Microkelvin 2013

Microkelvin Workshop 2013

Workshop – Review – Users’ Meeting of the EU Microkelvin Collaboration

Sannäs Manor House, Porvoo, Finland

9 – 13 September, 2013

The past 2012 Microkelvin Workshop was organized to provide a wide overview of latest research at ultra-low temperatures, emphasizing nanophysics applications. This time the 2013 Workshop is planned to

  • review Microkelvin research progress during the past four grant years
  • highlight promising future projects
  • introduce novel research which could be part of a future Microkelvin programme
  • prepare for a collection of research papers in Microkelvin Proceedings

The four workshop days 9-12 September are followed by the final public Review of the Microkelvin grant programme on Friday 13 September. The arrival to Sannäs is on Monday morning before the first session which starts at 11 am (You get the program by downloading a pdf-file, by clicking HERE). The departure is on Friday, either in the morning or after the Review at 4 pm. The venue is Sannäs Manor House, a convention centre [link] recently acquired by the Aalto University, which is located 60 km to the east from the Helsinki airport and 10 km from the small historic town of Porvoo/Borgå. Transportation from and to Helsinki airport will be available. Travel and boarding at Sannäs are provided from the Microkelvin grant.

The structure and topics of the workshop are organized as follows:


Monday 9 September

Workshop start Monday at 11 am with lunch (Welcome.ppt)

Refrigeration, thermalization, and thermometry – JRA1

Chair George Pickett <g.pickett(at)lancaster.ac.uk>

  • Refrigeration in the mK regime – George Pickett (Lancaster)
  • Cooling of nanoelectronics – Dominik Zumbühl (Basel)
  • Dominik Zumbuhl (University of Basel): Towards microkelvin quantum transport experiments in nanosamples

    I will present an overview of our activities working towards μK temperatures in electronic transport measurements in semiconductor nanostructures. The Basel approach employs individual, separate nuclear refrigerators to directly cool each of the electrical wires connecting the sample, thus enabling efficient thermal to a microkelvin bath. Recently, the nuclear refrigerators have cooled to 185±25 μK (in collaboration with the Pickett group in Lancaster), thus establishing the required experimental microkelvin platform. Nevertheless, cooling samples, their electrons and nuclear spins rather than only the nuclear stages, remains a formidable challenge. Recently, we have cooled a metallic Coulomb blockade thermometer to 5.2 +/- 0.2 mK (collaboration with the Pekola group in Aalto University). Though this is a new record and well below 10 mK, these temperatures are still clearly above the μK range. We are currently developing further strategies for reaching temperatures below 1 mK.

Already temperatures just below 10 mK are beginning to reveal exciting new physics. I will present striking data demonstrating the effects of density gradients on the integer and fractional quantum Hall effects in ultra-high mobility GaAs 2D electron gases. In ever cleaner samples measured at ever lower temperatures, gradient effects will inevitably appear and become important. Finally, I will also briefly outline ultralow temperature quantum transport experiments in GaAs quantum wires, presenting evidence for helical nuclear spin order in the Luttinger liquid regime – a novel quantum state of matter.

  • Aya Shibahara (Royal Holloway): Pulse-tube precooled and hyperfine-field-enhanced nuclear refrigeration with noise thermometry (.pptx)

    We report the experimental demonstration of the feasibility of reaching temperatures below 1 mK using cryogen-free technology. Our prototype system comprises an adiabatic nuclear demagnetisation stage, based on hyperfine-enhanced nuclear magnetic cooling, integrated with a commercial cryogen-free dilution refrigerator and 8 T superconducting magnet. Thermometry was provided by a current-sensing noise thermometer. The minimum temperature achieved at the experimental platform was 600 μK. The platform remained below 1 mK in excess of 24 hours, indicating a total residual heat-leak into the experimental stage of 5 nW. We also discuss improvements in the performance of current sensing noise thermometers. When optimised for speed 1% precision in 0.1 seconds measurement time was achieved. This work opens the way to widening the accessibility of temperatures in the microkelvin regime, of potential importance in areas from the application of strongly correlated electron states in nanodevices to quantum computing.

We present an advanced network of 16 parallel nuclear refrigerators operated on a BlueFors pulse-tube dilution refrigerator platform (cryogen-free, "dry" system) and demonstrate nuclear refrigeration, with the goal of cooling nanosamples for quantum transport experiments. Numerous measures were implemented to reduce vibrations originating mainly from the pulse-tube. Further, we show cooling of a metallic Coulomb blockade thermometer down to 5.2 \pm 0.2 mK on a similar nuclear stage operated in a traditional 4He bath ("wet") system. Striking deviations from electron-phonon cooling were observed and are strong indication of the effectiveness of cooling through the sample leads.

