Non-equilibrium electron transport induced by terahertz radiation in the topological and trivial phases of Hg_1−xCd_xTe

Abstract

Terahertz photoconductivity in heterostructures based on n-type Hg1− xCdxTe epitaxial films both in the topological phase (x < 0.16, inverted band structure, zero band gap) and the trivial state (x > 0.16, normal band structure) has been studied. We show that both the positive photoresponse in films with x < 0.16 and the negative photoconductivity in samples with x > 0.16 have no low-energy threshold. The observed non-threshold positive photoconductivity is discussed in terms of a qualitative model that takes into account a 3D potential well and 2D topological Dirac states coexisting in a smooth topological heterojunction.

Apparent PT-symmetric terahertz photoconductivity in the topological phase of Hg1−xCdxTe-based structures

“…The Hall bar samples were placed into an optical helium cryostat, the sample temperature was kept at 4.2 K. Photoconductivity was induced by an optically pumped gas laser generating ~ 100 ns long Thz pulses with line tunable frequencies in the range from 0.6 to 3.3 THz 24-28 . The photoconductivity has been measured in the 4-probe geometry with two opposite directions of the dc bias current in order to avoid a possible contribution of the photovoltaic signal 29,30 . The propagation direction of the incident radiation and magnetic field were normal to the sample surface.…”

“…These values were calculated in the assumption that the electron density is homogeneously distributed throughout the film. The energy distance between the Fermi level and the conduction band edge varied from 2 to 5 meV for different samples 29,30 . The latter value is less than the energy of a radiation quantum with the wavelengths of 280 and 496 μm.…”

Особенности транспорта в топологической фазе Hg-=SUB=-0.87-=/SUB=-Cd-=SUB=-0.13-=/SUB=-Te в условиях терагерцового фотовозбуждения

“…Отметим, что помимо быстрого отклика, следующего за возбуждающим лазерным импульсом, регистрируется задержанный сигнал. Подобная кинетика фотопроводимости, которая может быть описана суперпозицией нескольких вкладов, включающих задержанный отклик, ранее наблюдалась нами в аналогичных структурах [11,12].…”

“…В эпитаксиальных структурах на основе Hg 1−x Cd x Te с инверсным спектром нами ранее наблюдалась положительная фотопроводимость, стимулированная мощными терагерцовыми импульсами [11,12]. Появление положительного фотоотклика может быть обусловлено не только возбуждением носителей в объеме, но и неравновесными процессами в области гетерограниц структуры.…”

Terahertz Magnetospectroscopy of Cyclotron Resonances from Topological Surface States in Thick Films of CdxHg1−xTe

“…[ 33–35 ] Topological insulators (TIs) based on bulk films with Cd contents below the temperature‐dependent critical remain less studied so far. [ 36,37 ] These materials, however, have a number of advantages with respect to HgTe TIs. As demonstrated by Tomaka et al, [ 36 ] the topologically protected surface states in are characterized by 1) two times higher Dirac fermion velocity (approximately the same as in graphene) than in pure HgTe, and 2) a larger band gap and a higher position of the Dirac point on the energy scale than that obtained in strained HgTe films.…”