Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session J9: Transport in Semiconductors I |
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Sponsoring Units: DCMP Chair: Sergei Vitkalov, City College of New York Room: A105 |
Tuesday, March 16, 2010 11:15AM - 11:27AM |
J9.00001: Temperature Dependence of Valley Dynamics in a Six-fold Degenerate Si-(111) 2DEG Tomasz M. Kott , Binhui Hu , Robert N. McFarland , Bruce E. Kane Using a two-dimensional electron gas (2DEG) on hydrogen terminated Si-(111) surfaces, we have studied magnetic field dependent valley dynamics of this six-fold degenerate system [1]. High field (up to 12 T) magneto-transport measurements indicate field-dependent valley splitting from the nominally six-fold degenerate system. In a simple non-interacting picture, one would expect no odd filling factors; although crystal misorientation and strain can lead to $\nu=2$ and 4, no such mechanism exists for $\nu=1,3,5$ [2]. Our samples show easily resolvable filling factors of, amongst others, 3 and 5, as well as filling factor dependent resistance anisotropies which indicate possible many-body effects beyond the simple interaction-free model. I will show activation measurements of these filling factors, discuss the stability of the resulting energy gaps, and consider potential causes of the splitting. A comparison of these activation measurements across different densities ($n_s=3 - 8 \times 10^{11}$ cm$^{-2}$) and samples of varying mobility ($10,000 - 100,000$ cm$^2/$Vs) will also be made. [1] R. N. McFarland et al., {\it Phys.~Rev.~B} {\bf 80} 161310R (2009), [2] K. Eng, R. N. McFarland, and B. E. Kane, Phys. Rev. Lett. 99, 016801 (2007) [Preview Abstract] |
Tuesday, March 16, 2010 11:27AM - 11:39AM |
J9.00002: Effect of disorder on the valley splitting on hydrogen terminated silicon (111) surfaces Binhui Hu , Tomasz M. Kott , Robert N. McFarland , Bruce E. Kane High quality hydrogen terminated Si (111) surfaces provide us with a new material system to study a two-dimensional electron system with multi-valley interactions. In our field effect structure where a H-Si(111) substrate is bonded to a SOI substrate, two-dimensional electrons are confined at the hydrogen-terminated Si(111) surface with a vacuum barrier. In our previous work, a high-mobility ($\mu $=110,00cm2/Vs) sample shows sixfold degeneracy,[1] while on a sample with $\mu $=24,000cm2/Vs, the sixfold degeneracy is broken.[2] In order to find out the relationship between the electron mobility and the valley splitting, we have investigated a number of devices with mobility ($\mu $=10,000$\sim $25,000 cm2/Vs), and observed that most of them show the sixfold degeneracy, while a few of them show large asymmetry. Possible explanations will be presented. We will also compare the temperature dependence of the electron mobility between the high-mobility sample and a moderate-mobility sample, and discuss the possible different limiting factors behind them. [1] R. N. McFarland et al., Phys.Rev.B 161310R (2009). [2] K. Eng, et al., Phys. Rev. Lett. 99, 016801 (2007). [Preview Abstract] |
Tuesday, March 16, 2010 11:39AM - 11:51AM |
J9.00003: Probing valley-valley drag interactions in a sixfold valley-degenerate 2DES on H-Si(111) Robert N. McFarland , Luyan Sun , Tomasz M. Kott , Bruce E. Kane , Kevin Eng In 2D electron systems with multiple anisotropic valleys, multi-valley effects can significantly decrease the Hall coefficient $R_H$ relative to its classical value in the $B\to$0 limit [1]. Valley-valley drag interactions tend to suppress this behavior, making $R_H$ a particularly sensitive probe of electron-electron interactions in such systems. We report systematic measurements of this effect on H-terminated Si(111) surfaces with sixfold valley degeneracy and find that the temperature and density dependence (for 0.07 K $< T <$ 9 K and $2 < n_s < 7\times10^{11}$/cm$^2$) of the damping rate due to drag agrees well with Fermi liquid theory and possible weak disorder effects. However, we consistently observe a negative drag effect in the $T\to$0 limit that is not explained by these models. \\[4pt] [1] R.N. McFarland, T.M. Kott, L. Sun, K. Eng, \& B.E. Kane, Phys. Rev. B 80, 161310(R) (2009). [Preview Abstract] |
Tuesday, March 16, 2010 11:51AM - 12:03PM |
