Tentative. To be changed any time.
| Tue 4 Feb
at Korea Univ. |
Wed 5 Feb
at Korea Univ. |
Thu 6 Feb
at KIAS |
Fri 7 Feb
at KIAS |
|
| 09:00--09:15 | Opening | |||
| 09:15--10:00 |
Chemla | Kim (Philip) | Kuk (Young) | Cuevas |
| 10:00--10:45 |
Park (Q-Han) | Lee (Sanghoon) | Nitta | Ng |
| 10:45--11:00 | coffee break | |||
| 11:00--11:45 |
Nakamura | Chang (Kee Joo) | Lee (Yun Hi) | Nitta |
| 11:45--13:30 |
lunch break | |||
| 13:30--14:15 |
Choi (Han-Yong) | Strunk | Kim (Ju-Jin) | TOUR (13:00~17:00) |
| 14:15--15:00 |
Kim (Nam) | Chemla | Cuevas | |
| 15:00--15:30 |
coffee break | |||
| 15:30--16:15 |
Kim (Philip) | Nakamura | Ng | |
| 16:15--17:00 |
Kahng (Se Jong) | Kim (Gyu-Tae) | Strunk | |
| 17:00--17:45 |
Sim (Heung-Sun) | Banquet | ||
More than thirty years ago Keldysh and Kozlov [1] have shown that in the dilute limit, excitons should undergo Bose-Einstein condensation (BEC). Because of their very small mass at experimentally accessible densities the 3D critical temperature for exciton--BEC should be about five orders of magnitude higher than for atom--BEC. To date, however, exciton--BEC has not been observed.
We report on two sets of experiments where we have observed non-classical effects in highly degenerate exciton--gases. We exploit the properties of quasi--2D indirect excitons (i-X) in GaAs/AlGaAs coupled quantum wells (CQW) [2]: (i) long lifetime, (ii) efficient cooling via emission of bulk LA phonons, (iii) repulsive interaction, ∝r3, which favors condensation, limits screening and prevents collapse toward molecules and eventually droplets or plasmas.
Inspired by atom--BEC that is observed in atomic gases confined in potential traps, we collected i-Xs in an in-plane natural potential trap [3]. Spectrally and spatially resolved photoluminescence (PL) under uniform and localized excitation far away from the traps reveal i-X transport over distances l ? 300 ?m and collection the trap at densities, NXtrap≈1011cm2, corresponding a Bose occupation number of the lowest energy state ≈0.3→0.5.
Exploring the i-X PL away from any trap, we have observed a concentric-rings structure and a macroscopically ordered state of i-X appearing in the ring the most remote from the excitation spot [4]. The most interesting feature of that ring is its abrupt fragmentation at temperatures T<3K into a periodic array of circular fragments. The existence of this periodic ordering shows that the i-X state formed in the ring has a macroscopic coherence of length scale Lcoh≈1mm. This coherence appears spontaneously is not driven by a laser excitation.
Macroscopically ordered arrays of vortices in quantum liquids, such as superconductors, He-II, and atom BEC, demonstrate quantum macroscopic coherence in flowing superfluids. We note however that spontaneous macroscopic flow organization with periodic vortical structures is a general property of thermodynamically open systems described by nonlinear partial differential equations, including classical ones [5].
[1] Keldysh, L.V. and Kozlov, A.N. Zh. Eksp. Teor. Fiz. 54, 978 (1968) {Sov. Phys. JETP 27, 521 (1968)}.
[2] Butov, L.V. and Filin, A.I. Phys. Rev. B 58, 1980 (1998).
[3] Butov, L.V., Lai, C. W., Ivanov, A.L., Gossard, A. C., and Chemla D. S. Nature, 417, 47 (2002).
[4] Butov, L.V. Gossard, A. C., and Chemla D. S. Nature 418, 751 (2002).
[5] Taylor, G.I. Philos. Trans. R. Soc. London Ser. A-223, 289 (1923).
Quantum-coherent two-levels systems (or qubits) can be obtained in mesoscopic superconducting circuits containing small Josephson junctions. I will review recent activites on variety of Josephson- junction qubits and introduce experiments on a charge qubit and a flux qubit. Coherent control of the qubit and the decoherence in the experiment will be discussed.
Resonant transport and spin-dependent transport in carbon nanotube-based electronic device will be presented. A resonant peak in the differential conductance curve of the multi-walled carbon nanotubes in a crossed geometry was observed. The observed conductance peak was asymmetric and was well explained by Fano resonance originating from the scattering at the contact region of the two nanotubes. The conductance peak depended sensitively on the external magnetic field and exhibits Aharonov-Bohm-type oscillation. Also presented will be spin-dependent transport in carbon nanotube with ferromagnetic electrodes. Clear hysteresis in the magneto-resistance curve was observed, which was explaind by tunnel magneto-resistance effect.
