Small scale lateral superlattices in two-dimensional electron gases prepared by diblock copolymer masks

A poly(styrene-block-methylmethacrylate) diblock copolymer in the hexagonal cylindrical phase has been used as a mask for preparing a periodic gate on top of a Ga[Al]As-heterostructure. A superlattice period of 43 nm could be imposed onto the two-dimensional electron gas. Transport measurements show a characteristic positive magnetoresistance around zero magnetic field which we interpret as a signature of electron motion guided by the superlattice potential.Comment: 3 pages, 3 figure

Synthetic Beuys: on nano-materials and the aesthetics of imperceptibility

Aquest treball centra l'atenció sobre la paradoxal naturalesa dels nanoobjectes i analitza la relació entre una matèria imperceptible (massa petita per ser percebuda pels sentits) i les seves formes de manifestació ostensibles. El desenvolupament de les nanociències i la nanotecnologia no hauria estat possible si no s'hagués inventat una sèrie d'instruments de visualització (com el microscopi d'efecte túnel i el microscopi de força atòmica –STM i AFM segons les respectives sigles en anglès–). Des d'aquest punt de vista, podem parlar de nanopartícules com a «imatges-objectes» (Sacha Loeve), com a matèria mediadora. Si bé hi ha alguns estudis sobre la naturalesa de la investigació epistemològica en el camp de les nanociències i les nanotecnologies, encara és escàs l'interès pels canvis que es produeixen en el camp de l'estètica en relació amb la comprensió dels materials tractats mitjançant nanotecnologia. Tenint en compte els últims desenvolupaments en el terreny de la nanotecnologia aplicada als materials, aquest treball analitza com canvien l'estatus i la significació dels materials quan es tracten nanotecnològicament. Per a això proposem un escenari fictici relacionat amb algunes de les obres de Joseph Beuys en què substituïm els teixits «tradicionals» utilitzats per l'artista per altres de nous, produïts o tractats mitjançant nanotecnologia. Emprant el plantejament artístic de Joseph Beuys com a eina metodològica d'investigació crítica, estudiem com han de ser reevaluadas les categories perceptives, epistemològiques i semiòtiques quan tenen a veure amb plantejaments nanotecnològics. L'anàlisi ens ajudarà a formular algunes de les qüestions que artistes, dissenyadors i investigadors haurien de tenir en compte en tractar amb el que nosaltres anomenem «estètica de la imperceptibilitat» dels nanomaterials i les nanotecnologies

Spin States in Graphene Quantum Dots

We investigate ground and excited state transport through small (d = 70 nm) graphene quantum dots. The successive spin filling of orbital states is detected by measuring the ground state energy as a function of a magnetic field. For a magnetic field in-plane of the quantum dot the Zemann splitting of spin states is measured. The results are compatible with a g-factor of 2 and we detect a spin-filling sequence for a series of states which is reasonable given the strength of exchange interaction effects expected for graphene

Interactions and screening in gated bilayer graphene nanoribbons

The effects of Coulomb interactions on the electronic properties of bilayer graphene nanoribbons (BGNs) covered by a gate electrode are studied theoretically. The electron density distribution and the potential profile are calculated self-consistently within the Hartree approximation. A comparison to their single-particle counterparts reveals the effects of interactions and screening. Due to the finite width of the nanoribbon in combination with electronic repulsion, the gate-induced electrons tend to accumulate along the BGN edges where the potential assumes a sharp triangular shape. This has a profound effect on the energy gap between electron and hole bands, which depends nonmonotonously on the gate voltage and collapses at intermediate electric fields. We interpret this behavior in terms of interaction-induced warping of the energy dispersion.Comment: 6 pages, 4 figure

Statistics of conductance oscillations of a quantum dot in the Coulomb-blockade regime

The fluctuations and the distribution of the conductance peak spacings of a quantum dot in the Coulomb-blockade regime are studied and compared with the predictions of random matrix theory (RMT). The experimental data were obtained in transport measurements performed on a semiconductor quantum dot fabricated in a GaAs-AlGaAs heterostructure. It is found that the fluctuations in the peak spacings are considerably larger than the mean level spacing in the quantum dot. The distribution of the spacings appears Gaussian both for zero and for non-zero magnetic field and deviates strongly from the RMT-predictions.Comment: 7 pages, 4 figure

Transport properties of quantum dots with hard walls

Quantum dots are fabricated in a Ga[Al]As-heterostructure by local oxidation with an atomic force microscope. This technique, in combination with top gate voltages, allows us to generate steep walls at the confining edges and small lateral depletion lengths. The confinement is characterized by low-temperature magnetotransport measurements, from which the dots' energy spectrum is reconstructed. We find that in small dots, the addition spectrum can qualitatively be described within a Fock-Darwin model. For a quantitative analysis, however, a hard-wall confinement has to be considered. In large dots, the energy level spectrum deviates even qualitatively from a Fock-Darwin model. The maximum wall steepness achieved is of the order of 0.4 meV/nm.Comment: 9 pages, 5 figure

In-plane gate single-electron transistor in Ga[Al]As fabricated by scanning probe lithography

A single-electron transistor has been realized in a Ga[Al]As heterostructure by oxidizing lines in the GaAs cap layer with an atomic force microscope. The oxide lines define the boundaries of the quantum dot, the in-plane gate electrodes, and the contacts of the dot to source and drain. Both the number of electrons in the dot as well as its coupling to the leads can be tuned with an additional, homogeneous top gate electrode. Pronounced Coulomb blockade oscillations are observed as a function of voltages applied to different gates. We find that, for positive top-gate voltages, the lithographic pattern is transferred with high accuracy to the electron gas. Furthermore, the dot shape does not change significantly when in-plane voltages are tuned.Comment: 4 pages, 3 figure

Transport properties of quantum dots with hard walls

Quantum dots are fabricated in a Ga[Al]As-heterostructure by local oxidation with an atomic force microscope. This technique, in combination with top gate voltages, allows us to generate steep walls at the confining edges and small lateral depletion lengths. The confinement is characterized by low-temperature magnetotransport measurements, from which the dots' energy spectrum is reconstructed. We find that in small dots, the addition spectrum can qualitatively be described within a Fock-Darwin model. For a quantitative analysis, however, a hard-wall confinement has to be considered. In large dots, the energy level spectrum deviates even qualitatively from a Fock-Darwin model. The maximum wall steepness achieved is of the order of 0.4 meV/nm.Comment: 9 pages, 5 figure