In this presentation the current status of temperature scales at low temperatures is described. A new calibration service for the dissemination of the PLTS-2000, which PTB has started recently, is presented. Also an overview will be given of the activities of PTB in the temperature range below 1 mK, where no internationally agreed temperature scale exists at all.

Tuesday 10 September

Nanorefrigeration – JRA2

Chair Jukka Pekola <jukka.pekola(at)aalto.fi>

The topic of quantum fluctuations in quasi-1D superconductors, also called quantum phase slips (QPS), has attracted significant attention [1]. It has been shown that the phenomenon is capable to suppress the zero resistivity of ultra-narrow superconducting nanowires at low temperatures T << Tc [2-4] and quench persistent currents in tiny nanorings [5]. The coherent QPS effect enables the fabrication of a new generation of quantum logic devices – qubits [6]. It has been predicted that a superconducting nanowire in the regime of QPS is dual to a Josephson junction [7]. In the particular case of an extremely narrow superconducting nanowire imbedded in a high-impedance environment the duality leads to an intuitively controversial result: the superconductor can enter an insulating state [8].

Here we experimentally demonstrate that the I-V characteristics of such a wire indeed show Coulomb blockade which disappears with the application of a critical magnetic field and/or above the critical temperature proving that the effect is related to superconductivity [9]. Such a system can be considered as a junctionless single electron transistor (with charge 2e), where the QPSs provide the dynamic equivalent of weak links in conventional devices containing static (in space and time) tunnel junctions. Application of external RF radiation can be synchronized with the internal Bloch oscillations of the current-biased superconducting nanowire in the regime of QPS. The phenomenon is dual to the well-known Shapiro effect: the voltage steps for a Josephson junction are substituted by the current steps for a QPS wire: the proof-of-principle demonstration of the long-awaited metrological application - the quantum standard of electric current

[1] K.Yu. Arutyunov, D.S. Golubev, A.D. Zaikin, Phys. Rep. 464, 1 (2008)

[2] M. Zgirski, K.-P. Riikonen, V. Touboltsev, K. Arutyunov, NanoLett. 5, 1029 (2005)

[3] M. Zgirski, K.-P. Riikonen, V. Touboltsev, K.Yu. Arutyunov Phys. Rev. B. 77, 054508 (2008)

[4] J.S. Lehtinen, T. Sajavaara, K.Yu. Arutyunov, M.Yu. Presnjakov, A. Vasiliev, Phys. Rev. B 85, 094508 (2012)

[5] K.Yu. Arutyunov, T.T. Hongisto, J.S. Lehtinen, L.I. Leino, A.L. Vasiliev. Nature: Sci. Rep. 2, 293 (2012)

[6] O.V. Astafiev, L B. Ioffe, S. Kafanov, Yu.A. Pashkin, K.Yu. Arutyunov, D. Shahar, O. Cohen, J.S. Tsai, Nature 484, 355 (2012)

[7] J.E. Mooij, Yu.V. Nazarov, Nature Physics 2, 169 (2006)

[8] T.T. Hongisto, A.B. Zorin, Phys. Rev. Lett. 108, 097001 (2012)

[9] J. Lehtinen, K. Zakharov, K. Arutyunov, Phys. Rev. Lett. 109, 187001 (2012)

In this presentation we shall report the first experimental realization of a heat interferometer [6,7]. We investigate heat exchange between two normal metal electrodes kept at different temperatures and tunnel-coupled to each other through a thermal `modulator' [5] in the form of a DC-SQUID. Heat transport in the system is found to be phase dependent, in agreement with the original prediction. With our design the Josephson heat interferometer yields magnetic-flux-dependent temperature oscillations of amplitude up to ~ 21 mK, and provides a flux-to-temperature transfer coefficient exceeding ~ 60mK/Φ0 at 235 mK (Φ0 is the flux quantum). Besides offering remarkable insight into thermal transport in Josephson junctions, our results represent a significant step toward phase-coherent mastering of heat in solid-state nanocircuits, and pave the way to the design of novel-concept coherent caloritronic devices, for instance, heat transistors, thermal splitters and diodes [8] which exploit phase-dependent heat transfer peculiar to the Josephson effect.

In this latter context, we shall also present the concept for a further development of a Josephson heat interferometer based on a double superconducting loop [9] which allows, in principle, enhanced control over heat transport. We shall finally conclude presenting some preliminary results on a quite different prototypical thermal interferometer which could add complementary flexibility in mastering heat flux at the nanoscale.