In this presentation, I will discuss the recently developed novel methods to measure thermal transport properties of nanoscale materials using modern state-of-art microelectromechanical system (MEMS) technologies. Suspended mesoscopic devices that isolate heat flow from the experimental environment have been fabricated using MEMS technology. The thermal conductivity of carbon nanotubes and silicone nanowires bridging these suspended structures was determined. The observed thermal conductivity of carbon nanotubes is an order of magnitude higher than estimation from previous experiments that used macroscopic mat samples. Suppressed Umklapp phonon scattering and absence of boundary scattering result in long phonon mean free path comparable to the measured length of nanotube. Unlike nanotubes, strong reduction of thermal conductivity of silicon nanowires was observed as the diameters of silicon nanowire decreases. This reduction of thermal conductivity in silicon nanowires was ascribed by increasing phonon boundary scattering. In addition, the temperature distribution in carbon nanotubes heated by an electric current has been investigated with a scanning thermal probe microscope. An AFM tip with a microscopic thermocouple junction at the end of the tip probes the temperature distribution of the nanotube with a resolution less than 100 nm. The temperature distribution in the nanotube at high currents clearly indicates bulk dissipation of energy inside the tube and significant heat dissipation at the contact between the tube and electrodes as well. The implication of these experimental results for applications of carbon nanotubes as current and heat carrying wires and extending these experimental techniques to other nanoscale materials will be discussed.
Depending on atomic structure, single wall carbon nanotubes are either metallic or semiconducting. One can chemically or physically modify carbon nanotubes to produce novel materials with useful electronic properties. Recently, it was shown that fullerenes or endohedral metallofullerences can be encapsulated into single wall carbon nanotubes, forming peapod-like structures. We have studied atomic and electronic structures of nanotube peapods with low temperature (4.2K) scanning tunneling microscopy and scanning tunneling spectroscopy. We found that the encapsulated molecules strongly modify the electronic structures as well as physical structures of nanotubes. The encapsulated fullerenes are imaged as bright protrusions at some bias voltages and not at other bias voltages. We were able to correlate this behavior with measured local electronic structures. We further measured position dependent electronic states, as we move a tip along the axis of nanotubes. Measured electronic structures were discussed with the help of theoretical results.
We study shot noise for generic ballistic quantum dots whose classical phase space consists of both regular and chaotic regions. The noise is found to be systematically suppressed below the universal value of fully chaotic dots due to the deterministic nature of transport through regular regions and along short chaotic trajectories. From the analysis incorporating diffractive scatterings, it is also revealed that shot noise carries the information for the time scales of deterministic transport through the dots. We suggest that shot noise can be used as a probe of classical phase-space structures of ballistic quantum dots.
Reference: H.-S. Sim and H. Schomerus, Phys. Rev. Lett. 89, 066801 (2002).
Thermopower expresses the ability of a system of charged particles to generate an electromotive force when a temperature gradient is applied across the system. In mesoscopic systems, quantum transport nature of electrons through the narrow channel has been exhibited an oscillating thermopower as the conductance changes. In this presentation, I will discuss thermoelectric properties of individual single walled carbon nanotubes (SWNTs) that have been measured on mesoscopic scales. A micropatterned heater produces a temperature gradient on a substrate in contact with SWNTs. Microfabricated temperature sensors, voltage leads contacting an individual SWNT, and the presence of back gate electrode allow us to measure the thermoelectric power (TEP) of individual nanotubes at different chemical potentials. Strong modulation of measured TEP has been observed both in metallic and semiconducting SWNTs as a function of the gate voltage. Upon comparison with differential conductance measurements, we show the qualitative verification of the Mott formula in these mesoscopic systems.
Magnetic semiconductors hold promise for closing the gap between information storage and information processing in microelectronics.
Indeed, a completely new research area referred to as 'spintronics' has evolved around this idea [1,2].
The unique possibility to tailor electronic and magnetic properties independently in these materials has triggered a variety
of revolutionary device concepts, based on controlling the interaction of carrier spins and the spins of magnetic ions.
This includes suggestions for magnetic-quantum-dot-based spin filters and spin memories[3]
or the use magnetic single quantum dots in quantum computation[4].
Here we demonstrate an all-optical method to analyze statistical fluctuations of the magnetization on a 10 nanometer scale.
This is based on the sensitive dependence of the photoluminescence (PL) signal of a single electron-hole pair confined
in a magnetic single quantum dot on the spin alignment of the magnetic ions.
