Continuous Transformation from Skyrmion- to Pseudospin-type Excitation
A. Sawadaa, D. Terasawaa, N. Kumadaa, Z.F. Ezawaa, K. Murakib, T. Sakub, Y. Hirayamab
aDepartment of Physics, Faculty of Science, Tohoku University, Sendai 980-8578, Japan
bNTT Basic Research Laboratories, 3-1 Morinosato-Wakamiya, Atsugi 243-0198, Japan
We measured the activation energy of bilayer n = 1 quantum Hall states by changing the bias voltage and tilting samples in the magnetic field. By changing the density difference, the tilting behavior of the pseudospin(PS)-type activation energy at the equal density point gradually transforms into the Skyrmion(S)-type one at the monolayer density point. At the intermediate density difference, by increasing the tilting angle the activation energy starts to decrease as a PS-type excitation gap and then increase as a S-type excitation gap. It is impossible to explain this behavior in terms of the level crossing of the PS- and S-type excitations. The result of the overall behavior indicates the excitation in bilayer system possesses the both properties of the PS-type and S-type excitations.
Explanation of the tunneling phenomena between the edges of two lateral quantum Hall systems
Shinji Nonoyamaa,b, George Kirczenowa
aDepartment of Physics, Simon Fraser University, Burnaby, B.C., Canada V5A 1S6
bFaculty of Education, Yamagata University, Yamagata 990-8560, Japan
We use computer simulations to identify the physics behind the surprising results of recent measurements by Kang et al. [Nature 403, 59 (2000)] of electron transfer between the edges of two two-dimensional electron systems (2DES). We find that a consistent explanation of all of the observed phenomena is possible only if the barrier between the 2DES is surrounded by a strong potential well that supports quantum railroads of edge channels that, in the presence of disorder, exhibit directed localization. This together with the onset of electrical resistance as the system exits the perfectly conducting quantized Hall regime accounts for every aspect of the data. We propose direct experimental tests of our theory.
Neutral Collective Excitations in Striped Hall States
Nobuki Maeda, Takahiro Aoyama, Yumiko Ishizuka, Kenzo Ishikawa
Department of Physics, Hokkaido University, Sapporo 060-0810, Japan
In the absence of disorder and edges, the quantum Hall system has the magnetic translation and rotation symmetry. In the striped state, a magnetic translation in one direction is spontaneously broken to the discrete translation and the rotation is also spontaneously broken to the p-rotation. Using the conserved current, the property of the neutral collective excitations is studied. The spectrum of the neutral collective excitation is obtained in the single mode approximation numerically. The spectrum has a multiple line node structure and cusps. Furthermore, the spectrum has anisotropic feature, that is, in one direction the spectrum resembles the liquid Helium spectrum with the phonon and roton minimum, and in another direction it resembles the FQHS spectrum.
Goldstone Mode Kink-Solitons in Double Layer Quantum Hall Systems in the Absence of Tunneling
Ramaz Khomerikia, Kieran Mullenb
aDepartment of Physics, Tbilisi State University, Chavchavadze Ave. 3, Tbilisi 380028, Republic of Georgia.
bDepartment of Physics and Astronomy, University of Oklahoma, 440 West Brooks, Norman, Oklahoma 73019, USA
It is shown that in charge unbalanced double layer quantum Hall system with zero tunneling pseudospin Goldstone mode forms moving kink-soliton in weakly nonlinear limit. This charge-density localization moves with a velocity of gapless linear spin-wave mode and could be easily observed experimentally. We predict that mentioned Goldstone mode kink-solitons define diffusionless charge transport properties in double layer systems, where mentioned kink-solitons could be considered as transport carriers.
