Search

We analyze the magnetic mode structure of axially-magnetized, finite-length, nanoscopic cylinders in a regime where the exchange interaction dominates, along with simulations of the mode frequencies of the ferrimagnet yttrium iron garnet. For the bulk modes we find that the frequencies can be represented by an expression given by Herring and Kittel by using wavevector components obtained by fitting the mode patterns emerging from these simulations. In addition to the axial, radial, and azimuthal modes that are present in an infinite cylinder, we find localized "cap modes" that are "trapped" at the top and bottom cylinder faces by the inhomogeneous dipole field emerging from the ends. Semi-quantitative explanations are given for some of the modes in terms of a one-dimensional Schrodinger equation which is valid in the exchange dominant case. The assignment of the azimuthal mode number is carefully discussed and the frequency splitting of a few pairs of nearly degenerate modes is determined through the beat pattern emerging from them.

Magnetization reversals in magnetic recording media are largely carried out by brute force: a field is applied opposite to the existing magnetization direction of some bit that has sufficient magnitude to nucleate a seed that then grows into an oppositely magnetized bit. The fields used are generally quite large, $\sim 10$ kG, requiring elaborate magnetic circuitry to keep the fields localized so they do not spill over onto neighboring bits. This situation is to be contrasted with the resonant magnetization reversals performed in NMR spin echo experiments in which r.f. fields of a few Gauss coherently reverse the magnetization in the presence of static fields of a few kG, by applying a so-called Pi pulse; two such pulses restores the original alignment.

Stuck in the middle with you: The spin-wave mode localized in the contact area among macrospins in an artificial spin ice (ASI) is interesting for magnonic devices and applications. Detection and characterization of this mode is difficult, though. This study uses carefully designed lattices of individual macrospins with the right aspect ratio of individual nanoelements, the right geometry of macrospins on an ASI vertex, and the right configuration for clear detection of the spin dynamics. The localized mode is found to exhibit a peculiar sensitivity to the applied field's direction, which could impact applications such as magnonic position transducers.

We have observed the propagation of spin waves across a thin yttrium iron garnet film on (1 1 1) gadolinium gallium garnet for magnetic fields inclined with respect to the film plane. Two principle planes were studied: that for H in the plane defined by the wave vector k and the plane normal, n , with limiting forms corresponding to the Backward Volume and Forward Volume modes, and that for H in the plane perpendicular to k , with limiting forms corresponding to the Damon-Eshbach and Forward Volume modes. By exciting the wave at one edge of the film and observing the field dependence of the phase of the received signal at the opposing edge we determined the frequency vs. wavevector relation, ω = ω ( k ), of various propagating modes in the film. Avoided crossings are observed in the Damon-Eshbach and Forward Volume regimes when the propagating mode intersects the higher, exchange split, volume modes, leading to an extinction of the propagating mode; analysis of the resulting behavior allows a determination of the exchange parameter. The experimental results are compared with theoretical simulations.

Using a many spin micromagnetic simulation tool that directly integrates the Landau-Lifshitz equation, we demonstrate that by applying an r.f. pulse, generally referred to as a Pi pulse, it is possible to near-perfectly reverse the direction of the magnetization in a ferromagnet, provided that the sample is sufficiently small and the angular dependence of the precession frequency is continuously matched using an appropriately “chirped” r.f. pulse of the proper length. Simulations are carried out for “prolate” uniaxially symmetric cylindrical samples in the presence of dipole and exchange interactions. Such reversals can be performed in the presence of a static external magnetic field or, importantly, at zero field under the sample’s own internal demagnetization field. However, the ability to perform near-perfect Pi or two-Pi rotations is lost for samples above certain dimensions for which additional internal degrees of freedom are excited, particularly at higher static fields. In such larger samples the magnetization may still be reversed by utilizing damping, provided it can be rotated past a critical angle.