J9.00004: Ferromagnetic-paramagnetic transition in p-Si/SiGe due to Landau levels overlapping Alexey Suslov , I.L. Drichko , I.Yu. Smirnov , O.A. Mironov , D.R. Leadley The magnetoresistance $\rho_{xx}$ and $\rho_{xy}$ as well as attenuation and velocity change of surface acoustic waves were measured in a p-Si/SiGe sample with $p$=2$\times$10$^{11}$\,cm$^ {-2}$. The research was performed in the temperature range of 0.3-2 K and in the magnetic fields of up to 18 T tilted with respect to the two-dimensional (2D) channel plane. The dependence of the g-factor g*($\Theta$)/g*($0^{\circ}$) on the tilt angle was determined. The measurements of $\rho_{xx}$ and $\rho_{xy}$ in the tilted magnetic field showed that the anomaly in $\rho_{xx}$ observed at filling factor $\nu$ =3/2 is insignificant in the conductivity $\sigma_{xx}$. The anomaly in $\sigma_{xx}$ at $\nu$ =2 might be explained by overlapping of the levels with different spins 0$\uparrow$ and 1$\downarrow$ when the tilt angle of the applied magnetic field is changed. The overlapping occurs at $\Theta$ of about $60^{\circ}$ and causes a ferromagnetic-paramagnetic transition. [Preview Abstract] |
Tuesday, March 16, 2010 12:03PM - 12:15PM |
J9.00005: $\nu=1$ bilayer in a periodic potential Ganpathy Murthy , Jianmin Sun , Herbert Fertig , Noah Bray-Ali The clean $\nu = 1$ quantum Hall bilayer is an excitonic superfluid. Experimentally, due to disorder, the counterflow conductivity $\sigma_{CF}$ remains finite even at the lowest $T$ and the zero-bias peak has finite width. We mimic the nonperturbative effects of disorder [1] by a periodic potential[2] which couples, in a spin-only model, to the topological density ${\mathbf n} \cdot{\partial_x{\mathbf n}}\times{\partial_y{\mathbf n}}$(=charge density). We find a set of ground state phase transitions as the potential strength increases, with increasing local charge density. The transitions are weakly first order, with a new, quadratically dispersing, charge-carrying mode which represent incipient meron-antimeron pairs forming in regions of large potential gradient. These modes can become nearly gapless at and near the transition, which we argue leads to a strong suppression of the interlayer tunneling h. We demonstrate that near the transitions vortex-antivortex pairs become easy to create, leading to a strong suppression of $T_{KT}$ . We discuss an effective theory that incorporates both the Goldstone mode and the new, quadratically dispersion mode. \\[4pt] [1] H. A. Fertig and G. Murthy, prl {\bf 95}, 156802 (2005).\\[0pt] [2] G. Murthy and S. Sachdev, prl {\bf 101}, 226801 (2008). [Preview Abstract] |
Tuesday, March 16, 2010 12:15PM - 12:27PM |
J9.00006: Dissipationless Nernst effects Vadim Oganesyan , Doron Bergman We develop a theory of transverse thermoelectric (Peltier) conductivity, $\alpha_{xy}$, in finite magnetic field -- this particular conductivity is often the most important contribution to the Nernst thermopower. We demonstrate that $\alpha_{xy}$ of a free electron gas can be expressed purely and exactly as the entropy per carrier irrespective of temperature (which agrees with seminal Hall bar result of Girvin and Jonson). In two dimensions we prove the universality of this result in the presence of disorder which allows explicit demonstration of a number features of interest to experiments on graphene and other two-dimensional materials. We also exploit this relationship in the low field regime and to analyze the rich singularity structure in $\alpha_{xy}(B, T)$ in three dimensions; we discuss its possible experimental implications. [Preview Abstract] |
Tuesday, March 16, 2010 12:27PM - 12:39PM |
J9.00007: Quantum dot based refrigeration of a 2D electron gas Jonathan R. Prance , Charles G. Smith , Jonathan P. Griffiths , Simon J. Chorley , Dave Anderson , Geb A. C. Jones , Ian Farrer , Dave A. Ritchie The concept of cooling a 2D electron gas (2DEG) using energy-selective transport through quantum dots was first proposed by Edwards et al. [1]. Their scheme utilized two dots: one to remove hot electrons from an isolated 2DEG, the other to remove hot holes. The resulting current removes heat from the 2DEG and dissipates it in adjacent reservoirs. We will present measurements of a device designed to cool a 6$\mu $m$^{2}$ 2DEG using this scheme [2]. The measurements reveal a complication not previously considered: the charging energy of the cooled 2DEG itself. We will outline a model that accounts for this, and shows that the device can still achieve cooling. We will also show how the temperature of the cooled region can be inferred from the line-shape of the current through the device. By comparing measured line-shapes with predictions, we find the data to be consistent with cooling of the isolated 2DEG by over 90mK in the best case. [1] Edwards et al. Phys. Rev. B 52(8) p5714 (1995) [2] Prance et al. Phys. Rev. Lett. 102, 146602 (2009) [Preview Abstract] |