Anomalous Metallic Phase and Magnetism in a High-Mobility and Strongly Correlated 2D Electron System
Mitsuaki Ooya, Tohru Okamoto
Department of Physics, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
We report the transport properties of a low disorder (mmax ~ 30 m 2 / V s) two-dimenstional electron system in the Si/SiGe heterostructure, where the strong electron-electron interaction (U / eF ~ 10) causes the anomalous metallic temperature dependence of resistivity. We found a sharp kink in the resistivity r vs parallel magnetic field B// curve which corresponds to the complete spin polarization. The observed critical magnetic field Bc is smaller than that expected for the free electron system by a factor of 2 or 3. The metallic temperature dependence of r was observed even for B// > Bc where spin degree of freedom is frozen, while the metallic phase in Si-MOSFET's is suppressed by the parallel magnetic field (cf. Okamoto et al. PRL 82 (1999) 3875). B^-dependence of r are also investigated systematically.
Quantum Hall Ferromagnets in Magnetic Quantum Wells
Jan Jaroszy\'nskia,b, Tomasz Andrearczyka,b, Grzegorz Karczewskia, Jerzy Wróbela, Tomasz Wojtowicza, Ewa Papisc, Eliana Kami\'nskac, Anna Piotrowskac, Dragana Popovi\'cb, Tomasz Dietla
aInstitute of Physics, Polish Academy of Sciences, PL 02-668 Warsaw, Poland
bNational High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, USA
cInstitute of Electron Technology, PL 02-668 Warsaw, Poland
We report transport studies in a (Cd,Mn)Te/(Cd,Mg)Te:I heterostructure. Due to s-d coupling in this novel quantum Hall system, giant exchange Zeeman splitting strongly depends on magnetic field. Thus, even in moderate, perpendicular magnetic fields crossings of Landau levels occur. This makes it possible to probe for the first time formation of quantum Hall ferromagnetism in a magnetic quantum well. Magnetic ions not only dramatically modify electronic energy levels but also strongly enhance many body effects of the system. This makes ferromagnetic critical temperature as high as Tc » 2 K in this heterostructure.
Spin Peierls Effect in Polarization of Fractional Quantum Hall States
Shosuke Sasaki
Shizuoka Institute of Science and Technology
The magnetic dependence curve of the electron spin polarization in the fractional quantum Hall effect has many plateaus. Some of the plateaus have very small widths, and the others have wide widths. In the Landau gauge, the single electron orbitals are like straight lines with equal intervals. For a filling factor of 2/3, the electron-configuration with minimum energy is the sequence (filled, filled, empty, ...), which produces spin exchange interactions. When, we take the spin-Peierls effect into consideration, the intervals in the first sequence are changed to be wide, and the ones in the second sequence are changed to be narrow, .... This modulation creates energy gaps. The energy-decreasing by the gap is superior to the Coulomb energy increasing. Therefore, the gap really appears. Then, the magnetic dependences of the calculated spin polarization are well in accord with the experimental data after Kukushkin et al.
Negative Differential Conductance in a Benzene-Molecular Device
Maarten R. Wegewijsa, Matthias Hettlerb, Wolfgang Wenzelb, Herbert Schoellera
aInstitut für Theoretische Physik A, RWTH Aachen, 52056 Aachen, Germany
bForschungszentrum Karlsruhe, Institut für Nanotechnologie, 76021 Karlsruhe, Germany
We predict a negative differential conductance effect in the nonlinear transport through a molecular device containing a weakly coupled benzene ring. The effect is based on (1) the symmetry properties of the molecule with respect to the transport direction, (2) the strong local e-e interactions and (3) the coupling of electrons to photons. The effect is generic: other molecules may have similar current-switching properties. The flow of current is blocked by the spontaneous decay of the molecular anion from an excited state to a lower lying, stable state of the anion which does not couple to the electron tunneling. An effective interacting Hamiltonian for the p-electrons (derived from an electronic structure calculation) serves as input for the transport calculation.
