17% does not stabilize an axial phase.
New Journal of Physics, 2008
We have performed measurements of the temperature dependence of the magnetoresistance up to 9 T i... more We have performed measurements of the temperature dependence of the magnetoresistance up to 9 T in bulk single crystals of UPt 3 with the magnetic field along the b-axis, the easy magnetization axis. We have confirmed previous results for transverse magnetoresistance with the current along the caxis, and report measurements of the longitudinal magnetoresistance with the current along the b-axis. The presence of a linear term in both cases indicates broken orientational symmetry associated with magnetic order. With the current along the c-axis the linear term appears near 5 K, increasing rapidly with decreasing temperature. For current along the b-axis the linear contribution is negative.
Nature Physics, 2008
Order parameter collective modes are the fingerprint of a condensed phase. The spectroscopy of th... more Order parameter collective modes are the fingerprint of a condensed phase. The spectroscopy of these modes in superfluid 3 He and unconventional superconductors can provide key information on the symmetry of the condensate as well as the microscopic pairing mechanism responsible for the ground state and excitation energies. We report the discovery of a new collective mode in superfluid 3 He-B which we identify as an excited bound state of Cooper pairs. We use interferometry within an acoustic cavity that is very sensitive to changes in the velocity of transverse sound. Our measurements of sound velocity and mode frequency, together with the observation of acoustic birefringence indicate that this new mode is weakly bound with an excitation energy within 1% of the pair-breaking edge of 2∆. Based on the selection rules for coupling of transverse sound to a collective mode in 3 He-B, combined with the observation of acoustic birefringence near the collective mode frequency, we infer that the new mode is most likely a spin-triplet (S = 1), f -wave pair exciton (L = 3) with total angular momentum, J = 4. The existence of a pair exciton with J = 4 suggests an attractive, sub-dominant, f -wave pairing interaction in liquid 3 He.
Journal of the Physical Society of Japan, 2008
The discovery of superfluid 3 He in high porosity silica aerogels, and subsequent experimental an... more The discovery of superfluid 3 He in high porosity silica aerogels, and subsequent experimental and theoretical work, have led to a better general understanding of quasiparticle scattering from impurities in unconventional pairing systems. It is immensely helpful for understanding impurity effects in the case of superfluid 3 He that the structure of its order parameter is well-established. An overview of impurity effects is presented with emphasis on those experiments which have a quantitative interpretation in terms of theoretical models for homogeneous and inhomogeneous scattering. The latter can account successfully for most experimental results.
AIP Conference Proceedings, 2006
Porous aerogel is a source of elastic scattering in superfluid 3 He and modifies the properties o... more Porous aerogel is a source of elastic scattering in superfluid 3 He and modifies the properties of the superfluid, suppressing the transition temperature and order parameter. The specific heat jumps for the B-phase of superfluid 3 He in aerogel have been measured as a function of pressure and interpreted using the homogeneous and inhomogeneous isotropic scattering models. The specific heat jumps for other p-wave states are estimated for comparison.
AIP Conference Proceedings, 2006
We have designed an experiment to study the role of global anisotropic quasiparticle scattering o... more We have designed an experiment to study the role of global anisotropic quasiparticle scattering on the dirty aerogel superfluid 3 He system. We observe significant regions of two stable phases at temperatures below the superfluid transition at a pressure of 25 bar for a 98% aerogel.
Silica aerogel is a network of strands with a diameter of 3 nm and average separation xia 30 -- 1... more Silica aerogel is a network of strands with a diameter of 3 nm and average separation xia 30 -- 100 nm. Low-density aerogel can be used to introduce disorder in superfluid ^3He because the superfluid coherence length is of the same order as xia. We have developed novel sample growth and preparation techniques for producing aerogels for a variety of
ABSTRACT We have performed transverse acoustics experiments in superfluid ^3He-B, exploring the m... more ABSTRACT We have performed transverse acoustics experiments in superfluid ^3He-B, exploring the magnetic field splitting of the imaginary squashing mode (ISQ), a collective mode of the order parameter labelled by its total angular momentum J=2. We have compared theoretical calculationsootnotetextJ.A. Sauls and J.W. Serene, Phys. Rev. Lett. 49, 1183 (1982). of the Zeeman splitting, g2^-, and its dependence on the strength of f-wave pairing interactions, x3-1, with our recent experimental data, showing unexpected discrepancies. We suggest that the origin of these discrepancies can be traced to limits on the applicability of the theoretical calculations at high magnetic field and at frequencies some distance from the order parameter collective mode. We discuss the analysis done by Davis et al. in light of those limitations.ootnotetextJ.P. Davis et al., Phys. Rev. Lett. 100, 015301 (2008). This work was supported by the National Science Foundation DMR-1103625.