We demonstrate that the characteristic broadening of the PL emission from single magnetic quantum dots
can be used to directly monitor the statistical magnetic fluctuations on the nanometer scale.
The dynamical response of a paramagnetic spin system to the exchange field of quasi-zero dimensional electron-hole pairs
in semiconductor quantum dots is investigated by time-resolved spectroscopy.
The spin response time is extracted from the transient spectral shift of the photoluminescence signal caused
by the dynamical spin alignment of magnetic ions incorporated in the crystal matrix.
The formation of this ferromagnetically aligned spin complex is demonstrated to be surprisingly stable as compared to bulk systems,
evenat elevated temperatures and high external magnetic field.
[1] Prinz, G. Spin-polarized transport. Physics Today 48, 58 (1995).
[2] Awschalom, D.D. & Kikkawa, J.M. Electron spin and optical coherence in semiconductors. Physics Today 52, 33 (1999).
[3] Recher, P., Sukhorukov, E.V. & Loss, D. Quantum dot as spin filter and spin memory Phys. Rev. Lett. 85, 19621965 (2000).
[4] Loss, D. & DiVincenzo, D.P. Quantum computation with quantum dots. Phys. Rev. A 57, 120126 (1998).
I will talk about the transport properties of various nanotube hybrid systems, based on electronic structure calculations. For carbon nanotube hybrid systems such as nanotube quantum dots, peapods, and telescoping tubes, rich behavior is found in transport, with various resonances and antiresonances in transmission. Applications are demonstrated for nanodevices such as single electron transistors and field effect transistors. Finally, the spectral correlation is investigated in incommensurate multiwalled nanotubes, discussing the transport behavior, ballistic vs diffusive.
We report on shot noise created by charge quanta as opposed to shot noise created by flux quanta. The dual relationship between the two noise mechanisms is discussed. As an example of charge noise, we present data on the noise generated by open, ballistic cavities displaying fully chaotic dynamics. We find that the noise is suppressed with respect to the Poission noise by an universal Fano factor of 1/4 as predicted by theory. This noise originates from the quantum diffraction of electron trajectories inside the cavity. For long dwell times within the cavity a crossover to an electron heating regime is observed.
The vortex noise is measured in microbridges made from the amorphous superconductor NbGe. As a consequence of very low intrinsic pinning an extended vortex liquid state is present in this material. We study the vortex noise above and below the irreversibility line. Below the irreversibility line we find shot noise like behavior with Fano factors considerably smaller than unity, which vanishes at the irreversibility line. Above the irreversibility line, no shot noise is observed. At higher voltages, however, we measure a substantial amount of 1/f-noise which gradually decreases when approaching the upper critical field. We show that this noise is not related to residual pinning and attribute it to generic fluctuations of the vortex core radius.
Over the last decade a wealth of novel information on Coulomb correlation and electronic dynamics in semiconductors has been obtained in regimes where traditional assumptions fail [1]. The usual description of solids assumes a quasi-stationary limit in which a number of familiar approximations such as Fermi's Golden Rule, Botzmann's Kinetics and the Random Phase Approximation are valid. However, at low density and on a time- scale short compared to the time between quasi-particle collisions, not enough events happen over the time span of an experiment for a quasi-particle to interact with a substantial fraction of its neighbors. Thus it become possible to observed deviations from mean-field theory, a regime where the scattering fluctuations induce large fluctuations of the mean-field order parameters and high order correlations become dominant. Another regime where traditional assumptions fail is that of strongly correlated dynamics where it is no more justified to treat the interactions between quasi-particles as scattering events local in space and time, a key assumption of the Boltzmann theory. Recent coherent time resolved spectroscopy studies with time resolution much shorter than quasi-particle scattering time scales have revealed features related to genuine 4-particle and 6-particle correlations [2] and non-Markovian memory effects in electron-electron interaction in semiconductor nanostructures, in particular in the presence of a two-dimensional electron gas in the Quantum Hall Effect regime [3].
In this conference I give a comprehensive and balanced account of recent advances, both experimental and theoretical, in our understanding of dynamical Coulomb correlations and coherence in semiconductors. I shall try to focus on the most important physics and, as much as possible, give an intuitive picture of the new phenomena that have been observed.