Molecule-Based Single Electron Transistor
Hye Mi Soa, Jinhee Kimb, Wan Soo Yunb, Jong Wan Parka, Ju-Jin Kima, Do-Jae Wonc, Yongku Kangc, Changjin Leec
aDepartment of Physics, Chonbuk National University, Jeonju 561-756, Korea
bElectronic Device Group, Korea Research Institute of Standards and Science, Daejeon 305-600, Korea
cKorea Research Institute of Chemical Technology, Daejeon 305-600, Korea
We have fabricated molecular electronic devices combining Au nanoparticles and an organic self-assembled monolayer, and investigated their electrical transport properties. Current-voltage characteristic curves were nonlinear at temperatures below 70 K. Gate modulated current exhibited a periodic oscillation, attributed to the Coulomb oscillation, up to 40 K. The resonance peaks in differential conductance curves shifted with the increase of magnetic field, due to the Zeeman splitting of the electronic states of the Au nanoparticle.
Current-Voltage Characteristics for Point Contact Composed of Two Peierls Conductors at Finite Temperature
Kazuo Sano
School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo, 169-8555, Japan
Current-voltage (J-V) characteristics are numerically investigated at finite temperature for a point contact consisting of two Peierls conductors (P) separated by an insulator in the conventional tunnel Hamiltonian approach. Here P is a conductor with a charge density wave (CDW). The J-V characteristics depend on the CDW phases in the mean field approximation where the phases (j) and energy gaps (D) in both Peierls conductors are assumed to be equal, respectively. The current J is a periodic function of the phase j with a period p, and has a discontinuous jump at eV=2 D (j ¹ 0). The jump increases as the phase j increases. For 0 < eV < 2 D the current J decreases as the phase j increases, but while for eV > 2 D the current J increases as the phase j increases.
Electron Transport Properties of C60 Single Electron Transistor
Norihiko Nishiguchi
Department of Applied Physics, Hokkaido University, Sapporo 060-8628, Japan
Electron transport of a C60 single-electron transistor is studied theoretically, postulating the shuttle mechanism (L. Y. Gorelik et al., Phys. Rev. Lett.80, 4526(1998)) that the vibrating nanoparticle carries charges between the electrodes. Two types of gate voltage effect on the transport properties are demonstrated: one is direct modulation of the current via modification in the tunneling rate, giving rise to modifications in Coulomb staircase. Another is an indirect effect due to a shift in the range of the C60 molecule vibration induced by the gate voltage. The latter effect stops the shuttle mechanism at a large gate voltage, leading to the conduction gap which widens in proportion to the gate voltage. The findings of this work are consistent with those of the experimental study on the C60 single-electron transistors.(H. Park et al., Nature 407, 57(2000))
Longitudinal Magnetoresistance of Bismuth Nanowires
Tito E. Hubera, Michael J. Grafb
aLaser Laboratory, Howard University, Washington, DC20059
bDepartment of Physics, Boston College, Chestnut Hill, MA 02467
We study the electrical transport properties of 200-nm diameter and 30-nm single-crystal bismuth nanowire arrays embedded in an alumina matrix. Measurements have been carried out over a wide range of temperatures (1.8 K -300 K) and magnetic fields (0-18 T). The longitudinal magnetoresistance (B//I) is found to be negative at low fields, which is consistent with the model of size efects due to R.G. Chambers. The experiments have been carried out in our laboratories and in the U.S. National High Magnetic Field Lab. Support by NSF and ARO is acknowledged.
Rectifying diodes made of individual gallium nitride nanowires
Jae-Ryoung Kima, Hwangyou Oha, Hye Mi Soa, Ju-Jin Kima, Jinhee Kimb
aDepartment of Physics, Chonbuk National University, Jeonju 561-756, Korea
bElectronic Device Group, Korea Research Institute of Standards and Science, Daejeon 305-600, Korea
We have fabricated Schottky diodes with individual gallium nitride nanowires synthesized by chemical vapor deposition method. High quality rectifying diodes have been fabricated by using Al as the Schottky barrier and Ti/Au for the ohmic contact. The measured current-voltage characteristics showed clear rectifying behaviors and no reverse bias breakdown was observed up to the measured voltage, -5 V. The forward bias threshold voltage decreases linearly as the temperature was lowered from about 0.4 V at 280K to 1 V at 10 K.
Quantum wire arrays and dot arrays in mesoporous silica thin films.