[1] B. D. Josephson, Phys. Lett. 1, 251 (1962).

[2] J. Clarke and A.I. Braginski, The SQUID Handbook (Wiley-VCH, 2004).

[3] K. Maki and A. Griffin, Phys. Rev. Lett. 15, 921 (1965).

[4] G.D. Guttman, E. Ben-Jacob, and J. Bergman, Phys. Rev. B 57, 2717 (1998).

[5] F. Giazotto and M.J. Martínez-Pérez, Appl. Phys. Lett. 101, 102601 (2012).

[6] F. Giazotto and M.J. Martínez-Pérez, Nature 492, 401 (2012).

[7] R.W. Simmonds, Nature 492, 358 (2012).

[8] M.J. Martínez-Pérez and F. Giazotto, Appl. Phys. Lett. 102, 182602 (2013).

[9] M.J. Martínez-Pérez and F. Giazotto, Appl. Phys. Lett. 102, 092602 (2013).

[1] Tero T. Heikkilä, J. Tuorila, F.K. Massel, R. Khan, and M.A. Sillanpää, in preparation.

In this talk I will motivate our goal of cooling mesoscopic systems to sub-mK temperatures by discussing a recent experiment in which we investigated the electron-phonon coupling in a double-quantum-dot (DQD) charge-qubit. We performed Landau-Zener-Stueckelberg-Majorana interferometry in a GaAs-based DQD charged by two electrons. The coherence time of our charge qubit of > 200 ns (allowing for > 600 qubit operations) is limited by the electron-phonon interaction at our presently lowest temperature of 18 mK. The coherence could be further increased by further lowering the temperature which could make semiconductor charge qubits one of the strongest candidates for quantum information processing.

[1] O.-P. Saira et al., PRL 109, 180601 (2012)

[2] J.V. Koski et al., Nat. Phys. doi:10.1038/nphys2711 (2013)

[3] J.V. Koski, D.V. Averin et al., in preparation (2013)

[1] N. Vercruyssen, T.G.A. Verhagen, M.G. Flokstra, J.P. Pekola, T.M. Klapwijk, Phys. Rev. B 85, 224503 (2012)

Wednesday 11 September

Ultra-low temperatures – JRA3

Chair Henri Godfrin <henri.godfrin(at)grenoble.cnrs.fr>

At the lowest temperatures the lifetime of the magnon condensates reaches minutes. They become a very sensitive probe for the properties of superfluid 3He connected with the orbital texture or magnetic relaxation, including the properties of the free surface, vortex lines, and the interface between the A and B phases.

Understanding the quantum dynamics of many-body systems is one of the major goals of present physics. For this reason, many studies have been devoted to liquid 4He (bosons) and liquid 3He (fermions). The elementary excitations of a strongly interacting 2D Fermi liquid have been investigated by neutron inelastic scattering at very low temperatures [1,2]. The collective mode (zero-sound, or plasmon) traverses the particle-hole band in a Fermi system, an effect observed for the first time. The collective mode is mainly determined by the strong correlations, and not by quantum statistics. The results are compared to dynamic many-body theory and quantum Monte Carlo calculations. We also show new results on liquid 4He; the very rich dynamic behavior observed in a large momentum-energy domain challenges the current theoretical description of correlated bosons.

[1] H. Godfrin, M. Meschke, H.-J. Lauter, A. Sultan, H. Böhm, E. Krotscheck, M. Panholzer, Observation of a roton collective mode in a two-dimensional Fermi liquid, Nature, 483, 576-579 (2012)

[2] A. Sultan, M. Meschke, H.-J. Lauter, H. Godfrin, Static structure factor of two-dimensional liquid 3He adsorbed on graphite, J. of Low Temp. Phys. 169, 367-376 (2012)

[1] R.H. Salmelin, M.M. Salomaa, and V.P. Mineev: Phys. Rev. Lett. 63, 868-871 (1989).

[2] H. Ikegami, Y. Tsutsumi, and K. Kono: Science 341, 59-62.

I will mention certain further theoretical aspects of the modeling of this experiment, mostly the role of two-photon dissipative processes and the issue of backaction. Then I will discuss the connection between this experiment and other effects that rely on the instability of the quantum vacuum, namely the Hawking radiation, the Unruh effect, and the Schwinger effect.

