Anomalous Quasiparticle Reflection from the Surface of a

A free surface of a dilute ^{3}He-^{4}He liquid mixture is a unique system where two Fermi liquids with distinct dimensions coexist: a three-dimensional (3D) ^{3}He Fermi liquid in the bulk and a two-dimensional (2D) ^{3}He Fermi liquid at the surface. To investigate a novel effect generated by the interaction between the two Fermi liquids, the mobility of a Wigner crystal of electrons formed on the free surface of the mixture is studied. An anomalous enhancement of the mobility, compared with the case where the 3D and 2D systems do not interact with each other, is observed. The enhancement is explained by the nontrivial reflection of 3D quasiparticles from the surface covered with the 2D ^{3}He system.

Onset of Superfluidity in

We elucidate, for the first time, the overall behavior of the onset temperature of superfluidity in ^{3}He films for a wide range of film thicknesses d between 0.06 and 10 µm by taking advantage of the tunability of d implemented using microfabricated devices. We observe a suppression of the onset temperature of superfluidity T_{c}^{f} in a film from the bulk transition temperature as d decreases. In particular, T_{c}^{f} is strongly suppressed when d approaches the coherence length (∼77 nm). The observed T_{c}^{f} as a function of d is similar to that expected from the quasiclassical theory, but it unexpectedly deviates from the theoretical value by several percent when 0.5≲d≲5 µm.

Stick-Slip Motion of the Wigner Solid on Liquid Helium.

We present time-resolved transport measurements of a Wigner solid (WS) on the surface of liquid helium confined in a micron-scale channel. At rest, the WS is "dressed" by a cloud of quantized capillary waves (ripplons). Under a driving force, we find that repeated WS-ripplon decoupling leads to stick-slip current oscillations, the frequency of which can be tuned by adjusting the temperature, pressing electric field, or electron density. The WS on liquid He is a promising system for the study of polaronlike decoupling dynamics.

Electronic Magnetization of a Quantum Point Contact Measured by Nuclear Magnetic Resonance.

We report an electronic magnetization measurement of a quantum point contact (QPC) based on nuclear magnetic resonance (NMR) spectroscopy. We find that NMR signals can be detected by measuring the QPC conductance under in-plane magnetic fields. This makes it possible to measure, from Knight shifts of the NMR spectra, the electronic magnetization of a QPC containing only a few electron spins. The magnetization changes smoothly with the QPC potential barrier height and peaks at the conductance plateau of 0.5×2e^{2}/h. The observed features are well captured by a model calculation assuming a smooth potential barrier, supporting a no bound state origin of the 0.7 structure.

An incompressible state of a photo-excited electron gas.

Two-dimensional electrons in a magnetic field can form new states of matter characterized by topological properties and strong electronic correlations as displayed in the integer and fractional quantum Hall states. In these states, the electron liquid displays several spectacular characteristics, which manifest themselves in transport experiments with the quantization of the Hall resistance and a vanishing longitudinal conductivity or in thermodynamic equilibrium when the electron fluid becomes incompressible. Several experiments have reported that dissipationless transport can be achieved even at weak, non-quantizing magnetic fields when the electrons absorb photons at specific energies related to their cyclotron frequency. Here we perform compressibility measurements on electrons on liquid helium demonstrating the formation of an incompressible electronic state under these resonant excitation conditions. This new state provides a striking example of irradiation-induced self-organization in a quantum system.

Effect of Coulomb interaction on microwave-induced magnetoconductivity oscillations of surface electrons on liquid helium.

The experimental observation of the strong Coulombic effect on magneto-oscillations of the photoconductivity of surface electrons in liquid helium is reported. The observed broadening of the oscillations and shifts in positions of conductivity extrema with increasing electron density are in good agreement with the linear transport theory, which takes into account an internal electric field of fluctuational origin. These results provide important evidence for identification of the mechanism of the oscillations and zero-resistance states developed in their minima.

Evidence for reentrant melting in a quasi-one-dimensional Wigner crystal.