Noriaki Sugimotoa, Koji Tsukadaa, Yoko Kumaia, Atsushi Fukuokab, Masaru Ichikawab
aTOYOTA Central R &D Labs.,Inc., Nagakute, Aichi, 480-1192, Japan
bCatalysis Research Center, Hokkaido University, Sapporo 060-0811, Japan
The array of quantum metal wires and dots were synthesized by Ship-In-A-Bottle method with silica mesoporous thin films. The diameter of wire or dot can be designed from 2-6nm. The silica mesoporous film is self-assemble material with uniform diameter of mesopores like zeolite. The wire diameter and arrangement of wires are strictly controlled by Host materials. With the use of conventional semiconductor fabrication process, Host mesoporous film can cut under 500nm range. The electrical properties such as I-V characteristic and magneto-resistance were measured with sub-micron contact to wires. The UCF and electron wave interference effect will be discussed.
Electron Transport through Quasi-1D Atomic Chains on a Silicon Surface
Takashi Uchihashi, Urs Ramsperger
National Institute for Materials Science, 1 - 2 -1, Sengen, Tsukuba, Ibaraki, 305-0047, Japan
The self-assembling technique is ideal to fabricate high-quality metallic nanowires with atomic-scale dimensions on a semiconductor surface. We clarify electron conduction through quasi-one-dimensional (1D) indium atomic chains self-assembled on a silicon surface (the Si(111)-4×1-In reconstruction). Both incomplete structural growth of indium chains at surface steps and intentionally introduced defects in the middle of the chains are found to suppress conduction through the surface reconstruction. The surface conductivity, extracted from the total including the contribution of the subsurface space charge layer, decreases rapidly below 130 K, indicating a metal-insulator transition. The behavior is consistent with the recently found phase transition accompanying structural and electronic modulations. The validity of the Peierls transition picture is discussed based on the temperature dependence of conductivity below Tc.
Consistent size dependency of core-level binding energy shifts and single electron tunneling effects in supported gold nanoclusters
Taizo Ohgi, Daisuke Fujita
Nanomaterials Laboratory, Institute for Materials Science, 1-2-1, Sengen, Tsukuba, Ibaraki 305-0047
We report that the kinetic energy shift of the photoelectrons from the supported metal clusters has the origin in chemical potential of a cluster m(-1 « 0) determined by the capacitance between the cluster and substrate. Au deposited ocatnedithiol/Au(111) samples provide homogeneous Au nanoclusters, well defined tunneling barriers and atomically flat substrate, which enables us to compare experimental results with simple physical model and also minimizes the fluctuation of macroscopic data in photoelectron spectroscopy. We examined the prepared samples by photoelectron and tunneling electron spectroscopy independently and found that the results from both spectroscopies show consistent cluster-size dependency in the charging effect of supported metal clusters.
Transport and Magnetic Properties of Magnetic Alloy Atom Bridges
Hiroshi Nakanishia, Hideaki Kasaia, Tomoya Kishia, Fumio Komorib, Ayao Okijic
aDepartment of Applied Physics, Osaka University, Suita, Osaka 565-0871, Japan
bInstitute for Solid State Physics, University of Tokyo, Chiba 277-8587, Japan
cWakayama National College of Technology, Gobo, Wakayama 644-0023, Japan
We have investigated the magnetic properties of the atom bridge made from magnetic materials, which is the atom-scale wire constructed between a scanning tunneling microscopy tip and a solid surface, and how these magnetic properties affect the quantum transport of electrons through the bridge with ab initio calculations. In the case of FeNi alloy bridge, the atom bridge can be designed to have significantly larger magnetic moment than that in the corresponding alloy bulk and to provide the highly spin-polarized electron current through it. At the conference, we report the magnetic properties and the spin-dependent transport properties of various magnetic alloy atom bridges.