[1] P. Lähteenmäki, G.S. Paraoanu, J. Hassel, P.J. Hakonen, Dynamical Casimir effect in a Josephson metamaterial, Proc. Natl. Acad. Sci. U.S.A., 110, 4234 (2013)

[1] L.V. Levitin, R.G. Bennett, A. Casey, B. Cowan, D. Drung. Th. Schurig, J.M. Parpia, Science 340, 841 (2013)

[2] L.V. Levitin, R.G. Bennett, E.V. Surovtsev, J.M. Parpia, B. Cowan, A. Casey, J. Saunders, submitted to Phys. Rev. Lett.

In this talk we discuss characteristic properties of fermion spectra for different types of vortices in 3He-B. Due to the removed spin degeneracy singly quantized vortices in 3He-B have two anomalous branches crossing the Fermi level. In particular the spectral branches of nonsingular vortices intersect the Fermi level at finite angular momenta which leads to the appearance of a large number of fermionic zero modes. In the double-core vortices which appear in 3He-B at low pressure the manifold of zero energy states forms a highly anisotropic Fermi surface emerging inside vortex cores. We find a Lifshitz transition of the Fermi surface as a function of the momentum projection on the vortex axis and discuss its influence on the double-core vortex rotation dynamics.

(1) MIT bag model of hadrons, which is reproduced in magnon BEC confined in a textural trap. When the number of magnons increases, their zero point energy expels the texture, and the trap gradually transforms to a box with walls almost impenetrable for magnons. The resulting texture-free cavity filled by the magnon condensate wave function becomes the bosonic analog of the MIT bag, in which a hadron is seen as a cavity in the QCD vacuum, where the free quarks are confined in the ground or excited state.

(2) Higgs bosons - the amplitude modes, which have been experimentally investigated in condensed matter for many years, and now serve for the prediction of extra Higgs bosons. In He-B, the broken symmetry gives rise to 4 Goldstone modes and 14 Higgs modes. Based on the properties of the spectrum of these modes, Nambu suggested a general sum rule, which relates the masses of Higgs bosons to the masses of fermions. If this rule is applicable to the Standard Model, then one may expect that the observed Higgs boson with a mass of 125 GeV has a partner - another Higgs boson with mass 325 GeV. There is some evidence for such a particle.

(3) Majorana fermions, which exist in bulk 3He-A and on the boundaries, interfaces and inside the vortex cores in both phases.

Thursday 12 September

Measurement & devices – JRA4

Chair Christian Enss <enss(at)kip.uni-heidelberg.de>

In this talk I will describe the operation and performance of such a SQUID based NMR spectrometer in both a tuned and broadband configuration. We have demonstrated the coupling of these SQUIDs to receiver coils with a few 100 micron diameter, allowing the possibility for studies of features with a size on the order of the dipole healing length ~ 10 µm. PTB, Berlin, have designed and fabricated a new generation of novel SQUID current sensing devices with output current feedback, OCF, which allows operation at higher frequencies > 100 MHz. We have used these devices to measure the nuclear quadruple resonance response (NQR) of sodium chlorate at 30 MHz cooled to 10 mK on a cryogen-free dilution refrigerator. The bandwidth was previously limited by propagation delays to the room temperature control electronics which for our nuclear demagnetisation cryostats imposed a practical limit of a few MHz. This increased bandwidth should enable SQUIDs to be used in a wider range of applications in the future.

[1] A.J. Leggett Ann. of Phys. 85, 11 (1974)

[2] L.V. Levitin et al., Appl. Phys. Lett.91, 262507 (2007)

Amorphous dielectrics are non-equilibrium quantum systems whose low temperature properties are governed by atomic tunneling systems. Many aspects are well described within the phenomenological standard tunneling model. Via their elastic and electric dipole moments tunneling systems interact mutually and with external fields. The dynamics of tunneling systems can be investigated by polarization echo experiments. Here the echo amplitude is measured as a function of the delay time between the two excitation pulses. Different dephasing mechanisms contribute to the decay of the echo amplitude. In amorphous dielectrics at very low temperatures the dominating dephasing mechanism is spectral diffusion, which is the interaction of resonant tunneling systems with non-resonant thermally fluctuating ones.

We have performed such echo decay measurements with an improved setup allowing us to observe echoes at very long delay times where the echo has decayed five orders of magnitude from its original amplitude. The data obtained in this way allows a precision test of the model of spectral diffusion and the distribution of parameters of the tunneling systems given by the standard tunneling model. We will show experimental results from measurements on BK7 and will discuss them in the framework of spectral diffusion and the standard tunneling model.

