Non-volatile bistability effect based on electrically controlled phase transition in scaled magnetic semiconductor nanostructures

We explore the bistability effect in a dimensionally scaled semiconductor nanostruncture consisting of a diluted magnetic semiconductor quantum dot (QD) and a reservoir of itinerant holes separated by a barrier. The bistability stems from the magnetic phase transition in the QD mediated by the changes in the hole population. Our calculation shows that when properly designed, the thermodynamic equilibrium of the scaled structure can be achieved at two different configurations; i.e., the one with the QD in a ferromagnetic state with a sufficient number of holes and the other with the depopulated QD in a paramagnetic state. Subsequently, the parameter window suitable for this bistability formation is discussed along with the the conditions for the maximum robustness/non-volatility. To examine the issue of scaling, an estimation of the bistabiity lifetime is made by considering the thermal fluctuation in the QD hole population via the spontaneous transitions. A numerical evaluation is carried out for a typical carrier-mediated magnetic semiconductor (e.g., GaMnAs) as well as for a hypothetical case of high Curie temperature for potential room temperature operation.Comment: 9 pages, 7 figure

Re-entrant ferromagnetism in a generic class of diluted magnetic semiconductors

Considering a general situation where a semiconductor is doped by magnetic impurities leading to a carrier-induced ferromagnetic exchange coupling between the impurity moments, we show theoretically the possible generic existence of three ferromagnetic transition temperatures, T_1 > T_2 > T_3, with two distinct ferromagnetic regimes existing for T_1 > T > T_2 and T < T_3. Such an intriguing re-entrant ferromagnetism, with a paramagnetic phase (T_2 > T > T_3) between two ferromagnetic phases, arises from a subtle competition between indirect exchange induced by thermally activated carriers in an otherwise empty conduction band versus the exchange coupling existing in the impurity band due to the bound carriers themselves. We comment on the possibility of observing such a re-entrance phenomenon in diluted magnetic semiconductors and magnetic oxides.Comment: 4 pages, 3 figure

Bistability in a magnetic and nonmagnetic double-quantum-well structure mediated by the magnetic phase transition

The hole distribution in a double quantum well (QW) structure consisting of a magnetic and a nonmagnetic semiconductor QW is investigated as a function of temperature, the energy shift between the QWs, and other relevant parameters. When the itinerant holes mediate the ferromagnetic ordering, it is shown that a bistable state can be formed through hole redistribution, resulting in a significant change in the properties of the constituting magnetic QW (i.e., the paramagnetic-ferromagnetic transition). The model calculation also indicates a large window in the system parameter space where the bistability is possible. Hence, this structure could form the basis of a stable memory element that may be scaled down to a few hole regime.Comment: 9 pages, 3 figure

The role of the Coulomb interaction in the formation of superconducting and pseudogap states in cuprate metal-oxides

It is shown that the key role in the mechanism of high-Tc superconductivity in the layered cuprate metal-oxides with anisotropic quasi-two-dimensional electronic spectrum and d-wave symmetry of the superconducting order parameter is played by the retarded screened Coulomb interaction and many-body correlations. We argue that the pseudogap observed in these materials is the anisotropic dielectric gap, which appears due to the electron-hole pairing on the flat portions of the Fermi surface in the vicinity of the extended saddle points in the quasiparticle spectrum. This gap coexists with the superconducting gap and is partially suppressed by scattering of current carriers on the charged point defects. The suppression of dielectric gap is analogous to the suppression of superconducting gap by magnetic impurities in «gapless» superconductors. The complete destruction of the pseudogap by charged impurities is assumed to increase Tc significantly

Gap symmetry and charge density excitations in high-Tc superconductors with extended saddle points in electron spectrum

It is shown that the strong anisotropy of the one-particle electron spectrum, due to the presence of extended saddle-point features (ESPF) close to the Fermi level in the hole-type cuprates YBCO and BSCCO, leads to the occurrence of a low-frequency peak in the spectral function of the charge density fluctuations due to the presence of acoustic plasmon branch in the collective electron spectrum. The retarded anisotropic electron-plasmon interaction leads to the suppression of the static screened Coulomb repulsion for small transferred momenta and, consequently, to the effective attraction between electrons in the dx²-y²-wave channel of the Cooper pairing of current carriers. Breaking of C₄v symmetry in YBCO crystals leads to a possibility of a change of dx²-y²-wave symmetry of the gap to a mixed s − d gap symmetry for singlet Cooper pairs or to a p-wave gap symmetry for triplet pairs.Показано, що сильна анізотропія одночасткового електронного спектру веде, завдяки наявності подовжених сідлових особливостей біля рівня Фермі у купратах YBCO та BSCCO, до появи низькочастотного піку у спектральній функції флуктуацій зарядової густини, що є наслідком присутності гілки акустичних плазмонів у колективному електронному спектрі. Електрон-плазмонна взаємодія веде до значного зменшення статичного кулонівського відштовхування в області малих переданих імпульсів та, як наслідок, до ефективного притягнення між електронами у dx²-y²-хвильовому каналі куперівського спарювання носіїв струму. Порушення C₄v симетрії у кристалах YBCO призводить до можливості заміни dx²-y²-хвильової симетрії надпровідної щілини на змішану s − d симертрію для сінглетних куперівських пар або на p -хвильову симетрію щілини для триплетних пар