Magnetotransport in Double Quantum Well with Inverted Energy Spectrum: HgTe/CdHgTe

We present the first experimental study of the double-quantum-well (DQW) system made of 2D layers with inverted energy band spectrum: HgTe. The magnetotransport reveals a considerably larger overlap of the conduction and valence subbands than in known HgTe single quantum wells (QW), which may be regulated by an applied gate voltage $V_g$. This large overlap manifests itself in a much higher critical field $B_c$ separating the range above it where the quantum peculiarities shift linearly with $V_g$ and the range below with a complicated behavior. In the latter case the $N$-shaped and double-$N$-shaped structures in the Hall magnetoresistance $\rho_{xy}(B)$ are observed with their scale in field pronouncedly enlarged as compared to the pictures observed in an analogous single QW. The coexisting electrons and holes were found in the whole investigated range of positive and negative $V_g$ as revealed from fits to the low-field $N$-shaped $\rho_{xy}(B)$ and from the Fourier analysis of oscillations in $\rho_{xx}(B)$. A peculiar feature here is that the found electron density $n$ remains almost constant in the whole range of investigated $V_g$ while the hole density $p$ drops down from the value a factor of 6 larger than $n$ at extreme negative $V_g$ to almost zero at extreme positive $V_g$ passing through the charge neutrality point. We show that this difference between $n$ and $p$ stems from an order of magnitude larger density of states for holes in the lateral valence band maxima than for electrons in the conduction band minimum. We interpret the observed reentrant sign-alternating $\rho_{xy}(B)$ between electronic and hole conductivities and its zero resistivity state in the quantum Hall range of fields on the basis of a calculated picture of magnetic levels in a DQW.Comment: 15 pages, 13 figure

Широтные вариации структуры сообществ и продуктивности зоопланктона в Баренцевом море (лето 2013 г.)

The Barents Sea is regarded as one of the world’s most unique regions with its high summer productivity. However, there were a few studies dealing with estimation of daily secondary production in the Barents Sea. The aim of the present study was to conduct an investigation of the pelagic mesozooplankton assemblage in the Barents Sea, to calculate daily production levels of mesozooplankton and to analyze the relationship between the community structure and environmental drivers.Structure of zooplankton communities was investigated within the main water masses of the Barents Sea in the summer 2013 (June–July). Calanus finmarchicus presented as copepodites IV–V dominated in terms of the total zooplankton abundance and biomass in Murmansk Coastal Water and Atlantic Water. Spring phase of zooplankton succession cycle was found in Arctic Water where Copepoda nauplii were the most abundant and Calanus glacialis dominated by zooplankton biomass. Zooplankton of the Barents Sea Water had features of spring and summer phases of zooplankton succession cycle. Spatial variations in the zooplankton distribution were associated with hydrological conditions and phytoplankton density. Maximum of the mean zooplankton biomass was registered in Atlantic Water (30,7 mg dry mass/m3) while the minimum was found in Barents Sea Water (5,4 mg dry mass/m3). Calculated daily production of zooplankton ranged from 0,01 to 1,57 mg dry mass/m3 per day. Total zooplankton stock for the study area of 281 000 km2 was estimated to be > 740 000 t of dry mass. The obtained values of zooplankton biomass and stock were considerably lower than in previous years, which are probably connected with higher predator pressure — grazing of zooplankton by capelin and other predators as well as with possible climatic influence.Исследована структура зоопланктона в пределах основных водных масс Баренцева моря. В мурманской прибрежной и атлантической водной массах по численности и биомассе зоопланктона доминировал Calanus finmarchicus. В арктической водной массе преобладали науплии копепод, основу биомассы составлял Calanus glacialis. Пространственные вариации распределения зоопланктона были связаны с гидрологическими факторами и концентрацией фитопланктона. Наибольшая средняя биомасса отмечена в атлантических водах, наименьшая — в баренцевоморских. Общий запас зоопланктона для исследованной акватории площадью 281 тыс. км2 составил более 740 тыс. т сухой массы. Биомасса, продукция и запас зоопланктона были существенно ниже, чем в предшествующие годы, что, скорее всего, связано с более высоким прессом мойвы и других хищников, также возможным влиянием климатических факторов

Nonlocal transport near the charge neutrality point in a two-dimensional electron-hole system

Nonlocal resistance is studied in a two-dimensional system with a simultaneous presence of electrons and holes in a 20 nm HgTe quantum well. A large nonlocal electric response is found near the charge neutrality point (CNP) in the presence of a perpendicular magnetic field. We attribute the observed nonlocality to the edge state transport via counter propagating chiral modes similar to the quantum spin Hall effect at zero magnetic field and graphene near Landau filling factor $\nu=0$Comment: 5 pages, 4 figure

Cyclotron Resonance Assisted Photocurrents in Surface States of a 3D Topological Insulator Based on a Strained High Mobility HgTe Film

We report on the observation of cyclotron resonance induced photocurrents, excited by continuous wave terahertz radiation, in a 3D topological insulator (TI) based on an 80 nm strained HgTe film. The analysis of the photocurrent formation is supported by complimentary measurements of magneto-transport and radiation transmission. We demonstrate that the photocurrent is generated in the topologically protected surface states. Studying the resonance response in a gated sample we examined the behavior of the photocurrent, which enables us to extract the mobility and the cyclotron mass as a function of the Fermi energy. For high gate voltages we also detected cyclotron resonance (CR) of bulk carriers, with a mass about two times larger than that obtained for the surface states. The origin of the CR assisted photocurrent is discussed in terms of asymmetric scattering of TI surface carriers in the momentum space. Furthermore, we show that studying the photocurrent in gated samples provides a sensitive method to probe the effective masses and the mobility of 2D Dirac surface states, when the Fermi level lies in the bulk energy gap or even in the conduction band

Two-dimensional semimetal in wide HgTe quantum wells: Charge-carrier energy spectrum and magnetotransport

The magnetoresistivity and the Hall and Shubnikov-de Haas effects in heterostructures with a single 20.2-nm-wide quantum well made from the gapless semiconductor HgTe are studied experimentally. The measurements are performed on gated samples over a wide range of electron and hole densities. The data obtained are used to reconstruct the energy spectrum of electrons and holes in the vicinity of the extrema of the quantum-confinement subbands. It is shown that the charge-carrier dispersion relation in the investigated systems differs from that calculated within the framework of the conventional kp model. © 2013 Pleiades Publishing, Ltd

Effect of a boron implantation on the electrical properties of epitaxial HgCdTe with different material composition

In this work the experimental results of investigations of the dynamics of accumulation and spatial distribution of electrically active radiation defects when irradiating epitaxial films of Hg1-xCdxTe (MCT) with different material composition (x). The films, grown by molecular beam epitaxy (MBE) were irradiated by B ions at room temperature in the radiation dose range 1012 -1015 ions/cm2 and with ion energy 100 keV. The results give the differences in implantation profiles, damage accumulation and electrical properties as a function of the material composition of the film