We demonstrate, for the first time, that a quasi-one-dimensional Wigner crystal formed on superfluid (4)He with only a few electrons in the confined direction shows reentrant melting. By transport measurements, we find oscillations in current as a function of linear density measured at a fixed driving voltage at high temperatures, and detailed analyses of transport data reveal that the oscillations originate from the periodic reduction of the melting temperature as a function of linear density. Comparison with the structural phase diagram suggests that the reduction of the melting temperature occurs at the boundaries between the different structures as the structure evolves from a single, double, followed by a triple chain.

Photon-induced vanishing of magnetoconductance in 2D electrons on liquid helium.

We report on a novel transport phenomenon realized by optical pumping in surface state electrons on helium subjected to perpendicular magnetic fields. The electron dynamics is governed by the photon-induced excitation and scattering-mediated transitions between electric subbands. In a range of magnetic fields, we observe vanishing longitudinal conductivity σ(xx)→0. Our result suggests the existence of radiation-induced zero-resistance states in the nondegenerate 2D electron system.

Resonant correlation-induced optical bistability in an electron system on liquid helium.

We show that electrons on liquid helium display intrinsic bistability of resonant intersubband absorption. The bistability occurs for comparatively weak microwave power. The underlying giant nonlinearity of the many-electron response results from the interplay of the strong short-range electron correlations, the long relaxation time, and the multisubband character of the electron energy spectrum.

Nonlinear transport of the Wigner solid on superfluid 4He in a channel geometry.

Nonlinear transport of electron crystal floating on superfluid 4He is investigated in channels 8 and 15 mum in width, where the electron velocity and driving electric field are uniform. At a high excitation, we observe a jump in the velocity caused by the decoupling of the electrons from the underlying surface deformation. The obtained driving field at the jump indicates that the decoupling occurs from the dynamically deepened surface deformation as a result of the Bragg-Cherenkov scattering of surface waves. Our results also account for the unusual nonlinear transport reported by Glasson et al. [Phys. Rev. Lett. 87, 176802 (2001)10.1103/PhysRevLett.87.176802] considering the electrode geometry.

Novel radiation-induced magnetoresistance oscillations in a nondegenerate two-dimensional electron system on liquid helium.

We report the observation of novel magnetoresistance oscillations induced by the resonant intersubband absorption in nondegenerate 2D electrons bound to the surface of liquid 3He. The oscillations are periodic in B-1 and originate from the scattering-mediated transitions of the excited electrons into the Landau levels of the first subband. The structure of the oscillations is affected by the collision broadening of the Landau levels and by many-electron effects.

Microwave-resonance-induced resistivity: evidence of ultrahot surface-state electrons on liquid 3He.

Measurements of the dc resistivity of surface-state electrons on liquid helium exposed to microwave radiation are reported. It is shown that the resonant microwave excitation of surface-state electrons is accompanied by a strong increase in their resistivity, which is opposite to the result expected from the previously used two-level model. We show that even a very small fraction of electrons excited to the first excited state and decaying back due to vapor-atom scattering strongly heat the electron system, causing a population of higher subbands. The calculated resistivity change is in good agreement with the observed data.

Texture of superfluid 3He probed by a Wigner solid.

The resistivity of the Wigner solid floating on the free surface of superfluid 3He has been measured in both the A and B phases down to 200 microK in the magnetic field. The resistivity in the A phase shows the asymptotic behavior to the temperature-square dependence at low temperatures. The temperature dependence is successfully explained by the uniform l[over] texture oriented normal to the surface and by the specular scattering of quasiparticles excited along l[over] vector. In the B phase, the resistivity exhibits the exponential decrease at low temperatures. The steep increase of the resistivity observed at high magnetic field is attributable to the nonuniform texture of the field-distorted order parameter induced near the surface.

Nonlinear Wigner solid transport on the free surface of normal and superfluid 3He.

We present measurements and a theoretical concept of the nonlinear transport in the two-dimensional Wigner solid on the free surface of normal and superfluid 3He. We have observed the transition from Ohmic to nonlinear transport over a wide temperature range, from 200 microK to 200 mK at low electron velocities. The field-velocity (E-nu) characteristics show a nonmonotonic structure with a particular local maxima. We attribute this nonlinearity to the velocity-induced deformation of the periodic surface displacement due to the extremely strong damping in Fermi-liquid 3He. The calculated E-nu curves and thresholds are in good agreement with experiment.