Tuesday, March 16, 2010 12:39PM - 12:51PM |
J9.00008: Fermi edge singularity in a tunnel junction Jin Zhang , Yury Sherkunov , Nicholas d'Ambrumenil , Boris Muzykantskii We present results on the non-equilibrium Fermi edge singularity (FES) problem in tunnel junctions. The FES, which is present in a Fermi gas subject to any sudden change of potential, manifests itself in the final state many body interaction between the electrons in the leads [1]. We establish a connection between the FES problem in a tunnel junction and the Full Counting Statistics (FCS) for the device [2]. We find that the exact profile of the changing potential (or the profile for the barrier opening and closing in the tunnel junction case) strongly affects the overlap between the initial and final state of the Fermi gas. We factorize the contribution to the FES into two approximately independent terms: one is connected with the short time opening process while the other is concerned with the long time asymptotic effect, namely the Anderson orthogonality catastrophe. We consider applications to a localized level coupled through a tunnel barrier to a 1D lead driven out of equilibrium [3]. References: [1] G. Mahan, Phys. Rev. 163, 1612 (1967); P. Nozieres and C. T. De Dominicis, Phys. Rev. 178, 1079 (1969); P. Anderson, Phys. Rev. Lett. 18, 1049 (1967) [2] J. Zhang, Y. Sherkunov, N. d'Ambrumenil, and B. Muzykantskii, ArXiv:0909.3427 [3] D. Abanin and L. Levitov, Phys. Rev. Lett. 94, 186803 (2005) [Preview Abstract] |
Tuesday, March 16, 2010 12:51PM - 1:03PM |
J9.00009: Coulomb drag upturn in an undoped electron-hole bilayer in perpendicular and parallel magnetic fields Christian Morath , John Seamons , John Reno , Mike Lilly A low-temperature upturn of the Coulomb drag resistivity $\rho _{D }$measured in undoped electron-hole bilayer devices, possibly manifesting from formation of a superfluid condensate or density modulated state, was recently observed. Here the effects of perpendicular and parallel magnetic fields on the drag upturn are examined. Measurements of $\rho _{D}$ and drive layer resistivity $\rho _{xx-e}$ as a function of temperature and magnetic field in two uEHBL devices are presented. In B$_{\bot }$, the drag upturn was enhanced as the field increased up to roughly .2 T, beyond which oscillations in $\rho _{D}$ and $\rho _{xx-e}$, reflecting Landau level formation, begin appearing. A small phase offset between those oscillations, which decreased at higher fields and temperatures, was also observed. In B$_{\vert \vert }$, the drag upturn magnitude diminished as the field increased. Above the upturn regime, both $\rho _{D}$ and $\rho _{xx-e}$ were enhanced by B$_{\vert \vert }$, the latter via decreased screening of the uniform background impurities. This work has been supported by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract No. DE-AC04-94AL85000. [Preview Abstract] |
Tuesday, March 16, 2010 1:03PM - 1:15PM |
J9.00010: Ballistic transport in InSb quantum well nano-structures A.M. Gilbertson , D.E. Read , L.F. Cohen , S.A. Solin , M. Fearn , L. Buckle , A. Korm\'anyos , C.J. Lambert Electron mobilities in InSb quantum wells (QWs) have recently been reported in excess of 6 m$^{2}$V$^{-1}$s$^{-1}$ at 290 K translating to long mean free paths (mfp) of the order 650 nm [1]. For applications requiring high spatial resolution, device miniaturisation leads to the relevant dimensions becoming comparable to the mfp and ballistic transport becomes important. Whilst ballistic transport at 290 K has been demonstrated in a 200 nm Hall cross fabricated from GaAs QWs [2], the inherent complications in the growth of high quality InSb QWs has to-date limited this observation to 205 K in an equivalent device [3]. We report on the fabrication, modelling and measurement of devices patterned down to 50 nm on InSb QW material. The effect of device scaling and fabrication on the transport properties will be discussed. Features of ballistic transport are presented at elevated temperatures. \newline [2] A.M.Gilbertson, et al. PRB 79, 235333 (2009). \newline[3] Y.Hirayama et al., APL 63, 2366 (1993). \newline[4] N.Goel et al. Physica E 20,251 (2004). [Preview Abstract] |