We report experimental and theoretical results on the current gain in superconductor-ferromagnetic transistors (SFTs) with the SISFIFS structure [where S, I, and F denote a superconductor (Nb), an insulator ( ${\rm{AlO}}_{x}$ ), and a ferromagnetic material (Ni), respectively]. The Josephson critical current density $j_{{\rm{ca}}}$ of the acceptor (SISF) junction in the devices is above 9 kA/cm2, which is higher than that in our previous devices. The critical current gain is defined as $ \vert \delta I_{{\rm{ca}}}\vert / \vert \delta I_{i}\vert $ , where a change $\vert \delta I_{i}\vert $ in the injector (SFIFS junction) current produces a change $\vert \delta I_{{\rm{ca}}}\vert $ in the acceptor maximum Josephson current. We observed a small-signal gain as high as 7.8 and a large-signal current gain of about 2.2. In addition, we found that the Josephson current of the acceptor junction is sensitive to the state of the ferromagnetic layers. A theoretical model is proposed to describe the nonequilibrium processes in the SFT devices, which agrees with the experimental observations. The calculation shows that, in the present device configuration, the dominant contribution to the gap suppression in the middle Nb electrode is due to the quasiparticle injection; spin injection plays a secondary role. We demonstrate that proper device engineering allows one to efficiently control the maximum Josephson current in the SISF acceptor junction using the quasiparticle injection. We conclude that SFT devices can be used as input/output isolators and amplifiers for memory, digital, and RF applications.

We experimentally and theoretically characterize the angular-dependent microwave response of three-macrospin-vertex structures that can serve as a node in various spin ice lattices. The macrospins consist of patterned permalloy thin films with an elliptical cross section together with an in-plane aspect ratio allowing an Ising-like behavior together with bulk modes as low-frequency excitations in the field range of interest. Various branches of the frequency-magnetic field curves display atypical behaviors and discontinuities, together with softening due to macrospin reversals. The overall behavior observed accurately corresponds to a superposition of the spectra of the individual macrospins. The measured ferromagnetic resonance spectra are in good agreement with theoretical modeling. In particular, they reveal a close correlation between the field direction (relative to the individual macrospins), and the corresponding frequency-magnetic field curve, i.e., between the geometry and the magnetic response.

We report broadband ferromagnetic resonance measurements of the in-plane magnetic field response of three- and four-fold symmetric vertices formed by non-contacting permalloy nano-ellipses together with extended lattices constructed from them. Complementing the experimental data with simulations, we are able to show that, as far as the most intense FMR responses are concerned, the spectra of vertices and lattices can largely be interpreted in terms of a superposition of the underlying hysteretic responses of the individual ellipses, as elemental building blocks of the system. This property suggest that it is possible to understand the orientation of the individual magnetic dipole moments in a dipole network in terms of dynamic measurements alone, thereby offering a powerful tool to analyze the alignment statistics in frustrated systems that are exposed to various magnetic histories.

Excitations of free electrons and optical phonons are known to permit access to the negative real part of relative permittivities (ϵ^{'}

The rich physics associated with magnetism often centers around directional effects. Here we demonstrate how spin-transfer torques in general result in unidirectional ferromagnetic resonance dynamics upon field reversal. The unidirectionality is a direct consequence of both field-like and damping-like dynamic torques simultaneously driving the motion. This directional effect arises from the field-like torque being odd and the damping-like torque being even under field reversal. The directional effect is observed when the magnetization has both an in-plane and out-of-plane component, since then the linear combination of the torques rotates with a different handedness around the magnetization as the magnetization is tipped out-of-plane. The effect is experimentally investigated via spin-torque ferromagnetic resonance measurements with the field applied at arbitrary directions away from the interface normal. The measured asymmetry of the voltage spectra are well explained within a phenomenological torque model.

In complex networks of ferromagnetic macrospins, like artificial spin ices or artificial quasi-crystals, much interest has focused on the dynamics of Dirac strings, i.e. the apparently irregular path followed by macrospin reversals, producing a magnetic charge drift across the network.

