Time-resolved Transport Measurements Research Articles

Dynamical ordering in a two-dimensional electron crystal confined in a narrow channel geometry

We present both time-averaged and time-resolved transport measurements of a two-dimensional electron (Wigner) crystal on the surface of superfluid helium confined in a narrow microchannel. We find that the field-current characteristics of the driven crystal obtained by the time-averaged measurements exhibit oscillations and negative differential conductivity. This unusual transport behavior was observed previously by Glasson et al. [Phys. Rev. Lett. 87, 176802 (2001)] and was attributed to a nonequilibrium transition of the electron system to a novel dynamically ordered phase of current filaments aligned along the channels. Contrarily to this explanation, our time-resolved transport measurements reveal that oscillating field-current characteristics appear due to dynamical decoupling (slipping) and recoupling (sticking) of the uniform electron crystal to the liquid helium substrate. Our result demonstrates that this unusual non-linear transport effect is intrinsic, does not depend on the device geometry, and is associated with the dynamical interaction of Wigner crystal with the surface excitations of the liquid substrate.

Evidence for current-induced phase coexistence in Ca2RuO4 and its influence on magnetic order

Combining quasistatic and time-resolved transport measurements with X-ray and neutron diffraction experiments we study the non-equilibrium states that arise in pure and in Ti substituted Ca$_2$RuO$_4$ under the application of current densities. Time-resolved studies of the current-induced switching find a slow conductance relaxation that can be identified with heating and a fast one that unambiguously proves an intrinsic mechanism. The current-induced phase transition leads to complex diffraction patterns. Separated Bragg reflections that can be associated with the metallic and insulating phases by their lattice parameters, indicate a real structure with phase coexistence that strongly varies with temperature and current strength. A third contribution with a $c$ lattice constant in between those of metallic and insulating phases appears upon cooling. At low current densities, this additional phase appears below $\sim$100 K and is accompanied by a suppression of the antiferromagnetic order that otherwise can coexist with current carrying states. A possible origin of the intermediate phase is discussed.

Model Predictions for Time-Resolved Transport Measurements Made near the Superfluid Critical Points of Cold Atoms and K_{3}C_{60} Films.

Recent advances in ultrafast measurement in cold atoms, as well as pump-probe spectroscopy of K_{3}C_{60} films, have opened the possibility of rapidly quenching systems of interacting fermions to, and across, a finite temperature superfluid transition. However, determining that a transient state has approached a second-order critical point is difficult, as standard equilibrium techniques are inapplicable. We show that the approach to the superfluid critical point in a transient state may be detected via time-resolved transport measurements, such as the optical conductivity. We leverage the fact that quenching to the vicinity of the critical point produces a highly time dependent density of superfluid fluctuations, which affect the conductivity in two ways. First, by inelastic scattering between the fermions and the fluctuations, and second by direct conduction through the fluctuations, with the latter providing a lower resistance current carrying channel. The competition between these two effects leads to nonmonotonic behavior in the time-resolved optical conductivity, providing a signature of the critical transient state.

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.

Fractionalized wave packets from an artificial Tomonaga–Luttinger liquid

The model of interacting fermion systems in one dimension known as a Tomonaga-Luttinger liquid (TLL) provides a simple and exactly solvable theoretical framework that predicts various intriguing physical properties. Evidence of a TLL has been observed as power-law behaviour in electronic transport on various types of one-dimensional conductor. However, these measurements, which rely on d.c. transport involving electron tunneling processes, cannot identify the long-awaited hallmark of charge fractionalization, in which an injection of elementary charge e from a non-interacting lead is divided into the non-trivial effective charge e* and the remainder, e-e* (refs6, 7, 8). Here, we report time-resolved transport measurements on an artificial TLL composed of coupled integer quantum Hall edge channels, in which we successfully identify single charge fractionalization processes. A wave packet of charge q incident from a non-interacting region breaks up into several fractionalized charge wave packets at the edges of the artificial TLL, from which transport eigenmodes can be evaluated directly. These results are informative for elucidating the nature of TLLs and low-energy excitations in the edge channels.

