Szemináriumok

Fully tuneable coherence and control of acceptor qubits

Időpont: 
2017. 07. 11. 14:15
Hely: 
Building F, Entrance III, seminar room of Department of Theoretical Physics
Előadó: 
Dimi Culcer (UNSW Sydney)
A szeminárium részletei: 
http://physics.bme.hu/node/3830

Electrical control of quantum bits could pave the way for scalable quantum computation. An acceptor spin qubit in Si, based on spin-3/2 holes, can be controlled by electrical means using a gate electrode, which offers fast one- and two-qubit rotations and long coherence times at certain sweet spots. The relaxation time T1, while allowing >10^5 operations, is the primary limiting factor. I will show that, due to the interplay of the Td symmetry of the acceptor in the Si lattice and the spin-3/2 characteristic of hole systems, an applied in-plane magnetic field strongly enhances the performance and coherence properties of the qubit. An appropriate choice of magnetic field orientation leads to a near-total suppression of spin relaxation as well as full tunability of two-qubit operations in a parameter regime in which dephasing due to charge fluctuations can be eliminated. Interestingly for spintronic applications, an extreme in-plane anisotropy exists such that the in-plane g-factor can vanish under certain circumstances.

Zero energy edge states and topology in carbon nanotubes

Időpont: 
2017. 08. 29. 14:15
Hely: 
Building F, Entrance III, seminar room of Department of Theoretical Physics
Előadó: 
Wataru Izumida (Sendai)
A szeminárium részletei: 
http://physics.bme.hu/node/3836

In this talk, I will present our recent studies on zero energy edge states in finite-length carbon nanotubes from topological viewpoint.

 

An effective one-dimensional (1D) lattice model for a single-wall carbon nanotube (SWNT) is introduced to analyze the quantum system with boundary [1]. Open-ended SWNTs commonly host edge states whose energies lie in the bulk band gap. By analyzing the 1D lattice model a bulk-edge correspondence is proven, which shows that the number of edge states and a winding number topological invariant defined in the corresponding bulk system are identical [2]. Manipulation of the edge states by a magnetic field is suggested in terms of the topological phase transition [3].

 

We extend these studies to investigate the nature of edge states which arise when the SWNT is proximity coupled to a superconductor [4]. The zero energy edge states emerge as a combined effect of curvature-induced Dirac point shift and strong superconducting coupling between nearest-neighbor sites. A 1D continuum model reveals the topological origin of the zero energy edge states.

 

[1] W. Izumida, R. Okuyama, R. Saito, Phys. Rev. B 91, 235442 (2015)

[2] W. Izumida, R. Okuyama, A. Yamakage, R. Saito, Phys. Rev. B 93, 195442 (2016)

[3] R. Okuyama, W. Izumida, M. Eto, J. Phys. Soc. Jpn. 86, 013702 (2017)

[4] W. Izumida, L. Milz, M. Marganska, M. Grifoni, arXiv:1707.02934

Non-equilibrium steady states

Időpont: 
2017. 10. 02. 14:15
Hely: 
Building F, Entrance III., seminar room of the Department of Theoretical Physics
Előadó: 
Zoltán Zimborás (Wigner FK)
A szeminárium részletei: 
http://physics.bme.hu/node/3882

Title: "Non-equilibrium steady states: validation and falsification of CFT predictions"

Abstract: "We discuss the correlation properties of nonequilibrium steady states that arise as equilibrated states after local quenches in which two half-chains prepared at different temperatures are joined together. In particular, we treat the cases of free fermions, harmonic chains, and tranverse-field Ising models. We discuss the conformal field theoretic predictions, and show that some of these are (to some extent) violated. We also point out that these states give rise to the only known physically relevant counter-examples to the area law and the Rényi subadditivity property outside the zero-temperature regime. Finally, we close the talk with some ideas on future research motivated by further CFT predictions."

Quantum dots in graphene

Időpont: 
2017. 11. 06. 14:15
Hely: 
Building F, Entrance III, seminar room of Department of Theoretical Physics
Előadó: 
Péter Nemes-Incze (MTA EK)
A szeminárium részletei: 
http://physics.bme.hu/node/3890

Title: Magnetically confined quantum dots in graphene: capturing Dirac electrons using the tip of a scanning tunneling microscope

 

Cím: Kvantumpötty grafénben: hogyan ragadjunk meg egy Dirac elektront pásztázó alagútmikroszkóp segítségével?

 

Kivonat: Az előadásomban egy alternatív megoldást mutatok be Dirac-elektronok kvantumpöttybe zárására. Egy hexagonális bórnitrid hordozón fekvő grafén kristályt mágneses térbe helyezve, lehetséges tiltott sávot nyitni az elektron állapotokban, ez megakadályozza a Klein alagutazást. Ebben az esetben egy pásztázó alagútmikroszkóp tűjének az elektrosztatikus potenciálja segítségével kontrolláltan lokalizálhatóak a grafén töltéshordozói. Az ilyen módon kialakított „él-mentes” kvantumpöttyök egyrészecske szinttávolsága 4-10 meV, „völgy felhasadása” 3 meV, valamint az elektrosztatikus energia 10 meV. A kvantumpötty kialakulásának mechanizmusa, valamint az egyrészecske szinttávolságai kvantitatívan reprodukálhatóak szoros kötésű közelítésben. Kihasználva az STM tű mozgathatóságát, bemutatom miként használható egy ilyen kvantumpötty a minta lokális potenciál fluktuációinak térképezésére. Továbbá, hogy ezáltal milyen lehetőségek nyílnak meg grafén kvantumbitek kialakításában.

 

N.M. Freitag, L.A. Chizhova, P. Nemes-Incze, C.R. Woods, R. V. Gorbachev, Y. Cao, A.K. Geim, K.S. Novoselov, J. Burgdörfer, F. Libisch, and M. Morgenstern, Nano Lett. 16, 5798 (2016).

 

N. M. Freitag et al., Tunable giant valley splitting in edge-free graphene quantum dots on boron nitride. arXiv (2017) (available at http://arxiv.org/abs/1708.09170).

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