Previous studies of artificial spin ice have been largely restricted to periodic dot lattices. Ferromagnetic switching of segments in an applied magnetic field is stochastic in periodic spin ice systems, which makes emergent phenomena, such as the formation of vortex loops, hard to control or predict. We fabricated finite, aperiodic Penrose P2 tilings as antidot lattices with fivefold rotational symmetry in permalloy thin films. Measurements of the field dependence of the static magnetization reveal reproducible knee anomalies whose number and form are temperature dependent, which suggests they mark cooperative rearrangements of the tiling magnetic texture. Our micromagnetic simulations of the P2 tiling are in good agreement with experimental magnetization data and exhibit non-stochastic magnetic switching of segments in applied field, and vortex loops that are stable over an extended field interval during magnetic reversal.

We propose an arbitrary pattern lithography process using interference of Bose-Einstein condensates (BECs). A symmetric three-pulse Raman atom interferometer (AI) is used to implement the system. The pattern information, in the form of a phase-only mask, is optically encoded into the BEC order parameter in one of the AI arms. The lithographic pattern is represented by a two-dimensional intensity variation, and is transformed into a two-dimensional phase variation in the BEC order parameter via the use of ac-Stark shift induced by a pulsed laser field. The BEC probability distribution of the interference result at the end of the AI is proportional to the required pattern. In order to produce features smaller than the diffraction limit for the used optical elements, we employ a three-dimensional atomic lens system to scale down the resulting pattern. The operating conditions for this lens structure are investigated in order to identify practical constraints. Simulations of the overall system using the parameters of $^{87}\mathrm{Rb}$ BEC were performed to illustrate its functionality. The proposed process, while perhaps not suitable for general purpose usage, may enable the creation of special purpose patterns on a very small scale, with features as small as a few nanometers.

We report experiments which characterize spin wave propagation in a thin (111) yttrium iron garnet film for arbitrary angles between the in-plane magnetic field and the mode wavevectors. By measuring the magnetic field evolution of the phase of the wave traveling across the film, we deduce the frequency dependence of the wavevector, the dispersion relation, from which the mode velocity follows. Additionally, we observe multiple nodes in the regime of the propagating Damon-Eshbach mode; these arise from avoided crossings associated with the higher, exchange split, standing wave modes along the film normal, the positions of which correlate with the direct absorption measurements of their positions. This information allows a determination of the exchange parameter. Using this technique, we examine the nonreciprocity in spin wave propagation that results from an adjacent metal layer.We report experiments which characterize spin wave propagation in a thin (111) yttrium iron garnet film for arbitrary angles between the in-plane magnetic field and the mode wavevectors. By measuring the magnetic field evolution of the phase of the wave traveling across the film, we deduce the frequency dependence of the wavevector, the dispersion relation, from which the mode velocity follows. Additionally, we observe multiple nodes in the regime of the propagating Damon-Eshbach mode; these arise from avoided crossings associated with the higher, exchange split, standing wave modes along the film normal, the positions of which correlate with the direct absorption measurements of their positions. This information allows a determination of the exchange parameter. Using this technique, we examine the nonreciprocity in spin wave propagation that results from an adjacent metal layer.

Bulk spin Hall effects are well know to provide spin orbit torques, which can be used to drive magnetization dynamics [1]. But one of the reoccurring questions is to what extend spin orbit torques may also originate at the interface between materials with strong spin orbit coupling and the ferromagnets. Using spin torque driven ferromagnetic resonance we show for two systems, where interfacial torques dominate, that they can be large enough to be practically useful. First, we show spin transfer torque driven magnetization dynamics based on Rashba-Edelstein effects at the Bi/Ag interface [2]. Second, we will show that combining permalloy with monolayer MoS2 gives rise to sizable spin-orbit torques. Given the monolayer nature of MoS2 it is clear that bilk spin Hall effects are negligible and therefore the spin transfer torques are completely interfacial in nature. Interestingly the spin orbit torques with MoS2 show a distinct dependence on the orientation of the magnetization in the permalloy, and become strongly enhanced, when the magnetization is pointing perpendicular to the interfacial plane. This work was supported by the U.S. Department of Energy, Office of Science, Materials Science and Engineering Division. [1] A. Hoffmann, IEEE Trans. Mag. 49, 5172 (2013). [2] W. Zhang et al., J. Appl. Phys. 117, 17C727 (2015). [3] M. B. Jungfleisch et al., arXiv:1508.01410.