Electronic Transport through Benzene Molecule: Effect of Gold Contacts
Amir A. Farajian, Hiroshi Mizuseki, Yoshiyuki Kawazoe
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
Organic molecules are intriguing possible building blocks for nano-device applications due to their being self-assembled/abundant. In this work, we investigate electronic transport through one of the simplest possible organic molecules, i.e., benzene, which is attached to two Au(111) electrodes via sulfur clips. Ab initio model is used for both the organic molecule and the gold electrodes, by which the total Green's function of the system projected onto the molecule is determined. This will provide us with the transfer function across the benzene molecule, from which the conductance and current-voltage characteristics of the device are deduced. As our model takes into account the atomistic character of the electrodes, it provides a better understanding of the effect of electrodes on the transport properties of the device. The results are compared against the available experimental data.
A New Approach to Cooper Pair Pumping
José Aumentadoa, Mark W. Kellera, John M. Martinisa, Cristián Urbinab
aNational Institue of Standards and Technology, Boulder, CO 80305-3337
bCEA-Saclay, 91191 Gif-sur-Yvette CEDEX, France
The ability to pump single electrons in gated arrays of tunnel junctions has already been well established in normal state devices with metrological precision.1 However, similar attempts to pump Cooper pairs have not been so successful.2 At NIST we are attempting to modify the Josephson coupling directly at the single and multiple junction levels using environmental resistors. We have shown discrete Cooper pair pumping at zero bias in pumps and are in the process of quantifying the effects of having these resistors present.
Transport Properties of Quasiparticles in Semiconductor-Superconductor junctions with Magnetic Barrier at Interface
Ben-Yuan Gua, Young-Chong Hsueb, Tzong-Jer Yangb
aInstitute of Physics, Academia Sinica, P.O. Box 603, Beijing 100080, China
bDepartment of Electrophysics, National Chiao Tung University, Hsinchu, 30050, Taiwan
We present the numerical analyses of conductance of quasiparticles (QP's) in semiconductor-magnetic barrier-superconductor junctions. The total conductance as a function of the magnetic field strongly depends on the normal or superconductor state of the most right material and mismatch of Fermi energy and effective mass of QP's in different materials. It shows that the Andreev reflection plays a critical role. The conductance characteristics can be clearly interpreted by a phenomenological physical picture based on the classical cyclotron orbit of QP's under the magnetic field. We derive the explicit analytic expressions of the conductance and the analytic results coincide with numerical ones very well.
General models of Josephson circuits in the presence of linear quantum noise
G. Falcia, Elisabetta Paladinob, Fabio Taddeic, Gaetano Giaquintab
aDMFCI Universitá di Catania, Viale A. Doria 6, 95125 Catania, Italy
bNEST-INMF & DMFCI, Universitá di Catania, Viale A. Doria 6, 95125 Catania, Italy
cNEST-INMF, DMFCI Universitá di Catania, Viale A. Doria 6, 95125 Catania, Italy, & ISI, Torino Italy
Single electron tunneling systems are influenced by noise coming from quantum and thermal fluctuations of the surrounding circuit. The coupling with this environment is usually described in phenomenological terms. We derive effective models by direct transformations of a bath of harmonic oscillators which describes an arbitrary LC line. We derive explicitely couterterms and discuss the Lamb shift induced by the environment, which is important in the multiqubit system dynamics.
DC Josephson current through the nanographite ribbon junctions
Katsunori Wakabayashi
Department of Quantum Matter Science, Graduate School of Advanced Sciences of Matter (ADSM), Hiroshima University, Higashi-Hiroshima 739-8526, Japan
The DC Josephson currents through a nano-graphite ribbon sandwiched by two conventional superconductors are theoretically studied by using the thermal Green function techniques based on the tight binding model. It is found that the behavior of the DC Josephson current crucially depends on the electronic states of nano-graphite ribbons. Because the electronic states of nano-graphite ribbons strongly depend on their shapes of edges, it is shown that the temperature, Josephson junction length, and mangetic field dependence of the DC Josephson currents strongly depend on the edge structures of nano-graphite ribbons.
