Classical spins in topological insulators

Abstract

Following the recent theoretical proposal and experiment on quantum spin Hall effect in HgTe/CdTe quantum wells, we consider a single magnetic impurity localized in the bulk of the system, which we treat as a classical spin. It is shown that there are always localized excited states in the bulk energy gap for arbitrarily strong impurity strength in inverted region, while the localized excited states vanish for very strong impurity strength in normal region. Similar conclusion also applies to three-dimensional topological insulators. This distinct difference serves as another novel criterion for the conventional and topological insulating phases when the time-reversal symmetry is broken, and can be easily experimentally observed through the STM and/or ARPES experiments.Comment: 5 pages, 4 figures

Incoherentc-Axis Interplane Response of the Iron ChalcogenideFeTe0.55Se0.45Superconductor from Infrared Spectroscopy

“…σc(ω) in the far-infrared region becomes clearly depressed with decreasing temperature and the suppressed spectral weight seems not to be recovered by 5000 cm 45 (as well as in the underdoped cuprates) is transferred to higher energies and not to the region below the low-frequency cutoff of our data. The latter effect would have resulted in drastic increases of R c (ω) and the c axis dc conductivity, which is not supported either by our own reflectance or resistivity data (of FeTe 0.6 Se 0.4 compound [26]). The oscillator strength sum rule implies a conservation of the global spectral weight.…”

Decoherence of a quantum system coupled to an XY spin chain: Role of the initial state of the spin chain

“…The effects of the effective Hamiltonian on the decoherence has been studied in many papers [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. Especially, dramatic manifestation of the decoherence has often been found in the vicinity of the quantum critical point of the effective Hamiltonian.…”

Quantum impurity in the bulk of a topological insulator

“…The bound state, on the other hand, is induced when the system is topologically nontrivial. Even a potential scattering 18 or a vacancy 19 could produce the in-gap bound state in the bulk of TIs. Therefore, the in-gap bound state could be considered as a degree of freedom, while its energy is determined by the impurity.…”

“…[23][24][25][26][27] For these reasons, it is important to study how the quasiparticle states are affected when vacancies or impurities are localized in the bulk of the system. It has been shown that classical spins 18 or vacancies 19,20 localized in the bulk of TIs could result in the coexistence of in-gap bound states and boundary states. For a quantum impurity, the quantum fluctuations of its internal degree of freedom play an important role, making it significantly differ from classical impurities.…”

“…7,8 So far, the effects of impurity scattering on the surface of TIs have been investigated extensively. [9][10][11][12][13][14][15][16][17][18][19][20][21][22] In the presence of classical spins which break time-reversal symmetry, it was predicted that the impurity could open up a local gap and suppress the local density of states. 9,10 The suppression of backscattering around nonmagnetic impurities on surfaces with strong spin-orbit coupling has been confirmed by scanning tunneling miscroscope experiments.…”