This text covers the basic physics of the solid state starting at an elementary level and advancing, in stages. In addition to treating the fundamental elastic, electrical, thermal, magnetic, structural, electronic, transport, optical, mechanical, and compositional properties, the text also discusses topics like superfluidity and superconductivity, along with special topics such as strongly correlated systems, high temperature superconductors, the quantum Hall effects, and graphene. Particular emphasis is given to so-called first principles calculations utilizing modern density functional theory, which for many systems now allow accurate calculations of the electronic, magnetic, and thermal properties.

Devices with multiple insulating ($I$), ferromagnetic ($F$), and superconducting ($S$) layers in various configurations have been studied for some time, both in basic research and to exploit quantum effects for improved electronics, and lately have enjoyed renewed interest. This systematic study addresses Cooper-pair tunneling out of equilibrium in a hybrid $S\phantom{\rule{0}{0ex}}I\phantom{\rule{0}{0ex}}S\phantom{\rule{0}{0ex}}F\phantom{\rule{0}{0ex}}I\phantom{\rule{0}{0ex}}F\phantom{\rule{0}{0ex}}S$ sandwich that acts as a transistor. The results of the analytical model agree well with experiment, providing an important tool for designing tomorrow's ultralow-power computing hardware.

Magnetization dynamics in an artificial square spin-ice lattice made of Ni80Fe20 with magnetic field applied in the lattice plane is investigated by broadband ferromagnetic resonance spectroscopy. The experimentally observed dispersion shows a rich spectrum of modes corresponding to different magnetization states. These magnetization states are determined by exchange and dipolar interaction between individual islands, as is confirmed by a semianalytical model. In the low field regime below 400 Oe a hysteretic behavior in the mode spectrum is found. Micromagnetic simulations reveal that the origin of the observed spectra is due to the initialization of different magnetization states of individual nanomagnets. Our results indicate that it might be possible to determine the spin-ice state by resonance experiments and are a first step towards the understanding of artificial geometrically frustrated magnetic systems in the high-frequency regime.

Due to its transverse nature, spin Hall effects (SHE) provide the possibility to excite and detect spin currents and magnetization dynamics even in magnetic insulators. Magnetic insulators are outstanding materials for the investigation of nonlinear phenomena and for novel low power spintronics applications because of their extremely low Gilbert damping. Here, we report on the direct imaging of electrically driven spin-torque ferromagnetic resonance (ST-FMR) in the ferrimagnetic insulator Y$_3$Fe$_5$O$_{12}$ based on the excitation and detection by SHEs. The driven spin dynamics in Y$_3$Fe$_5$O$_{12}$ is directly imaged by spatially-resolved microfocused Brillouin light scattering (BLS) spectroscopy. Previously, ST-FMR experiments assumed a uniform precession across the sample, which is not valid in our measurements. A strong spin-wave localization in the center of the sample is observed indicating the formation of a nonlinear, self-localized spin-wave `bullet'.

We report the microwave response of symmetric and asymmetric threefold clusters with nearly contacting segments that can serve as the node in a Kagome artificial spin ice lattice. The structures are patterned on a coplanar waveguide and consist of elongated and nearly-contacting ellipses with uniform thickness. Branches of the ferromagnetic resonance spectra display mode softening that correlates well with the calculations, whereas agreement between the measured and simulated static magnetization is more qualitative.