Quantum Fluctuations and Dissipative Phase Transition in One-Dimensional Josephson Junction Arrays
Hisao Miyazaki, Takahide Yamaguchi, Akinobu Kanda, Youiti Ootuka
Institute of Physics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8571, Japan
In small Josephson junction arrays, the charging effect competes with the Josephson effect and makes the superconducting phases of islands fluctuate quantum-mechanically. On the other hand, the dissipation introduced by the resistance shunting the junction suppresses the fluctuation of the phases. We fabricated one-dimensional Josephson junction arrays in which each junction was shunted by an ohmic resistor and measured their transport properties at low temperatures. We observed a clear change of behavior from the insulating to the superconducting one as a function of the Josephson coupling and the shunt resistance, and obtained the phase diagram at T® 0. The SI transition due to dissipation took place at the shunt resistance nearly equal to the quantum resistance RQ( @ 6.5 kW) when the Josephson coupling is weak.
Spin relaxation and tunnel magnetoresistance of a ferromagnet/ superconductor / ferromagnet single-electron tunneling transistor
Hiroshi Imamura, Yasuhiro Utsumi, Hiromich Ebisawa
Graduate School of Information Sciences, Tohoku University, Sendai 980-8579, Japan
We theoretically study the tunnel magnetoresistance(TMR) of ferromagnet / superconductor / ferromagnet single-electron tunneling transistors with a special attention to the parity effect. It is shown that in the plateau region, there is no spin accumulation in the island even at finite bias voltage. However, the information of injected spin is carried by an excess electron and thus the TMR exists. The spin relaxation rate of the excess electron can be estimated from the TMR. We also show that the TMR increases with decreasing the size of the superconducting island.
Scalable quantum computing using persistent current qubits with Josephson junctions
Mun Dae Kim, Dongkwan Shin, Jongbae Hong
School of Physics, Seoul National University, Seoul 151-747, Korea
We study quantum logic gates using two persistent current states as a qubit in the superconducting ring with Josephson junction. We use an rf SQUID as a single persistent current qubit. An effective double-well potential, where quantum tunneling is possible, exists in the single qubit. The degeneracy of the two persistent current states is lifted by tunneling. The coupling between qubits is performed by the inductive current connecting qubits. A scalable qubit is constructed by making quantum circuit connecting many qubits in a topologically same way. Switching each qubit is possible by inserting dc SQUID into the line coming out of qubit.
System-Size Dependences and Correlation Effects of Josephson current through One-Dimensional Josephson networks
Takeo Kato
Department of Applied Physics, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka, 558-8585, Japan
Josephson junction networks have attracted interest for many years as interacting Boson systems. In order to clarify correlation effects in this system, I discuss Josephson junctions hybridized with an array of superconducting islands. Assuming large on-site Coulomb interactions, a particle number and a Josephson critical current are calculated as functions of voltages of two leads based on a hard-core Boson model, which is equivalent to a spin system with boundary fields. The size-dependences of Josephson current are discussed by studying resonant tunneling peaks in critical currents and phase-dependence of Josephson currents.
Numerical study of unconventional superconductor / a quantum dot / unconventional superconductor junction
Koichi Kusakabea, Yukio Tanakab, Yasunari Tanumac
aGraduate School of Science and Technology, Niigata University, Niigata 950-2181, Japan
bDepartment of Applied Physics, Nagoya University, Nagoya, 464-8603 Japan
cGraduate School of Natural Science and Technology, Okayama University, Okayama 700-8530, Japan
We analyze numerically a Josephson junction with a quantum dot which is attached to two d-wave superconductors. Various interfaces including those made from a (100) surface and/or a (110) surface of a d-wave superconductor are considered. The DC Josephson current is evaluated by a quantum Monte Carlo method. Interplay of correlation effects in the dot, i.e. the Coulomb blockade and the Kondo effect, and effects from the surface geometry, i.e. formation of the zero-energy states, is seen in temperature dependence of the current, which is controlled by the gate voltage on the dot.