Charge-cluster glass in an organic conductor

Geometrically frustrated spin-systems do not order magnetically even at absolute zero, forming instead a spin liquid or a glassy state. An organic conductor in which the charges, rather than spins, are frustrated now shows a similar absence of long-range order, resulting in a charge-cluster glass at low temperature. Geometrically frustrated spin systems often do not exhibit long-range magnetic ordering, resulting in either quantum-mechanically disordered states, such as quantum spin liquids1, or classically disordered states, such as spin ices2,3 or spin glasses4. Geometric frustration may play a similar role in charge ordering5,6, potentially leading to unconventional electronic states without long-range order; however, there are no previous experimental demonstrations of this phenomenon. Here, we show that a charge-cluster glass evolves on cooling in the absence of long-range charge ordering for an organic conductor with a triangular lattice. A combination of time-resolved transport measurements and X-ray diffraction reveals that the charge-liquid phase has two-dimensional charge clusters that fluctuate extremely slowly (<10–100 Hz) and heterogeneously. On further cooling, the cluster dynamics freezes, and a charge-cluster glass is formed. Surprisingly, these observations correspond to recent ideas regarding the structural glass formation of supercooled liquids7,8,9,10. Glassy behaviour has often been found in transition-metal oxides, but only under the influence of randomly located dopants11,12. As organic conductors are very clean systems, the present glassy behaviour is probably conceptually different.

Charge carrier velocity distributions in field-effect transistors

The measurement of the distribution of charge carrier velocities in a field-effect transistor can provide considerable insight into charge transport mechanisms and structure-property relationships. We have developed such a method and have applied it to study temperature-dependent velocity distributions in solution-processed zinc-tin oxide thin-film transistors. Two distinct transport pathways, each with a different activation energy, have been observed, in contrast to a single activation energy yielded by steady-state measurements. Our results show that more insight into charge transport behavior and phenomena can be obtained with such time-resolved transport measurements.

Nitrogen-doped carbon nanotube structure tailoring and time-resolved transport measurements in a transmission electron microscope

A dynamical response of the current-voltage characteristics of ropes and individual structures of multiwalled nitrogen-doped carbon nanotubes has been observed inside a transmission electron microscope. The drastic change of the current transport properties is thought to be due to purging of contaminating gaseous surface adsorbed species. In addition, an in situ methodology was developed to obtain individually pure and length-tailored nanotubes. By carefully controlling the current flow across the nanotubes, residual surface-decorating and encapsulated catalyst particles were eliminated, and short sections of the nanotubes (200to500nm) were cut.

Onset of Motion and Dynamic Reordering of a Vortex Lattice

Time resolved transport measurements on a driven vortex lattice in an undoped 2H-NbSe2 crystal show that the response to a current pulse is governed by healing of defects as the lattice evolves from a stationary to a moving steady state and that the response time reflects the degree of order in the initial vortex state. We find that stationary field cooled vortex lattices become more ordered with decreasing temperature and identify a temperature below which a qualitative change in the response signals the disappearance of topological defects.

Probing the edge of a 2DEG by time-resolved transport measurements

Abstract We have studied time-resolved transport in a two-dimensional electron gas in the integer and fractional quantum Hall regime. In samples with a smooth edge and an additional screening electrode, the propagation velocity of high-frequency signals depends on the number and width of the involved edge channels, and thus can be used to obtain an approximate electron density profile. The amplitude of the transmitted signals oscillates with respect to the applied magnetic field with maxima appearing close to integer and fractional bulk filling factors. While the data around filling factor 1 3 are qualitatively similar to those around filling factors 1 and 2, deviations appear around filling factor 2 3 . At odd filling factors, signal propagation in partially decoupled edge channels is observed.

Photoconductive switches for time-resolved transport measurements at low temperatures and high magnetic fields

Metal-semiconductor-metal photodiodes were optimized as pulse generators for time-resolved electrical measurements at low temperatures and high magnetic fields, using a laser diode as the light source. For the photoconductor, a heterostructure consisting of high-quality GaAs and low-temperature grown GaAs was used. The dependence of the photocurrent pulses on the applied magnetic field was determined. A two-dimensional electron gas in a magnetic field served as the device under test for the time-resolved measurements. We observed a splitting of the incident pulse into two transmitted pulses resulting from modal dispersion in the two-dimensional electron gas.

Time-resolved measurements of transport in edge channels.

In the quantum Hall regime the influence of edge channels on the evolution of the current through the device has been analyzed by time-resolved transport measurements. The observed nonlinear dependence of the time-resolved current on applied voltage is qualitatively in agreement with the model describing edge channels as compressible strips separated by incompressible regions.