We report systematic measurements of the dispersion of long wavelength spin waves for a wide range of wave vectors for the magnetic field along the three principal directions defining the forward volume, backward volume and Damon-Eshbach modes of a 9.72 μm thick film of an yttrium iron garnet obtained using lithographically patterned, multi-element, spatially resonant, antennas. Overall good agreement is found between the experimental data for the backward volume and Damon-Eshbach modes and the magnetostatic theory of Damon and Eshbach. Also, good agreement is found between the experimental data for the forward volume mode and the theory of Damon and van de Vaart.

Techniques for studying the combined temporal and spatial response of 3He are few in number. Here we describe, qualitatively, some strategies to probe single particle and collective responses in normal and superfluid 3He in a frequency and wave-vector specific manner with the goal of stimulating such experiments in the future.

Combined experimental and theoretical studies are reported of the Fermi surface, band structure, generalized magnetic susceptibility, electron-phonon enhancement factor λ and superconducting transition temperature TT of LaB6. Whereas the unusually large λ values, ranging from 1.0 to 2.5, are expected to result in high TT values, TT is observed to be only 0.122°K. These results further emphasize the need for appropriate theoretical formulations for these systems.

We report characteristics of CeCoIn 5 /Al/AlO x /Nb and CeCoIn 5 /Al/AlO x /Al tunnel junctions fabricated on the (0 0 1) surface of CeCoIn 5 crystal platelets. The main result of this work is the observation of a low Josephson current (as compared with that expected from the Ambegaokar–Baratoff formula), which is consistent with idea that the order parameter in the heavy-fermion superconductor CeCoIn 5 has unconventional pairing symmetry.

Using resonant two-photon excitation, we create a coherently propagating orthoexciton–polariton wave packet and measure its reflectance ( R ) and transmittance ( T ) at the boundary opposite the incoming surface. We find an enhanced reflection of orthoexciton–polaritons from sample surfaces such that the ratio T / R deviates significantly from the present theory based on the additional boundary condition. This anomalous boundary effect most likely arises from the quadrupole excitonic component of orthoexciton–polaritons, resulting in an enhanced reflection at boundaries.

The lifetime of dense orthoexcitons in Cu 2 O is severely limited by dark molecule formation. In order to investigate this two-body process for orthoexciton-polaritons, we employ two-photon excitation and resonantly create orthoexciton-polaritons at 2 K, which are the coherent quantum superposition of orthoexcitons and the light field. Based on the steady-state analysis, we find that this density-dependent process is an order of magnitude reduced for resonantly created orthoexciton-polaritons compared with thermalized orthoexcitons at 2 K. This most likely arises from the photonic character of an orthoexciton-polariton. Considering the greatly reduced effective mass of the orthoexciton-polariton, this implies that the experimentally achievable orthoexciton-polariton densities can be above the critical density for Bose–Einstein condensation. However, insufficient elastic scattering seems to keep this propagating quantum ensemble from internal equilibrium.

Non-linear excitation of non-uniform spin waves (Suhl instability) is of profound importance when it comes to magnetization reversal using r.f. fields. Here we numerically show that this instability can be suppressed by choosing the applied field and/or sphere diameter in such a way that the frequencies of the modes that can be excited through non-linear processes are off-resonance. While the results cannot be explained by a traditional model based on plane waves, they can be understood by projecting the actual state onto the small amplitude spin wave eigenfunctions.

We present a detailed comparison between the results of ferromagnetic resonance (FMR) measurements on a square array of permalloy nanodots and a numerical simulation based on the eigenvalues of the linearized Landau–Lifshitz equation. Of particular importance is the implementation of a recently developed technique for numerically calculating the absorption spectrum from the associated eigenvectors as a function of the orientation, frequency, and position dependence of the applied RF field. The method is also used to predict results of some possible future experiments.