We have developed a concept for the low-noise, wide-band microwave reflection-amplifier which utilizes the intrinsic negative resistance of a single Josephson junction. In our first generation amplifiers, the negative resistance of the shunted single junction is singled out on the amplification band using a band stop filter. A gain of 30 dB at 2.8 GHz was measured over a band width of 1 MHz, and the noise temperature was found to be within a factor of three from the quantum limit (½ \hbar \omega). Since the regular Langevin analysis is insufficient with frequency-dependent dissipation, we have resorted to numerical simulations using the fundamental AC and DC Josephson relations. According to the simulations, the device relies heavily on noise compression, accomplished via diminishing down mixing and increasing up mixing when enough power gain is available.

In the second class of amplifiers, we have operated parametric devices in which pumped metamaterial made of arrays of SQUIDs is placed in a coplanar transmission line cavity with a high quality factor. As such systems can be quickly tuned, they allow for the creation of two-photon microwave states and the observation of elusive phenomena such as the dynamic Casimir effect. Under flux-pumping at double frequency, these devices provide a gain up to 25 dB and a noise temperature of ~ 200 mK at 5 GHz. At detuned pumping conditions, we have analyzed two-mode correlations, which indicate strong increase of dissipation with increasing pump power. Dissipation, presumably in the silicon nitride dielectric, limits the achieved squeezing of the two mode correlations in the dynamic Casimir pair production.

We also present new types of NanoSQUIDs with improved sensitivity allowing fast readout down to the lowest temperatures, ideally suited for integration in present and future NanoSQUID microscopes.

In collaboration with P. Castellazzi, Z.S. Wang, D. Hykel, D. Hazra, J.R. Kirtley.

Friday 13 September

Microkelvin grant review

Chair Matti Krusius <mkrusius(at)neuro.hut.fi>

Workshop finish Friday at 16:00


Posters

[1]. S. Autti et al., Phys. Rev. Lett. 108, 145303 (2012).

[2]. S. Autti et al., JETP Lett., 95, 611 (2012).

[3]. Yu. M. Bunkov, G. E. Volovik, Phys. Rev. Lett. 98, 265302 (2007).

We have employed shot noise thermometry in combination with ac conductance measurement for the determination of the electron-phonon coupling in high quality suspended graphene monolayers. The substrate, acting as a thermal bath, was kept at 50 mK using a dilution refrigerator while the electrons in our two-terminal graphene devices were heated up to Te = 100 - 600 K by Joule heating. In the regime Te > 200 K, we found that the electron-phonon coupling became the most important thermal relaxation channel with only minor contributions by the electronic heat diffusion along the current leads. Moreover, the chemical potential of the sample was varied using a back gate in our experiments.

At Te < 100 K, we observe power-law behavior characteristic to electronic diffusion with an exponent δ ≈ 1.5 - 2.0. Around Te = 200 K, there is a transition to a region with enhanced thermal relaxation. When Te > 200 K, we observe power-law dependence, δ ≈ 3 - 5, and the coupling constant increases as ∑ ~ μ2, which indicates a crucial role of two-phonon scattering events, reminiscent to the supercollision events analyzed by J. Song et al [2].

[1] J.K. Viljas, T.T. Heikkilä, Phys. Rev. B 81, 24504 (2010)

[2] J.C.W. Song, M.Y. Reizer, L.S. Levitov, Phys. Rev. Lett. 109, 106602 (2012)

In our recent experiments, we demonstrate quantized Cooper-pair pumping down to the level of a single pair. We study the crossover between adiabatic and nonadiabatic regimes and provide a characterization of Landau-Zener transitons in our pump. Finally, by operating the pump in a different way, we observe signatures of pure quantum pumping in our device.

[1] M. Möttönen, J.J. Vartiainen, J.P. Pekola, Phys Rev Lett 100, 177201 (2008).

[2] S. Gasparinetti, P. Solinas, J.P. Pekola, Phys Rev Lett 107, 207002 (2011)

[3] S. Gasparinetti, P. Solinas, Y. Yoon, J.P. Pekola, Phys Rev B 86, 060502(R) (2012)

[4] S. Gasparinetti, I. Kamleitner, Phys Rev B 86, 224510 (2012)

The LUMINEU project aims at developing a pilot double b decay experiment using scintillating bolometers based on ZnMoO4 crystals enriched in 100Mo. In the next months, regular deliveries of large-mass ZnMoO4 crystals are expected from the Nikolaev Institute of Inorganic Chemistry (Novosibirsk, Russia). It is therefore crucial for the LUMINEU program to test systematically and in real time these samples in terms of their bolometric properties, light yield and internal radioactive contamination. In this paper we describe an above-ground cryogenic facility based on a dilution refrigerator coupled to a pulse-tube cooler capable performing these measurements. A 23.8 g ZnMoO4 crystal was fully characterised in this setup. We show also that macro-bolometers can be operated with high signal-to-noise ratio in liquid-free dilution refrigerators.