Tuesday, March 16, 2010 1:15PM - 1:27PM |
J9.00011: Pump-and-probe measurements of the nuclear spin relaxation time in a two-subband electron system Hong Pan , Hongwen Jiang Nuclear spin relaxation measurements are emerging as an effective tool to study the low-frequency spin dynamics of two-dimensional electrons in quantum Hall systems. We present such a measurement in a two-dimensional electron system consisting of two filled subbands. A recently developed pump-and-probe technique [1] was used to obtain the nuclear spin relaxation time in a region where the two sets of Landau levels, corresponding to the two subbands, were nearly degenerate. The pump-and-probe technique allowed us to measure the relaxation time over a much broader range of magnetic fields and electron densities than that in an earlier nuclear magnetic resonance study [2]. An array of interesting observations will be reported,including phase space and temperature dependence study. The project is supported by the NSF under Grant No. DMR-0804794. \\[4pt] [1] N. Kumada, K. Muraki, and Y. Hirayama, Science 313, 329 (2006). \\[0pt] [2] X. C. Zhang, G. D. Scott and H. W. Jiang, Phys. Rev. Lett. 98,246802 (2007). [Preview Abstract] |
Tuesday, March 16, 2010 1:27PM - 1:39PM |
J9.00012: Observation of glassy dynamics in high quality GaAs two-dimensional hole systems using transport and RF reflectometry A.R. Hamilton , L.J. Taskinen , A.P. Micolich , M.Y. Simmons , D.A. Ritchie , M. Pepper We have used standard low frequency ac transport measurements, as well as large bandwidth rf reflectometry measurements, to study the non-equilibrium relaxation of the resistance of strongly interacting, high quality 2D GaAs hole systems at milliKelvin temperatures. We observe logarithmic relaxation of the resistance over 7 orders of magnitude in time (from 1ms to 10,000s) following a discontinuous step in the gate voltage. This is characteristic of glassy behaviour, and may be evidence for the formation of a Coulomb glass. A comparison of the logarithmic behaviour observed in different samples provides clues as to the origins of these slow glassy dynamics. [Preview Abstract] |
Tuesday, March 16, 2010 1:39PM - 1:51PM |
J9.00013: 0.7 anomaly due to the Rashba interaction in a nonuniform electric field J.H. Hsiao , K.M. Liu , S.Y. Hsu , T.M. Hong We demonstrate that three conductance features, 0.5 and 0.7G0 plateaus and a dip at 0.5G0, observed in quantum point contacts (QPCs) can be consistently explained by the Rashba interaction in the nonuniform electric field created by the side gates along the transport direction. A quantity is defined which depends on the extent of this nonuniformity and the electron density. A short QPC tends to have a small . Only when is large will the Rashba interaction produce a potential well deep enough to localize the electron. This provides the bound state that forms the Kondo resonance with the tunneling electrons. We propose to compare the medium/long QPC to small/large quantum dots, which are governed by the Kondo physics and the Coulomb blockade, respectively. The relation between 0.7 anomaly and the side-gate voltage, length of QPC, and temperature can all be determined to agree qualitatively with the experiments. [Preview Abstract] |
Tuesday, March 16, 2010 1:51PM - 2:03PM |
J9.00014: Giant piezoresistance in silicon/metal hybrid resistors Alistair Rowe We report a giant room temperature piezoresistance in planar metal/semiconductor hybrid resistors fabricated from Aluminum and Silicon, with gauge factors $\sim $1000 for strains up to 10$^{-5}$. This new effect [1] is shown to be due to the geometric arrangement of the metal and the semiconductor, and results from a stress induced redirection of the injected current from the metallic shunt into the semiconductor. Since there is a large difference in the electrical conductivity of these materials, this yields a large increase in the device resistance. This ``extraordinary piezoconductance'' will be compared and contrasted with the extraordinary magnetoresistance previously observed in metal/semiconductor hybrid resistors. \\[4pt] [1] A.C.H. Rowe et al., Phys. Rev. Lett. 100, 145501 (2008) [Preview Abstract] |
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