We demonstrate the generation and detection of spin-torque ferromagnetic resonance in $\text{Pt/}{\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ (YIG) bilayers. A unique attribute of this system is that the spin Hall effect lies at the heart of both the generation and detection processes and no charge current is passing through the insulating magnetic layer. When the YIG undergoes resonance, a dc voltage is detected longitudinally along the Pt that can be described by two components. One is the mixing of the spin Hall magnetoresistance with the microwave current. The other results from spin pumping into the Pt being converted to a dc current through the inverse spin Hall effect. The voltage is measured with applied magnetic field directions that range from in plane to nearly perpendicular. When compared with theory, we find that the real and imaginary parts of the spin mixing conductance have out-of-plane angular dependences.

The Rashba-Edelstein effect stems from the interaction between the electron's spin and its momentum induced by spin-orbit interaction at an interface or a surface. It was shown that the inverse Rashba-Edelstein effect can be used to convert a spin current into a charge current. Here, we demonstrate the reverse process of a charge- to spin-current conversion at a Bi/Ag Rashba interface. We show that this interface-driven spin current can drive an adjacent ferromagnet to resonance. We employ a spin-torque ferromagnetic resonance excitation/detection scheme which was developed originally for a bulk spin-orbital effect, the spin Hall effect. In our experiment, the direct Rashba-Edelstein effect generates an oscillating spin current from an alternating charge current driving the magnetization precession in a neighboring permalloy (Py, ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$) layer. Electrical detection of the magnetization dynamics is achieved by a rectification mechanism of the time dependent multilayer resistance arising from the anisotropic magnetoresistance.

We investigated the spin-wave propagation in a micro-structured yttrium iron garnet waveguide of $40$ nm thickness. Utilizing spatially-resolved Brillouin light scattering microscopy, an exponential decay of the spin-wave amplitude of $(10.06 \pm 0.83)$ $\mu$m was observed. This leads to an estimated Gilbert damping constant of $\alpha=(8.79\pm 0.73)\times 10^{-4}$, which is larger than damping values obtained through ferromagnetic resonance measurements in unstructured films. The theoretically calculated spatial interference of waveguide modes was compared to the spin-wave pattern observed experimentally by means of Brillouin light scattering spectroscopy.

We analyze the performance of a particle detector based on a double-barrier Nb/Al/AlOx/Al/AlOx/Al/Nb junction where the Al trapping layer is embedded between two insulating AlO/sub x/ barriers. Within this layer, a very sharp Andreev bound state (ABS) level E/sub 0/ is formed. Because of a very long recombination time, the charge amplification factor is expected to be 10-100 times larger in the double-barrier detector compared to ordinary single-barrier devices. The double barrier detector also has much higher idle-state subgap resistance at low temperatures. Because the middle Al quasiparticle trap layer is separated from the adjacent layers by two insulating tunneling barriers, the proximity effect is minimized. This provides much better alignment between the maximum of nonequilibrium quasiparticle distribution function and the maximum in the electron density of states. All the factors listed result in much higher sensitivity and the energy resolution of the double barrier Nb-based STJ as compared to their single-barrier analogs.

The switching dynamics of a double-barrier Josephson junction is analyzed as a function of the microscopic properties of its electrodes. In particular, it is found that the nonstationary behavior of the Josephson phase difference is very sensitive to dissipation mechanisms acting inside the intrinsic shunt. The leading factor that determines the dissipation is the local electron density of states N(E) inside the electrodes. The roles of junction geometry, electrode purity, and interface quality are discussed and how they affect the details of N(E), hence the resulting phase dynamics. The microscopic analyses allow optimization of the performance of double-barrier Josephson junction-based rapid-single-flux-quantum circuits in two ways: 1) decreasing the switching time of Josephson elements and 2) reducing the excess wiring. Such an analysis is facilitated with the aid of a lumped circuit representation which generalizes the nonlinear resistive-shunted-junction model.

We present SPNR and DC magnetization studies of non-uniformly distributed vortices in Nb/Al multilayers for fields applied near-parallel to the film surface. Peaks are observed in the M–H curves that are shown to correspond to vortex row-transitions and the field values of the transitions agree well with free energy calculations. An additional peak is observed at an applied field smaller than the first row-transition field and this is shown to arise from the lower critical field parallel to the surface. Demagnetization effects are discussed. SPNR measurements performed at low field give the London penetration length and measurements in the mixed state are consistent with a single row of vortices residing in the film center, but with positional fluctuations amounting to 1/4 of the film thickness. It is also shown that cycling the applied field leads to a surface-induced reorientation of the vortex magnetic field, which points perpendicular to the surface in zero field.