Waves on the surface of a fluid in a gravitational field are among the most ubiquitous phenomena in nature. We report the first observation of gravity waves on the surface of superfluid 3He-B at temperatures below 0.2 Tc in the ballistic regime of quasiparticle motion [1]. At higher temperatures gravity waves are damped by the large viscosity of the normal component, and only third sound waves in a thin film have been observed. We excite the waves by vibrating a vertical cylindrical container filled partially with 3He-B. The oscillating free surface is coupled to a magnon Bose-Einstein condensate in a magneto-textural trap [2]. In the magnon BEC the magnetization of 3He precesses with coherent phase and common frequency, which is determined by the trapping potential. The oscillating surface modifies the shape of the trap and modulates the frequency. By measuring the precession frequency of the magnetization we have identified the two lowest surface wave modes of our system. Our measurements show that the damping of the waves decreases with temperature linearly with the density of the normal component and extrapolates to a finite value at zero temperature. We have also observed an enhancement of the relaxation rate of the trapped magnon condensate when the surface waves modulate the trap, whereas a similar modulation of the trap with the magnetic field does not affect the relaxation. We discuss the possibility that both the finite damping at T = 0 and the enhanced magnon relaxation could be related to surface-bound Majorana states expected to exist at the free surface of the topological superfluid 3He-B.

[1] V.B. Eltsov et al., arXiv:1302.0764

[2] S. Autti et al., Phys. Rev. Lett. 108, 145303 (2012)

According to current understanding Kelvin waves have a significant role in the energy dissipation of quantum turbulence in the zero temperature limit. The identification of a helical distortion on a straight vortex or a vortex ring is straightforward. In more complicated situations, such as Kelvin waves on a curved vortex or in case of a vortex tangle, it becomes a challenging task. Here we review the methods used to identify Kelvin waves within the vortex filament model. We test their suitability by using vortex configurations with known Kelvin spectra. We find that none of the methods is accurate enough, such that the verification of the theoretically predicted spectrum is not possible.

We report on experimental studies of dynamic nuclear polarization (DNP) and relaxation of 31P donors in natural silicon. After the pioneering work of Feher [1] the recent interest towards this system has been raised by the proposal of Kane [2] to utilize impurity atoms for quantum computing. The samples were studied in strong magnetic fields and temperatures below 1 K. At these conditions the donor electron spins are fully polarized and the relaxation times of electrons and nuclei are very long. Pumping the allowed electron spin transitions of the sample with very low RF power (<1 μW) efficiently creates DNP of 31P and 29Si. Pumping the forbidden transition of 29Si (solid effect) creates narrow peaks in the ESR spectrum, which correspond to polarization of 29Si in certain resolved lattice sites around the donors.

[1] G. Feher, Phys. Rev. 114, 1219 (1959)

[2] B.E. Kane, Nature, 393, 133 (1998)

We present experimental progress toward ultrasensitive microwave power measurements using the superconductor proximity effect in a superconductor/normal-metal/superconductor (SNS) circuit. In our devices three aluminium leads directly contact a gold-palladium nanowire with a small volume ([100 nm]^3) in order to form long (700 nm) and short (300 nm) SNS junctions on the same nanowire. To calibrate the power sensor, we use the long junction as a local dc heater of the electronic system while simultaneously measuring the switching current probability distribution in the short junction. First results show resolution of the switching current statistics down to local dc heating powers of 1 fW at bath temperatures < 50 mK. Operated as a bolometer, the resulting noise equivalent power (NEP) is ~100 aW/sqrt(Hz) when ramping the current bias over 10 ms. By employing an RF measurement scheme and by further reducing heat capacity and thermal conductance of the nanowire, we aim to reach a NEP sufficient for detection of individual photons in the microwave regime.

One of the challenges of the modern research on the dynamics of quantized vortices is the identification of dissipation mechanisms in superfluids with almost no normal component. It is generally believed that an essential part of the dissipation is the energy cascade of Kelvin waves, helical excitations on vortex lines, which transfer the kinetic energy from macroscopic scales larger than the inter-vortex distance to small scales where microscopic dissipation mechanisms like quasiparticle emission by vortex cores terminate the cascade. So far the experimental verification of this picture is missing.

