The Physics of Low-dimensional Semiconductors

Davies, John H.

doi:10.1017/cbo9780511819070

Cited by 593 publications

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“…Unfortunately this technique is far from being readily available as a versatile tool for broad investigations. Hence the most obvious approach to achieve one-dimensional physics in solids would be to utilize advanced semiconductor technology (Davies 1998). Besides the enormous technological effort, this approach has the disadvantage that these structures are embedded in bulk materials and not easily accessible to further experiments.…”

Section: Realization Of One-dimensional Structuresmentioning

confidence: 99%

Ordering phenomena in quasi-one-dimensional organic conductors

Dressel

2007

Naturwissenschaften

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Low-dimensional organic conductors could establish themselves as model systems for the investigation of the physics in reduced dimensions. In the metallic state of a one-dimensional solid, Fermi-liquid theory breaks down and spin and charge degrees of freedom become separated. But the metallic phase is not stable in one dimension: as the temperature is reduced, the electronic charge and spin tend to arrange themselves in an ordered fashion due to strong correlations. The competition of the different interactions is responsible for which broken-symmetry ground state is eventually realized in a specific compound and which drives the system towards an insulating state. Here we review the various ordering phenomena and how they can be identified by optic and magnetic measurements. While the final results might look very similar in the case of a charge density wave and a charge-ordered metal, for instance, the physical cause is completely different. When density waves form, a gap opens in the density of states at the Fermi energy due to nesting of the one-dimension Fermi surface sheets. When a one-dimensional metal becomes a charge-ordered Mott insulator, on the other hand, the short-range Coulomb repulsion localizes the charge on the lattice sites and even causes certain charge patterns. We try to point out the similarities and conceptional differences of these phenomena and give an example for each of them. Particular emphasis will be put on collective phenomena which are inherently present as soon as ordering breaks the symmetry of the system.

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Section: Realization Of One-dimensional Structuresmentioning

confidence: 99%

Ordering phenomena in quasi-one-dimensional organic conductors

Dressel

2007

Naturwissenschaften

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“…24 The corresponding carrier area density at the low field is 2.6 × 10 13 cm -2 . The nonlinear dependence of RH on the B field may be ascribed to the presence of the surface states, previously masked by excessive bulk carriers in a high density sample.…”

mentioning

confidence: 98%

Topological Insulator Nanowires and Nanoribbons

Kong¹,

Randel

Peng³

et al. 2009

Nano Lett.

313

330

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Recent theoretical calculations and photoemission spectroscopy measurements on the bulk Bi 2 Se 3 material show that it is a three-dimensional topological insulator possessing conductive surface states with nondegenerate spins, attractive for dissipationless electronics and spintronics applications. Nanoscale topological insulator materials have a large surface-to-volume ratio that can manifest the conductive surface states and are promising candidates for devices. Here we report the synthesis and characterization of high quality single crystalline Bi 2 Se 3 nanomaterials with a variety of morphologies. The synthesis of Bi 2 Se 3 nanowires and nanoribbons employs Au-catalyzed vapor-liquid-solid (VLS) mechanism. Nanowires, which exhibit rough surfaces, are formed by stacking nanoplatelets along the axial direction of the wires. Nanoribbons are grown along [11][12][13][14][15][16][17][18][19][20] direction with a rectangular crosssection and have diverse morphologies, including quasi-one-dimensional, sheetlike, zigzag and sawtooth shapes. Scanning tunneling microscopy (STM) studies on nanoribbons show atomically smooth surfaces with 1 nm step edges, indicating single Se-Bi-Se-Bi-Se quintuple layers. STM measurements reveal a honeycomb atomic lattice, suggesting that the STM tip couples not only to the top Se atomic layer, but also to the Bi atomic layer underneath, which opens up the possibility to investigate the contribution of different atomic orbitals to the topological surface states. Transport measurements of a single nanoribbon device (four terminal resistance and Hall resistance) show great promise for nanoribbons as candidates to study topological surface states.Bi 2 Se 3 is a narrow gap semiconductor, previously studied for infrared detectors and thermoelectric applications. 1 Recently, research on Bi2Se3 and related compounds (Bi2Te3 and Sb2Te3) has attracted much interest because they are predicted to be three-dimensional (3D) topological insulators (TIs), a new class of quantum matter possessing conducting surface states with nondegenerate spins. 2 In TIs, the strong spin-orbit coupling dictates robust, nontrivial surface states, which are topologically protected against back scattering from time-reversal invariant defects and impurities. Angleresolved photoemission spectroscopy (ARPES) measurements on bulk single crystals of BixSb1-x, Bi2Se3, and Bi2Te3 have verified the existence of the 3D TI phase. [3][4][5][6][7] In particular, the surface states of Bi2Se3 forms a single Dirac cone inside a large bulk band gap of 0.3 eV, thus being suggested as the reference material for the 3D TIs. 2,4,8 The unique properties of the Bi2Se3 TI may pave the way for dissipationless quantum electronics and room temperature spintronics applications.1 To date, the surface properties of the 3D TIs have been mainly investigated by ARPES measurements on the cleaved surface of bulk crystals. [3][4][5][6][7] Single crystalline nanostructure, on the other hand, offers an attractive alternative system to study the surface sta...