Three ultrasonic spectrometers have been constructed for use with the continuous and pulsed magnetic field facilities of the NHMFL at the Tallahassee and Los Alamos sites. The first of these, designed for use with the dc magnets, is a heterodyne spectrometer with the capabilities of high velocity resolution and near-simultaneous measurements at several frequencies (typically near transducer harmonics). The other two spectrometers are designed for the short-pulsed magnets, with the capability of fast data acquisition. These spectrometers have been used in the study of metamagnetism in the heavy fermion materials.

A qubit system exploiting manipulations of the Andreev bound state (ABS) levels in the SINIS junction by applying appropriate bias voltages and transport currents is suggested. The parameters of the SINIS setup may be chosen in such a way that only two ABS levels are present; this is in agreement with our experimental data obtained using Nb/Al double-barrier junctions. In the qubit Hamiltonian, H , the two ABS levels are presented as the ‘↑’ and ‘↓’ spin states, while the controlling physical parameters (voltage across one of the barriers and the transport current) are mapped to the ‘magnetic fields’ B x n and B z n . The phase decoherence time is estimated.

This volume continues the presentation of recent results on superfluids, including novel metallic systems, superfluid liquids, and atomic/molecular gases of bosons and fermions, particularly when trapped in optical lattices. Since the discovery of superconductivity (Leyden, 1911), superfluid4 He (Moscow and Cambridge, 1937), superfluid3 He (Cornell, 1972), and observation Bose–Einstein condensation (BEC) of a gas (Colorado and MIT, 1995), the phenomenon of superfluidity has remained one of the most important topics in physics. Again and again, novel superfluids yield surprising and interesting behaviors. The many classes of metallic superconductors, including the high temperature perovskite-based oxides, MgB2, organic systems, and Fe-based pnictides, continue to offer challenges; the technical applications grow steadily. What the temperature and field limits are remains elusive. Atomic nuclei, neutron stars, and the Universe itself all involve various aspects of superfluidity; clearly the lessons learned have had a broad impact on physics as a whole.

We report measurements of the attenuation of a beam of orthoexciton polaritons by a photoionizing optical probe. Excitons were prepared in a narrow resonance by two photon absorption of a 1.016 eV, 54 ps pulsed light source in cuprous oxide (${\mathrm{Cu}}_{2}\mathrm{O}$) at 1.4 K. A collinear, 1.165 eV, 54 ps probe delayed by 119 ps was used to measure the photoionization cross section of the excitons. Two photon absorption is quadratic with respect to the intensity of the pump and leads to polariton formation. Ionization is linear with respect to the intensity of the probe. Subsequent carrier recombination is quadratic with respect to the intensity of the probe, and is distinguished because it shifts the exciton momentum away from the polariton anticrossing; the photoionizing probe leads to a rise in phonon-linked luminescence in addition to the attenuation of polaritons. The evolution of the exciton density was determined by variably delaying the probe pulse. Using the probe irradiance and the reduction in the transmitted polariton light, a cross section of $(3.9\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}22}$ ${\mathrm{m}}^{2}$ was deduced for the probe frequency.

The field dependent transport properties of superconducting niobium films, which are modulated by a regular array of non-magnetic and magnetic normal-metal inclusions (dots), have been investigated. Strong peaks in the critical current are seen for fields at which the vortex density in the superconductor is some rational or sub-rational multiple of the dot density. This commensurate peak effect is enhanced for the magnetic dot arrays when the dots are magnetized in a direction parallel to the applied magnetic field and suppressed when they are anti-aligned. Qualitative information on the strength of this dot-vortex interaction is inferred from the commensurate peaks present and missing for different regular lattices.