We have studied the librating motion of a cylindrical sample of superfluid 3He-B, that is rotation of the sample around its axis with a periodically modulated angular velocity, in the temperature range 0.14 – 0.20 Tc. The modulation excites inertial waves in the liquid and Kelvin waves on vortex lines as seen from the decrease of vortex polarization. The polarization is determined from its influence on the order-parameter texture, probed with Bose-Einstein condensates of magnon quasiparticles. When the modulation of rotation velocity is stopped, the energy stored in the inertial waves is dissipated and the vortex polarization is restored. By calibrating the energy using the known free energy difference in solid-body rotation at different velocities we can extract the dissipation rate per vortex line in absolute units. We present dissipation measurements as a function of temperature, pressure, and rotation velocity, and discuss the relation of our results to the picture of the Kelvin-wave cascade.

The conditions for Fabry-Pérot resonances in rectangular graphene sheets with nonperfect contacts were recently analyzed by Gunlycke and White [1] who showed that, under certain conditions evenly spaced groups of resonances, separated by ΔE ~ hvF/2L, can emerge. These collective resonances originate owing to simultaneous participation of modes in nonequivalent channels that are facilitated by transversely quantized states with small energy separation. Such collective resonances should not be confused with the ordinary two-channel Fabry- Pérot resonances observed in single-wall carbon nanotubes.

We report the analysis of Fabry-Pérot type interferences in high-mobility suspended graphene using both shot noise and conductance measurements. Differential conductance shows definite Fabry-Pérot patterns emerging, by taking the derivative of the conductance the visibility is improved. The Fourier transform of the data shows three sets of peaks which are identified as resonances of 1) clean suspended part bordered by pn junctions, 2) full length of the sample with scattering from the contacts, and 3) width of the sample. The Fabry-Pérot pattern is also visible in our shot noise measurements. Their analysis reveals again three sets of Fourier peaks, which nearly coincide with those obtained from our conductance measurements. A correlation analysis between conductance and shot noise indicates rather weak correlation. This analysis demonstrates that the observed Fabry-Pérot pattern originates from more than two channels in contrast to interference phenomena in single walled carbon nanotubes.

[1] D. Gunlycke and C. T. White, Appl. Phys. Lett. 93, 122106 (2008)

In general, it is impossible to observe shot noise in macroscopic resistors but in small mesoscopic systems this is feasible at low temperatures. The criterion for seeing shot noise is the small size of the sample; the scale has to be below the electron–phonon scattering length of the material at the measuring temperature. Fano factor and cross correlations tell about the type of noise in the sample as well as about the physical phenomena behind the fluctuations. An interesting type of cross correlation measurement in a four port diffusive system is the determination of the Hanbury-Brown–Twiss (HBT) exchange correction factor in a manner proposed by Blanter and Büttiker [1]. The correction factor can be used to characterize the geometry of the sample; the resulting factor differs in sign depending on whether the sample forms effectively a diffusive cross or box structure.

In this work we present the measured shot noise relations of a thin-film copper cross in the cold and hot electron regimes. In the analysis of the experimental data the theoretically expected slope relations given by [2] in various configurations were found to match the data quite well. The exchange correction factor ΔS = |SC| - |SA| - |SB| is defined by subtraction of the cross correlation slopes in voltage following the original definitions by Blanter & Büttiker and Shukhorukov & Loss [1,2]. Theoretically, based on Pauli Virtanen’s simulation according to general principles of [2] it can be seen that in the cold regime ΔS = 0 whereas in the hot-electron regime ΔS < 0 should be clearly evident, and indeed this is the case in our measurements.

[1] Ya.M. Blanter, M. Büttiker, Phys Rev B 56,2127 (1997)

[2] E.V. Shukhorukov, D. Loss, Phys Rev B 59, 13054 (1999)

The dependences of thermal conductivity on temperature for pure carbon monoxide and solid solution of carbon monoxide-nitrogen for different concentrations of nitrogen in the temperature range 1.5 – 40 K are presented. The CO-N2 solution is a unique object for investigation, due to the absence of mass mismatch. The solution allows us to investigate the interaction of phonons with impurities directly, without isotopic effect. The obtained dependences of the thermal conductivity on temperature show a typical behavior for a dielectric crystal. This dependence is determined by the mechanisms of phonon scattering. The contribution of the various mechanisms of phonon scattering to the thermal conductivity of CO-N2 solid solution at different concentrations was calculated. The crystals with higher concentration of nitrogen admixture exhibit lower thermal conductivity. The excess thermal resistivity is calculated. Some discussion of the results is given.