ABSTRACT It was proposed by Volovik that ^3He-A in aerogel will be a superfluid glass owing to th... more ABSTRACT It was proposed by Volovik that ^3He-A in aerogel will be a superfluid glass owing to the Larkin-Imry-Ma (LIM) effect where arbitrarily small amounts of disorder can disrupt long range directional coherence of a vector order parameter in a condensed system. Several reports of NMR experiments in ^3He-A have been interpreted as evidence for this phenomenon. However it is not trivial to distinguish the LIM effect induced from disorder on a microscopic scale from macroscopic non-uniformity or anisotropy in the aerogel sample. Order parameter disorder from these two possible mechanisms have very different distributions of order parameter orientations directly observable in the width of the NMR spectrum. If a complete LIM effect is operative there should be no contribution to the line width, contrary to previous reports. On warming from the low temperature isotropic state, we find NMR spectrum shifts characteristic of the dipole-locked axial state, i.e. no sign of a LIM superfluid glass. However, on cooling from the normal state this same phase is fully disordered in a LIM state. We will discuss the origin of the different order parameter structures in superfluid ^3He-A that result when prepared from the normal state, as compared with warming from the B-phase.
Physical Review B, 2007
In the Ginzburg-Landau theory of superfluid 3 He, the free energy is expressed as an expansion of... more In the Ginzburg-Landau theory of superfluid 3 He, the free energy is expressed as an expansion of invariants of a complex order parameter. Strong coupling effects, which increase with increasing pressure, are embodied in the set of coefficients of these order parameter invariants 1,2 . Experiments can be used to determine four independent combinations of the coefficients of the five fourth order invariants. This leaves the phenomenological description of the thermodynamics near Tc incomplete. Theoretical understanding of these coefficients is also quite limited. We analyze our measurements of the magnetic susceptibility and the NMR frequency shift in the B-phase which refine the four experimental inputs to the phenomenological theory. We propose a model based on existing experiments, combined with calculations by Sauls and Serene 3 of the pressure dependence of these coefficients, in order to determine all five fourth order terms. This model leads us to a better understanding of the thermodynamics of superfluid 3 He in its various states. We discuss the surface tension of bulk superfluid 3 He and predictions for novel states of the superfluid such as those that are stabilized by elastic scattering of quasiparticles from a highly porous silica aerogel.
Physical Review Letters, 2014
In recent work it was shown that new anisotropic p-wave states of superfluid 3 He can be stabiliz... more In recent work it was shown that new anisotropic p-wave states of superfluid 3 He can be stabilized within high porosity silica aerogel under uniform positive strain . In contrast, the equilibrium phase in an unstrained aerogel, is the isotropic superfluid B-phase [2]. Here we report that this phase stability depends on the sign of the strain. For negative strain of ∼20% achieved by compression, the B-phase can be made more stable than the anisotropic A-phase resulting in a tricritical point for A, B, and normal phases with a critical field of ∼ 100 mT. From pulsed NMR measurements we identify these phases and the orientation of the angular momentum.
Physical Review Letters, 2006
Precision measurements of collective mode frequencies in superfluid 3 He-B are sensitive to quasi... more Precision measurements of collective mode frequencies in superfluid 3 He-B are sensitive to quasiparticle and f -wave pairing interactions. Measurements were performed at various pressures using interference of transverse sound in an acoustic cavity. We fit the measured collective mode frequencies, which depend on the strength of f -wave pairing and the Fermi liquid parameter F s 2 , to theoretical predictions and discuss what implications these values have for observing new order parameter collective modes.
Physical Review Letters, 2006
High resolution measurements of the specific heat of liquid 3 He in the presence of a silver surf... more High resolution measurements of the specific heat of liquid 3 He in the presence of a silver surface have been performed at temperatures near the superfluid transition in the pressure range of 1 to 29 bar. The surface contribution to the heat capacity is identified with Andreev bound states of 3 He quasiparticles that have a range of half a coherence length.
Physical Review Letters, 2005
We have measured the 14 N(p, γ) 15 O excitation function for energies in the range E lab p = 155-... more We have measured the 14 N(p, γ) 15 O excitation function for energies in the range E lab p = 155-524 keV. Fits of these data using R-matrix theory yield a value for the S-factor at zero energy of 1.64 ± 0.07 (stat) ±0.15 (sys) keV·b, which is significantly smaller than the result of a previous direct measurement. The corresponding reduction in the stellar reaction rate for 14 N(p, γ) 15 O has a number of interesting consequences, including an impact on estimates for the age of the Galaxy derived from globular clusters.