Quantum dynamics of underdamped current-biased Josephson junctions
O. Buissona, F. Balestroa, J. P. Pekolab, F. Faurec, F. W. J. Hekkingc
aCRTBT-CNRS, 25 Av. des Martyrs, BP 166, 38042 Grenoble Cedex 9, France
bDepartment of Physics, University of Jyväskylä, P. O. Box 35, 40351 Jyväskylä, Finland
cLPM2C-UJF-CNRS, 25 Av. des Martyrs, BP 166, 38042 Grenoble Cedex 9, France
We have measured escape rate of a current-biased Josephson junction and of a hysteretic DC-SQUID using DC-pulses with duration between 100 ps and 1 ms. For longer pulses, with decreasing temperature, we observe first the thermally activated regime, then the thermally assisted macroscopic quantum tunnelling (MQT) and finally the pure MQT regime. We are currently performing one shot readout quantum measurements using MQT escape with ultra-short pulses on a current-biased DC-SQUID alone or capacitively coupled to a Cooper pair box.
Electron tunneling in smal-area junctions
Alexander I. Khachaturova, Eiji Hattab, Vladimir M. Svistunova
aDonetsk A.Galkin Institute for Physics & Technnology, Academy of Sciences, Donetsk 83114, Ukraine
bNanoelectronics Laboratory, Hokkaido University, Sapporo 060-0813, Japan
Electron tunneling in small-area junctions has been treated. It is found that at appropriate successful sets of barrier parameters and electronic characteristics of electrodes quantization of the latteral component of the wave vector manifests itself in the differential conductance as quite noticeable minima at low temperature. The second derivative of the tunnel current versus applied voltage should contain dictinct periodic structure. The caracteristic feature of the effects is reduced sensetively to thermal smearing. It is pointed out that possibility of manifestation of size-quantum effect should be taken into spectroscopic studies.
Two quantum dots with three-electrons
Lomidze Archila, Archil Lomidzea, Shalva Tsiklaurib
aDepartment of Physics, Tbilisi State University, av.3 Chavchavadze, Tbilisi, 380028, Georgia.
bDepartment of Physics, Tbilisi State University, av.3 Chavchavadze, Tbilisi, 380028, Georgia.
In this work two quantum dots (QD) with three-electron in 2D spaces using the method of hyperspherical functions have been investigated theoretically. To study the total wave function of three-electron QD in 2D spaces first of all we investigate to taken into account by invers square potential between particles. Harmonic oscillator was use as confinement potential. The term of interchanging interaction for two quantum dots with three-electrons has been obtained use the received expression for total wave function.
Tunneling Effects and Electron Transport in Quantum Dot Structures
Lukas Pichla, Juri Horacekb, Vladimir Mitinc, Victor Ryzhiia
aUniversity of Aizu, Aizu-Wakamatsu 966-8580, Japan
bCharles University Prague, Prague 18000, Czech Republic
cWayne State University, Detroit 48202, USA
We report the development of a theoretical model for low-temperature effects in multiple-array QD structures similar to those used in quantum dot infrared photodetectors. The model takes into account the peculiar features of the electron tunneling from QDs in realistic structures. These features are associated with a flattened (disk-like) shape of QDs resulting in a substantially anisotropic electric-field distribution around each QD that, in turn, leads to the electron tunneling primarily through the QD edges, a strong dependence of the tunneling rate on the QD charge (i.e., the number of electrons residing in the QD), and the character of the transport of unbound electrons limited by the QD space charge.
Scattering Phase Approach for Energy Spectrums in Quantum Dots
Hsiu-Hau Lin, Wei Chen, Tzay-Ming Hong
Department of Physics, National Tsing-Hua University, Hsinchu 300, Taiwan, Republic of China
We present a semiclassical approach to evaluate quantum energy levels in asymmetrical quantum dots and wells, where the analytical solution for Schrödiger equation is not possible. In spatial regions where the potential profile is steep, the wave function is locally solved and gives rise to a momentum-dependent phase correction f(k) as shown in the figure. For smooth profiles, the usual WKB approximation works. Combining scattering phases accumulated in both steep and smooth regimes, we arrive at a generalized EBK quantization rule that can be solved algebraically to obtain the energy levels. We present several examples and show that this semiclassical approximation actually works very well, even for low-lying excitations.