[1] L.V. Levitin, R.G. Bennett, A. Casey, B. Cowan, D. Drung. Th. Schurig, J.M. Parpia, Science 340, 841 (2013)

[2] L.V. Levitin, R.G. Bennett, E.V. Surovtsev, J.M. Parpia, B. Cowan, A.Casey, J. Saunders, submitted to Phys. Rev. Lett.

Damping in macroscopic mechanical resonators can generally be described by a linear damping force. Present-day advances in nanofabrication, however, have made it possible to explore damping in systems with one or more atomic-scale dimensions. Recently, damping in graphene mechanical resonators was found to depend non-linearly on the amplitude of motion in measurements using frequency-modulated mixing techniques [1]. The drawback of such mixing techniques is that it is hard to know the amplitude of motion for quantitative analysis. In this work, we have employed capacitive detection methods, in which the graphene mechanical resonator, positioned on top of a counter-electrode, acts as part of an electrical cavity (LC) resonator. In our measurements, the change in the resonator capacitance can be calculated quite accurately, which allows a precise determination of the amplitude of vibrational motion [2]. At large vibration amplitudes, we observe both softening and hardening Duffing behavior on different single layer graphene resonators. These effects are typically caused by nonlinear external potentials and geometric effects, and can be modeled by a force that is proportional to the cube of the resonator displacement x3. At large amplitudes, we also observe the effect of nonlinear damping (force proportional to x2•dx/dt). From the response amplitude curve, it is difficult to distinguish between linear and nonlinear damping, but when we look at the responsivity (amplitude divided by driving force) of the resonator, we can clearly see a decrease for large amplitudes. As linear damping does not cause any change in responsivity, we can use this to determine the strength of nonlinear damping in our resonators.

[1] A. Eichler, J. Moser, J. Chaste, M. Zdrojek, I. Wilson-Rae, A. Bachtold, Nature Nanotech 6, 339 (2011)

[2] X. Song, M. Oksanen, M. Sillanpää, H. Craighead, J. Parpia, P. Hakonen, Nano Lett. 12, 198 (2012)

In this work we present the first ESR study of H/D-impurities stabilized in solid H2 and D2 matrices below 1 K. We demonstrate that the chemical reactions of isotopic exchange play an important role and help in reaching even higher densities of atomic hydrogen. We found that the quantum isotopic exchange reactions D+H2=H+HD, D+HD=H+D2 proceed with sufficiently high rate even at temperatures below 1 K and effectively convert atomic deuterium into hydrogen. It turned out that H densities after such conversion reached record high densities of 8^19 cm-3, twice higher than ever before. At such densities the effects of the dipole-dipole interactions between atoms lead to substantial homogeneous broadening of the ESR lines with the line widths exceeding 10 Gauss. High, nearly 100 % nuclear polarization of H was created by means of solid- and Overhauser effects within one hour of irradiation by <1 µW of mm-wave power. Pumping the second line of D (center of ESR spectrum) we created simultaneously positive DNP of D and negative DNP of H. We discuss possible explanations of this effect, the nuclear polarization transfer between H and D, or strong exchange effects between clusters of H atoms.

Recently there is a new trend in SQUID magnetometry [2]: nanometer-sized SQUID loops defined with e-beam lithography are tested for switching current – a current at which the SQUID switches to a finite voltage state due to thermal fluctuations. We present the new measuring protocol which involves probing a SQUID with a series of current pulses. The switching for a current pulse of given duration exhibits stochastic character: the SQUID may switch with probability p or not switch with probability 1-p. Hence it resembles a coin for which a “heads and tails” experiment is performed. However, unlike in a fair coin experiment our “coin” probabilities p can be set with the magnetic flux &phi threading the SQUID loop, p = p(&phi). The number of switchings n for N probing pulses is governed by the binomial distribution. The width of the distribution compared against p(&phi) gives the ultimate resolution of the pulse technique for DC-SQUID magnetometry.

In the future we are planning to apply this technique for magnetization measurements of clusters and nanowires.

[1] SQUID Handbook, edited by J. Clarke and A. Braginski (2004)

[2] W. Wernsdorfer, Supercond. Sci. Technol. 22, 064013 (2009)

General information

The workshop will host oral and poster sessions. Participation is expected to be around 70 persons. Presentations by visiting Microkelvin Users are particularly encouraged. Currently the programme is under construction. Please, contact the appropriate chairman and the coordinator Matti Krusius [mkrusius(at)neuro.hut.fi] about presentations and participation.

Information about the Microkelvin Collaboration (EU grant no. 228464 under FP7 research infrastructures) is available starting from the address [link].

Browse the links to the right for more information.

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