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**세미나**

2015.05.18 09:21

다음과 같이 세미나 모임을 개최합니다. 많은 참석 부탁합니다.

날짜: 2015/ 5/16 (토) 발표는 10:30부터 시작합니다.

장소: 삼성 학술 정보관 7층 Creative Learning Room

연사: 정석범 교수 (서울대)

제목: Searching for Majorana fermions

초록:

At the dawn of modern physics, there were two different attempts to reconcile quantum mechanics with special relativity for electrons. When it was recognized that the earlier attempt by Dirac required the existence of the anti-particle, a colleague of Fermi, Ettore Majorana, came up with a solution that does not require anti-particles. While this concept for a long time had been considered merely as a possible theory for neutrinos, there recently has been active interest in possibility of Majorana fermions in condensed matter system as an instance of electron fractionalization. The host system for such Majorana fermions are the superconducting analogues of the topological insulators, known as topological superconductors. I will discuss both the conceptual issues and the experimental progresses of this topological superconductor both in 1D and in higher-D.

연사: 조길영 박사 (과학원)

제목: Geometry of Fractional Quantum Hall states

초록:

Fractional quantum Hall (FQH) fluids of two-dimensional electron gases (2DEG) in large magnetic fields are fascinating topological states of matter. As such they are characterized by universal properties such as their fractional quantum Hall conductivity, fractionally charged anyonic excitations and a degeneracy of topological origin on surfaces with the topology of a torus. Quite surprisingly these topological fluids also couple to the geometry on which the 2DEG resides and have universal responses to adiabatic changes in the geometry. These responses are given by a Wen-Zee term (which describes the coupling of the currents to the spin connection of the geometry) and a gravitational Chern-Simons term which reflects the universal energy and momentum transport along the edges of the FQH state. We use a field theory of the FQH states to derive these universal responses [1, 2]. To account for the coupling to the background geometry, we show that the concept of flux attachment needs to be modified and use it to derive the geometric responses from Chern-Simons theories. We show that the resulting composite particles minimally couple to the spin connection of the geometry [1]. Taking account of the framing anomaly of the quantum Chern-Simons theories [2], we derive a consistent theory of geometric responses from the Chern-Simons effective field theories and from parton constructions, and apply it to both abelian and non-abelian states. If time allows, we will also discuss the implications [3-4] of the geometric responses on the so-called nematic fractional quantum Hall states which is recently found in experiments.

[1] **GYC**, You, and Fradkin, PRB,
90, 115139 (2014)

[2] Gromov, **GYC**, You, Abanov,
and Fradkin, PRL, 114, 016805 (2014)

[3] **GYC**, Parrikar, You, Reigh,
and Hughes, PRB, 91, 035122 (2014)

[4]
You, **GYC**, and Fradkin, PRX, 4, 041050, (2015)