Physical Review Letters, 2008
We present the first measurements of the attenuation of transverse sound in superfluid 3 He-B. We... more We present the first measurements of the attenuation of transverse sound in superfluid 3 He-B. We use fixed path length interferometry combined with the magneto-acoustic Faraday effect to vary the effective path length by a factor of two, resulting in absolute values of the attenuation. We find that attenuation is significantly larger than expected from the theoretical dispersion relation, in contrast to the phase velocity of transverse sound. We suggest that the anomalous attenuation can be explained by surface Andreev bound states.
Physical Review Letters, 2008
We have used the acoustic Faraday effect in superfluid 3 He to perform high resolution spectrosco... more We have used the acoustic Faraday effect in superfluid 3 He to perform high resolution spectroscopy of an excited state of the superfluid condensate, called the imaginary squashing mode. With acoustic cavity interferometry we measure the rotation of the plane of polarization of a transverse sound wave propagating in the direction of the magnetic field from which we determine the Zeeman energy of the mode. We interpret the Landé g factor, combined with the zero-field energies of this excited state, using the theory of Sauls and Serene, to calculate the strength of f-wave interactions in 3 He.
Physical Review Letters, 2011
Motivated by the recent prediction that uniaxially compressed aerogel can stabilize the anisotrop... more 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 Tc, 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 C, 2002
The 14N(p,gamma)15O reaction regulates the energy produced by the CN cycle in main-sequence stars... more The 14N(p,gamma)15O reaction regulates the energy produced by the CN cycle in main-sequence stars and in red giants. Recently, preliminary evidence was presented for a new resonance in this reaction, which would significantly increase the reaction rate for temperatures near 108 K. We have attempted to confirm this result and find no indication of a resonance near Elabp=127 keV. Our
Physical Review B, 2008
It has been suggested that anisotropic quasiparticle scattering will stabilize anisotropic phases... more It has been suggested that anisotropic quasiparticle scattering will stabilize anisotropic phases of superfluid 3 He contained within highly porous silica aerogel. For example, global anisotropy introduced via uniaxial compression of aerogel might stabilize the axial state, which is called the A-phase in bulk superfluid 3 He. Here we present measurements of the phase diagram of superfluid 3 He in a 98% porous silica aerogel using transverse acoustic impedance methods. We show that uniaxial compression of the aerogel by 17% does not stabilize an axial phase.
New Journal of Physics, 2008
We have performed measurements of the temperature dependence of the magnetoresistance up to 9 T i... more We have performed measurements of the temperature dependence of the magnetoresistance up to 9 T in bulk single crystals of UPt 3 with the magnetic field along the b-axis, the easy magnetization axis. We have confirmed previous results for transverse magnetoresistance with the current along the caxis, and report measurements of the longitudinal magnetoresistance with the current along the b-axis. The presence of a linear term in both cases indicates broken orientational symmetry associated with magnetic order. With the current along the c-axis the linear term appears near 5 K, increasing rapidly with decreasing temperature. For current along the b-axis the linear contribution is negative.
Nature Physics, 2008
Order parameter collective modes are the fingerprint of a condensed phase. The spectroscopy of th... more Order parameter collective modes are the fingerprint of a condensed phase. The spectroscopy of these modes in superfluid 3 He and unconventional superconductors can provide key information on the symmetry of the condensate as well as the microscopic pairing mechanism responsible for the ground state and excitation energies. We report the discovery of a new collective mode in superfluid 3 He-B which we identify as an excited bound state of Cooper pairs. We use interferometry within an acoustic cavity that is very sensitive to changes in the velocity of transverse sound. Our measurements of sound velocity and mode frequency, together with the observation of acoustic birefringence indicate that this new mode is weakly bound with an excitation energy within 1% of the pair-breaking edge of 2∆. Based on the selection rules for coupling of transverse sound to a collective mode in 3 He-B, combined with the observation of acoustic birefringence near the collective mode frequency, we infer that the new mode is most likely a spin-triplet (S = 1), f -wave pair exciton (L = 3) with total angular momentum, J = 4. The existence of a pair exciton with J = 4 suggests an attractive, sub-dominant, f -wave pairing interaction in liquid 3 He.
Journal of the Physical Society of Japan, 2008
The discovery of superfluid 3 He in high porosity silica aerogels, and subsequent experimental an... more The discovery of superfluid 3 He in high porosity silica aerogels, and subsequent experimental and theoretical work, have led to a better general understanding of quasiparticle scattering from impurities in unconventional pairing systems. It is immensely helpful for understanding impurity effects in the case of superfluid 3 He that the structure of its order parameter is well-established. An overview of impurity effects is presented with emphasis on those experiments which have a quantitative interpretation in terms of theoretical models for homogeneous and inhomogeneous scattering. The latter can account successfully for most experimental results.