scispace - formally typeset
Search or ask a question
Author

C. R. Becker

Bio: C. R. Becker is an academic researcher from University of Würzburg. The author has contributed to research in topics: Molecular beam epitaxy & Band gap. The author has an hindex of 23, co-authored 100 publications receiving 2659 citations.


Papers
More filters
Journal ArticleDOI
TL;DR: In this paper, the Raman-active lattice vibrations of Bi2Se3, Bi2Te3, Sb2T3, and SbT3 were investigated by Raman scattering and three of the four expected Raman modes, Eg and A1g, could be determined.
Abstract: The Raman-active lattice vibrations of Bi2Se3, Bi2Te3, Sb2Te3, and their solid slutions, whose symmetries correspond to the R3m space group, are investigated by Raman scattering. Three of the four expected Raman modes, Eg and A1g, could be determined. The FIR optical properties of Bi2Te3 crystal surfaces of improved quality (E ∥ c and E ⟂ c) and Bi2Se3 (E ⟂ c) are re-examined near helium and room temperature with a Fourier spectrometer, allowing a determination of the infrared-active mode frequencies with higher accuracy. The results for Bi2Te3 are compared to the predicted frequencies from the lattice dynamical model, given by Jenkins et al. The frequncy shifts of the Raman-active modes in the mixed crystals show single-mode and two-mode behaviour, which is in agreement with simple models for the substitution of antimony and selenium atoms for bismuth and tellurium, respectively, in Bi2Te3.

473 citations

Journal ArticleDOI
TL;DR: This work fabricated all II-VI semiconductor resonant tunneling diodes based on the (Zn,Mn,Be)Se material system, containing dilute magnetic material in the quantum well, and studied their current-voltage characteristics to interpret evidence of tunneling transport through spin polarized levels.
Abstract: We have fabricated all II-VI semiconductor resonant tunneling diodes based on the (Zn,Mn,Be)Se material system, containing dilute magnetic material in the quantum well, and studied their current-voltage characteristics. When subjected to an external magnetic field the resulting spin splitting of the levels in the quantum well leads to a splitting of the transmission resonance into two separate peaks. This is interpreted as evidence of tunneling transport through spin polarized levels, and could be the first step towards a voltage controlled spin filter.

247 citations

Journal ArticleDOI
TL;DR: In this article, the band structure of semimagnetic type-III quantum wells (QW's) was calculated using an eight-band model in an envelope function approach, where the mutual influence of the two-dimensional confinement and the exchange interaction on the transport properties of QW's with low Mn concentrations was analyzed.
Abstract: The band structure of semimagnetic ${\mathrm{Hg}}_{1\ensuremath{-}y}{\mathrm{Mn}}_{y}\mathrm{Te}∕{\mathrm{Hg}}_{1\ensuremath{-}x}{\mathrm{Cd}}_{x}\mathrm{Te}$ type-III quantum wells (QW's) has been calculated using an eight-band $\mathbit{k}∙\mathbit{p}$ model in an envelope function approach. Details of the band structure calculations are given for the Mn-free case $(y=0)$. A mean-field approach is used to take the influence of the $sp\text{\ensuremath{-}}d$ exchange interaction on the band structure of QW's with low Mn concentrations into account. The calculated Landau level fan diagram and the density of states of a ${\mathrm{Hg}}_{0.98}{\mathrm{Mn}}_{0.02}\mathrm{Te}∕{\mathrm{Hg}}_{0.3}{\mathrm{Cd}}_{0.7}\mathrm{Te}$ QW are in good agreement with recent experimental transport observations. The model can be used to interpret the mutual influence of the two-dimensional confinement and the $sp\text{\ensuremath{-}}d$ exchange interaction on the transport properties of ${\mathrm{Hg}}_{1\ensuremath{-}y}{\mathrm{Mn}}_{y}\mathrm{Te}∕{\mathrm{Hg}}_{1\ensuremath{-}x}{\mathrm{Cd}}_{x}\mathrm{Te}$ QW's.

223 citations

Journal ArticleDOI
TL;DR: In this article, the authors associate nonmonotonic phase changes with the Aharonov-Casher effect, which is confirmed by comparison with numerical calculations of the magnetoconductance for a multichannel ring structure within the Landauer-Buttiker formalism.
Abstract: Ring structures fabricated from $\mathrm{HgTe}/\mathrm{HgCdTe}$ quantum wells have been used to study Aharonov-Bohm type conductance oscillations as a function of Rashba spin-orbit splitting strength. We observe nonmonotonic phase changes indicating that an additional phase factor modifies the electron wave function. We associate these observations with the Aharonov-Casher effect. This is confirmed by comparison with numerical calculations of the magnetoconductance for a multichannel ring structure within the Landauer-B\"uttiker formalism.