[1] D. S. Chemla and J. Shah, Nature 411, 549 (2001) and references therein
[2] S.R. Bolton, U. Neukirch, L.J. Sham, D.S. Chemla and V.M. Axt, Phys. Rev. Lett 85, 2002 (2000) and V.M. Axt, S.R. Bolton, U. Neukirch, L.J. Sham and D.S. Chemla, Phys. Rev. B. 63, 115303 (2001)
[3] N. A. Fromer, C. E. Lai, and D. S. Chemla, I. E. Perakis, D. Driscoll and A. C. Gossard Phys. Rev. Lett. 89, 067401 (2002), N. A. Fromer, C. Shuller, C. E. Lai, D. S. Chemla, I. E. Perakis, D. Driscoll and A. C. Gossard, Phys. Rev. B 66, 205314 (2002) and I. E. Perakis and D. S. Chemla, Phys. Stat. Sol.234, 242 (2002)
Many electronic devices made of synthetic nanofibers have been demonstrated since the first report on the carbon nanotube field effect transistor in 1997. With the advent of compound semiconductor nanowires, the various combinations of different nanofibers were also reported as possible nanoscale electronic devices. Depending on the different electronic properties of individual nanofibers, the characteristic current-voltage curves were observed which are different from those of the conventional semiconductor devices. In this talk, the possibility and the issues of the applications of synthetic nanofibers as electronic devices will be discussed with the critical points of view.
The structural inversion asymmetry (SIA) causes a spin-orbit interaction in semiconductor heterostructure. This is so-called Rashba spin-orbit interaction. We have experimentally found that the spin-orbit interaction in InGaAs can be controlled by SIA of the quantum well and by the gate voltage[1],[2]. A spin interference device is proposed based on the gate controlled spin-orbit interaction [3]. We will discuss the gate-controlled Aharonov-Bohm experiment in the presence of the spin-orbit interaction.
[1] J. Nitta, T. Akazaki, H. Takayanagi, and T. Enoki, Phys. Rev. Lett. 78 1335 (1997).
[2] T. Koga, J. Nitta, T. Akazaki, and H. Takayanagi, Phys. Rev. Lett. 89 046801 (2002).
[3] J. Nitta, F. Meijer, and H. Takayanagi, Appl. Phys. Lett. 75 695 (1999).
We describe an approach to wire up carbon nanotubes, which were bridged between the prepatterned catalytic contact electrodes using direct lateral growth technique. Normally, the floating CNT channels fabricated by this simple method exhibit a capability of current control due to the gate voltage. The response characteristics of the devices with the gate voltage showed typical FET behavior at high temperature regime and Coulomb blockade in low temperature regions. The observation results strongly suggest that the catalytic ferromagnetic electrode - floating CNT structure made by direct lateral growth may be promising bridge for nano electronics and nano-electro-mechanical systems (NEMS).
One-dimensional nanostructures, such as nanowires and nanotubes play an important role as fundamental building blocks for nano-electronics. In particular, the predictable and controllable transport characteristics would be critical to develop new nano-scale electronic devices. Here we have studied the electron and spin transport properties of GaN based nanowire and carbon nanotubes. The electrical measurements on individual nanowire and tube with the mesoscopic non-magnetic or magnetic metal electrodes show that these materials behave as the good conductor with long phase and spin coherence length, p-type and n-type semiconductor depending on their crystal structure and doping level. The detailed low temperature electronic and spin transport properties of nanotube and wires will be presented.
Present trends in the miniaturization of electronic devices suggest that ultimately single molecules may be used as electronically active elements in a variety of applications. Recent advances in the manipulation of single molecules now permit to contact an individual molecule between two electrodes and measure its electronic transport properties. In contrast to single-electron transistors based on metallic islands, molecular devices have a more complicated, but in principle tunable, electronic structure. In addition to generic principles of nanoscale physics, e.g. Coulomb blockade, the chemistry and geometry of the molecular junction emerge as the fundamental tunable characteristics of molecular junctions.
In my talk, I will present our theoretical efforts to bridge traditional concepts of mesoscopic and molecular physics to describe the electronic transport through single molecules. Our approach, based on the combination of ab initio quantum chemistry methods and non-equilibrium Green functions techniques, allows us to show how the electronic structure of individual molecules is reflected in their conduction properties, and it constitutes the first step towards a quantitative theory for the a priori design of molecular devices. In particular, I will discuss three examples of special experimental interest: (i) the transport through single-atom contacts [1,2], (ii) the conductance of a hydrogen molecule [3,4], and (iii) the current through organic molecules [5,6].
[1] J.C. Cuevas, A. Levy Yeyati and A. Martin-Rodero, Phys. Rev. Lett. 80, 1066 (1998).
[2] E. Scheer, N. Agrait, J.C. Cuevas, et al., Nature 394, 154 (1998).
[3] R.H.M. Smit et al., Nature 419, 906 (2002).