Broadening of charge state transitions in a single-electron box
Roland Schäfer, Bernhard Limbach, Peter vom Stein, Christoph Wallisser
Forschungszentrum Karlsruhe, Institut für Festkörperphysik, Postfach 3640, 76201 Karlsruhe, Germany
We report on measurements on a sample consisting of two roughly identical single-electron transistors the islands of which are coupled capacitively. One transistor at a time is operated as electron box. The remaining transistor is used as an electrometer to measure the charge on the box gate. While ramping up the box gate voltage transitions between states occur periodically which differs in the charge on the box island by the elementary charge e. This shows up in jumps of the electrometer current. The coupling between the box and the measuring device causes a broadening of the transition width not included in the formulae for an isolated box. This is evident in our data as well as from a thorough analysis of the system in the framework of the sequential tunneling model. The sample is studied in the superconducting as well as in the normal state.
Strong tunneling in metallic double island structures
Bernhard Limbach, Peter vom Stein, Christoph Wallisser, Roland Schäfer
Forschungszentrum Karlsruhe, Institut für Festkörperphysik, Postfach 3640, 76201 Karlsruhe, Germany
We report on measurements of metallic single electron tunneling systems in the intermediate and strong tunneling regime (g » 0.1 ... 5 with g=G/GK, GK=e2/h), where significant deviations from the orthodox theorie can be observed. We studied single and double island structures with variations of the coupling between the islands and the leads, respectively. Our sample designs enable us to determine the conductance of each single tunnel junction, while in common layouts the ratio of conductances remains unknown.
Conductance and DC-current measurements were carried out at varying temperatures and the results are compared to different theoretical predictions (pertubation theory, nonperturbative semiclassical approach, Monte-Carlo simulation).
Quantum Interference on Electron Wave Spreading over a Coupled Dot
Nobuyuki Aokia, Daisuke Oonishia, Yoshikazu Iwasea, Kohji Ishibashib, Jonathan P. Birdc, David K. Ferryc, Yuichi Ochiaia
aDepartment of Materials Technology, Chiba University, 1-33 Yayoi, Inage, Chiba 263-8522, Japan
bSemiconductors Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
cDepartment of Electrical Engineering, Arizona State University, Tempe, AZ 85287-5706, USA
The characteristics of the fluctuations, observed in the low-temperature magnetoconductance of an open quantum-dot molecule formed from a pair of split-gate quantum dots have been studied. The evolution of these fluctuations suggests a decrease in the typical area for coherent interference with decreasing the coupling strength between the two dot. We discuss this behavior in terms of a transition from multi- to single-dot interference as a function of the inter-dot coupling. Moreover, an existanve of interference trajectories independent of the dot coupling was also found in our analysis.
A model for ferromagnetic nanograins with discrete electronic states
Silvia Kleff, Jan von Delft
Sektion Physik and Center for NanoScience, Ludwig-Maximilians-Universität, Theresienstr.37, 80333 München, Germany
We discuss a simple phenomenological model [1-3] for an ultrasmall ferromagnetic grain, formulated in terms of the grain's discrete energy levels. We compare the model's predictions with recent measurements of the discrete tunneling spectrum through such a grain [3,4]. The model can qualitatively account for the observed features if we assume (i) that the anisotropy energy varies among different eigenstates of one grain, and (ii) that nonequilibrium spin accumulation occurs.
[1] S. Kleff, J. von Delft, M. Deshmukh, and D. C. Ralph, Phys. Rev. B. Rapid Comm. 64, 220401(R) (2001); [2] S. Kleff and J. von Delft, submitted to PRB, cond-mat/0110348; [3] M. Deshmukh, S. Kleff et al. PRL 87, 226801 (2001); [4] S. Guéron et al., PRL 83, 4148 (1999).
1M. W. Keller et al., Appl. Phys. Lett. 69, 1804 (1996)
2L.G. Geerligs et al., Z. Phys. B 85, 349 (1991)