Ultrasound velocity and attenuation measurements were performed on single crystals of the heavy fermion compound ${\mathrm{UPt}}_{3}$ in magnetic fields up to 33 T and at temperatures ranging from 2.4 K to below 0.1 K. With longitudinal sound propagated in the crystallographic basal plane, parallel to the applied field, the familiar elastic softening is observed at the metamagnetic transition field $H\ensuremath{\sim}20.2\mathrm{T}.$ More complicated structure emerges at low temperatures, including quantum acoustic oscillations and a second velocity minimum at \ensuremath{\sim}21.6 T. A weak frequency dependence (dispersion) is observed in the velocity. The ultrasonic data are analyzed using the Landau-Khalatnikov formalism, from which temperature- and field-dependent relaxation times are deduced.

We have experimentally investigated electron transport in double-barrier Nb/Al/AlO x /Al/AlO x /Nb devices. At low temperatures, the devices reveal a novel magnetic-field-sensitive subgap structure in the current-voltage characteristics, which is interpreted as a manifestation of Andreev bound states. A correlation between phase-coherent and nonequilibrium properties is suggested.

The angular dependence of the magnetic moment of the heavy fermion compound UPt3 has been measured near the metamagnetic transition by using a vibrating sample magnetometer for angles between an external magnetic field and the basal plane in the range 0°–50°. The value of magnetic field at which the metamagnetic transition occurs increases with the angle between the applied field and the basal plane. A planar geometry of the transition explains this behavior.

The relative change of the microwave surface resistance has been measured as a function of temperature in the superconducting and normal states of UPt 3 using a cavity perturbation technique. With our frequency range of ∼9–38 GHz, the microwave photon energy (ℏ ω ) is ∼20–95% of the estimated zero-temperature (BCS) energy gap (2 Δ 0 ) of UPt 3 . The surface resistance shows a change in slope as the signature of T c and has a significantly weaker temperature dependence in the superconducting state than would be predicted by the Mattis–Bardeen theory. At higher frequencies, the slope tends to change by a smaller amount. No indication of collective modes at any of the frequencies was observed down to the lowest temperatures measured (∼0.1 K).

We examine the solutions of the non-linear equations governing the behavior of a current biased Josephson junction. Both inline and overlap current bias geometries are considered. The solution space is investigated analytically and using numerical techniques. We characterize the types of solutions expected analytically and find good approximations for large magnetic fields. For small magnetic fields the solution space is large and its stability is entangled. We study this space and its stability as a function of magnetic field and applied bias current. Selective results are presented that characterize the general behavior of the solution space.

We report on free excitons coexisting with exciton magnetic polarons (EMPs) in bulk semimagnetic semiconductors of Cd 1− x Mn x Te for 0.04 ⩽ x ⩽ 0.36 at 2 K under nonresonant two-photon excitation. This two-photon excitation not only generates free excitons but also more efficiently creates EMPs compared with ordinary one-photon excitation. Stimulated emission from free excitons is demonstrated under strong two-photon excitation.

A simple technique involving transmission through a copper-wire meander line is utilized to probe the microwave absorption of the heavy-fermion superconductor CeCoIn5 in the frequency range below 20 GHz. The measurements do not show features identifiable with absorption into order parameter collective modes, as observed earlier in UBe13, but instead show a rather rapid change in the transmitted signal vs. temperature in the range 8.5–10.0 GHz, possibly implying the opening a low-energy dissipation channel in CeCoIn5.

We describe a variable path acoustic interferometer for use at cryogenic temperatures. Movement is enabled without mechanical coupling via two piezoelectric bimorphs wired and mounted in a manner that preserves the parallelism of two ultrasonic transducers that define the acoustic path. A certain degree of in situ alignment can also be accomplished. Path length sweeps from 0 to 180 μm have been made at cryogenic temperatures and preliminary sound velocity measurements in liquid 4He and gaseous 3He near 4 K are presented which agree well with past measurements.