[4] J. Heurich, F. Pauly, J.C. Cuevas, W. Wenzel and G. Schon, submitted to Phys. Rev. Lett. (November, 2002).
[5] J. Reichert et al., Phys. Rev. Lett. 88, 176804 (2002).
[6] J. Heurich, J.C. Cuevas, W. Wenzel and G. Schon, Phys. Rev. Lett. 88, 256803 (2002).
Recent experiments on shot noise in diffusive mesoscopic conductors are presented. After discussion of the significance of current noise as a probe of non-equilibrium states in mesoscopic structures, special emphasis is put on superconductor(S)/normal(N) metal hybrid structures. We observe the signatures of multiple Andreev reflection in the differential conductance of relatively long Cu wires between Al reservoirs. The noise measurements reveal that multiple Andreev reflections at the SN-interfaces lead to a pronounced broadening of the quasiparticle distribution function, even at very small applied voltages V. This broadening corresponds to finite shot noise. When the energy gap Δ of the superconductor greatly exceeds the applied voltage, many Andreev reflections are required to transfer quasiparticles between the reservoirs. Each incident quasiparticle leads to an avalanche of Andreev reflections, transferring a bunch of charge q*≈2ne = 2Δ/V, where n is the number of Andreev reflections. If the inelastic scattering is weak, noise corresponding to charge avalanches with more than ≈50 electron charges can be observed. If the inelastic scattering is important, it reduces the nonequilibrium noise by narrowing the quasiparticle distribution function. This is verified on measurements on Nb/Au samples.
Our understanding of the electronic transport through superconducting nanostructures has experienced a notable development in last few years. Partly, this has been due to the appearance of the metallic atomic-size contacts, which can be produced by means of scanning tunneling microscope and break-junction techniques [1]. These nanowires have turned out to be an ideal test-bed for the modern transport theories in mesoscopic superconductors. Thus, for instance Scheer and coworkers [2,3] found a quantitative agreement between the measurements of the current-voltage characteristics of different atomic contacts and the predictions of the theory for a single-channel superconducting contact [4]. These experiments not only helped to clarify the structure of the subgap current in superconducting contacts, but also showed that the set of the transmission coefficients in an atomic-size contact is amenable to measurement. This unique possibility has recently allowed a set of experiments that confirm the theoretical predictions for transport properties such as shot noise [5,6]. From these combined theoretical and experimental efforts a coherent picture of transport in superconducting point contacts has emerged with multiple Andreev reflections (MAR) as a central concept.
In my talk I will review the MAR theory which describes quantitatively the electronic transport in superconducting quantum point contacts, paying special attention to three important properties: (i) the current-voltage characteristics, (ii) the shot noise, and (iii) the Shapiro steps [7].
[1] For a recent review see N. Agrait, A. Levy Yeyati and Jan M. van Ruitenbeek, cond-mat/0208239.
[2] E. Scheer et al., Phys. Rev. Lett. 78, 3535 (1997).
[3] E. Scheer et al., Nature 394, 154 (1998).
[4] J.C. Cuevas, A. Martin-Rodero and A. Levy Yeyati, Phys. Rev. B 54, 7366 (1996).
[5] J.C. Cuevas, A. Martin-Rodero and A. Levy Yeyati, Phys. Rev. Lett. 82, 4086 (1999).
[6] R. Cron et al., Phys. Rev. Lett. 86, 4104 (2001).
[7] J.C. Cuevas, J. Heurich, A. Martin-Rodero, A. Levy Yeyati and G. Schon, Phys. Rev. Lett. 88, 157001 (2002).
Ferromagnetic metal (FM)/semiconductor (SM) hybrid structures combine the field of magnetism and semiconductor physics. The important application of FM/SM/FM is so-called spin field effect transistor proposed by Datta and Das [1]. However, the conductivity mismatch between FM and SM forms a major obstacle for spin injection [2]. Our simple model based on BTK theory shows that the tunnel barrier improves spin injection considerably [3]. This is opposite to the case of Cooper pair injection in Superconductor/SM junctions. We will discuss about our spin-injection experiments in FM/SM/FM structures and will show a proposal of spin filter [4].
[1] S. Datta and B. Das, Appl. Phys. Lett. 56 665 (1990).
[2] G. Schmidt, D. Ferrand, L. W. Molenkamp, A. T. Filip, and B. J. van Wees, Phys. Rev. B62 R4790 (2000).
[3] H. B. Heersche, Th. Schaepers, J. Nitta, and H. Takayanagi Phys. Rev. B64 161307(R) (2001).
[4] T. Koga, J. Nitta, H. Takayanagi, and S. Datta, Phys. Rev. Lett. 88 126601 (2002).