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“…Our model for the potential landscape incorporates known properties of 2DEGs in GaAs/AlGaAs heterostructures 14,15 and replicates the physical parameters of the experimental system. To model the full in-plane potential, treating the 2DEG as an idealized δ-layer, we consider two contributions.…”

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confidence: 99%

Coherent branched flow in a two-dimensional electron gas

Topinka

LeRoy²,

Westervelt

et al. 2001

Nature

460

573

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Images of electron flow from the quantum point contact (QPC) are obtained by raster scanning a negatively charged SPM tip above the surface of the device and simultaneously measuring the position-dependent conductance of the device. The negatively charged tip capacitively couples to the 2DEG, creating a depletion region that backscatters electron waves. When the tip is positioned over areas with high electron flow from the QPC the conductance is decreased, whereas when the tip is over areas of relatively low electron flow the conductance is unmodified. By raster scanning the tip over the sample and simultaneously recording the effect the tip has on device conductance, a two dimensional image of electron flow can be obtained.The quantum point contact sample is mounted in an atomic force microscope and cooled to liquid He temperatures. The QPC is formed in the 2DEG inside a GaAs/AlGaAs heterostructure by negatively biasing two gates on the surface -a negative potential on these gates creates two depletion regions that define a variable width channel between them as shown in Fig. 1a. The conductance of the QPC is measured using an ac lock-in amplifier at 11kHz. The heterostructure for the devices used in this experiment was grown by molecular beam epitaxy on an n-type GaAs substrate.The 2DEG resides 57 nm below the surface with mobility µ = 1.0x10 6 cm 2 /Vs and density n = 4.5x10 11 /cm 2 . These values of mobility and density correspond to a mean free path l = 11 µm, Fermi wavelength λ F = 37 nm, and Fermi energy E F = 16 meV. The root mean square voltage across the QPC was chosen so as to not heat electrons -0.2 mV for 1.7K scans. The conductance of the quantum point contact, shown in Fig. 1b, increases as the width of the channel is increased (by changing the gate voltage V g ) and shows well defined conductance plateaus at integer multiples of the conductance quantum 2e 2 /h 1,2 . When probing the electron flow, the SPM tip was held at a negative potential relative to the 2DEG and was scanned at 10nm above the surface of the heterostructure. Figures 2a and 2b show images of electron flow from two different quantum point contacts at the temperature 1.7K; both QPCs are biased on the G = 2e 2 /h conductance plateau. Figure 2b shows the flow patterns on each side of a quantum point contact (the gated region at the center was not scanned), and Figure 2a shows a higher-resolution image of flow from one side of a different QPC.In both these images, the current exits the point contact in a central lobe, as would be expected from an exact quantum-mechanical calculation of electron flow through an ideal QPC without impurities or non-uniform distributions of dopant atoms. Rather than continuing out as a smoothly widening fan, it quickly forks into several different paths and continues to branch off into ever smaller rivulets for the full width of the scan. This branching behavior was observed in all of the 13 QPC exit patterns observed so far. Previously, there have been suggestions of an unexpected narrowness in observe...

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The Physics of Low-dimensional Semiconductors

Cited by 593 publications

References 0 publications

Ordering phenomena in quasi-one-dimensional organic conductors

Ordering phenomena in quasi-one-dimensional organic conductors

Topological Insulator Nanowires and Nanoribbons

Coherent branched flow in a two-dimensional electron gas

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