169 citations

Journal Article
TL;DR: Nonmonotonic phase changes indicating that an additional phase factor modifies the electron wave function are associated with the Aharonov-Casher effect.

129 citations


Cited by
More filters
Journal ArticleDOI
TL;DR: Topological superconductors are new states of quantum matter which cannot be adiabatically connected to conventional insulators and semiconductors and are characterized by a full insulating gap in the bulk and gapless edge or surface states which are protected by time reversal symmetry.
Abstract: Topological insulators are new states of quantum matter which cannot be adiabatically connected to conventional insulators and semiconductors. They are characterized by a full insulating gap in the bulk and gapless edge or surface states which are protected by time-reversal symmetry. These topological materials have been theoretically predicted and experimentally observed in a variety of systems, including HgTe quantum wells, BiSb alloys, and Bi2Te3 and Bi2Se3 crystals. Theoretical models, materials properties, and experimental results on two-dimensional and three-dimensional topological insulators are reviewed, and both the topological band theory and the topological field theory are discussed. Topological superconductors have a full pairing gap in the bulk and gapless surface states consisting of Majorana fermions. The theory of topological superconductors is reviewed, in close analogy to the theory of topological insulators.

11,092 citations

Journal ArticleDOI
TL;DR: Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems as discussed by the authors, where the primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport.
Abstract: Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors discuss in detail spin decoherence mechanisms in metals and semiconductors. Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties. Experimental work is reviewed with the emphasis on projected applications, in which external electric and magnetic fields and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics.

9,158 citations

Journal ArticleDOI
15 Dec 2006-Science
TL;DR: In this article, the quantum spin Hall (QSH) effect can be realized in mercury-cadmium telluride semiconductor quantum wells, a state of matter with topological properties distinct from those of conventional insulators.
Abstract: We show that the quantum spin Hall (QSH) effect, a state of matter with topological properties distinct from those of conventional insulators, can be realized in mercury telluride–cadmium telluride semiconductor quantum wells. When the thickness of the quantum well is varied, the electronic state changes from a normal to an “inverted” type at a critical thickness d c . We show that this transition is a topological quantum phase transition between a conventional insulating phase and a phase exhibiting the QSH effect with a single pair of helical edge states. We also discuss methods for experimental detection of the QSH effect.

5,187 citations

Journal ArticleDOI
02 Nov 2007-Science
TL;DR: The quantum phase transition at the critical thickness, d = 6.3 nanometers, was independently determined from the magnetic field–induced insulator-to-metal transition, providing experimental evidence of the quantum spin Hall effect.
Abstract: Recent theory predicted that the quantum spin Hall effect, a fundamentally new quantum state of matter that exists at zero external magnetic field, may be realized in HgTe/(Hg,Cd)Te quantum wells. We fabricated such sample structures with low density and high mobility in which we could tune, through an external gate voltage, the carrier conduction from n-type to p-type, passing through an insulating regime. For thin quantum wells with well width d 6.3 nanometers), the nominally insulating regime showed a plateau of residual conductance close to 2e(2)/h, where e is the electron charge and h is Planck's constant. The residual conductance was independent of the sample width, indicating that it is caused by edge states. Furthermore, the residual conductance was destroyed by a small external magnetic field. The quantum phase transition at the critical thickness, d = 6.3 nanometers, was also independently determined from the magnetic field-induced insulator-to-metal transition. These observations provide experimental evidence of the quantum spin Hall effect.

4,343 citations

24 Apr 2014
TL;DR: In this article, the quantum spin Hall effect was observed in HgTe/(Hg,Cd)Te quantum wells with well width d 6.3 nanometers and the residual conductance was independent of sample width, indicating that it is caused by edge states.
Abstract: Recent theory predicted that the quantum spin Hall effect, a fundamentally new quantum state of matter that exists at zero external magnetic field, may be realized in HgTe/(Hg,Cd)Te quantum wells. We fabricated such sample structures with low density and high mobility in which we could tune, through an external gate voltage, the carrier conduction from n-type to p-type, passing through an insulating regime. For thin quantum wells with well width d 6.3 nanometers), the nominally insulating regime showed a plateau of residual conductance close to 2e(2)/h, where e is the electron charge and h is Planck's constant. The residual conductance was independent of the sample width, indicating that it is caused by edge states. Furthermore, the residual conductance was destroyed by a small external magnetic field. The quantum phase transition at the critical thickness, d = 6.3 nanometers, was also independently determined from the magnetic field-induced insulator-to-metal transition. These observations provide experimental evidence of the quantum spin Hall effect.

2,958 citations