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.

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Spintronics: Fundamentals and applications

The science and technology of ultracapacitors are reviewed for a number of electrode materials, including carbon, mixed metal oxides, and conducting polymers. More work has been done using microporous carbons than with the other materials and most of the commercially available devices use carbon electrodes and an organic electrolytes. The energy density of these devices is 3¯5 Wh/kg with a power density of 300¯500 W/kg for high efficiency (90¯95%) charge/discharges. Projections of future developments using carbon indicate that energy densities of 10 Wh/kg or higher are likely with power densities of 1¯2 kW/kg. A key problem in the fabrication of these advanced devices is the bonding of the thin electrodes to a current collector such the contact resistance is less than 0.1 cm2. Special attention is given in the paper to comparing the power density characteristics of ultracapacitors and batteries. The comparisons should be made at the same charge/discharge efficiency.

Ultracapacitors: Why, How, and Where is the Technology

http://ltl.tkk.fi/ice/Kevo/noisereview.pdf

Shot noise in mesoscopic conductors

The magnetoresistance effect in WTe2, a layered semimetal, is extremely large: the electrical resistance can be changed by more than 13 million per cent at very high magnetic fields and low temperatures. Apply a magnetic field to a magnetoresistive material and its electrical resistance changes — a technologically useful phenomenon that is harnessed, for example, in the data-reading sensors of hard drives. Mazhar Ali and colleagues have now identified a material (tungsten ditelluride or WTe2) in which the magnetoresistance effect is unusually large: the electrical resistance can be changed by more than 13 million per cent. Its remarkable magnetoresitance is evident at very high magnetic fields and at extremely low temperatures, so practical applications are not yet in prospect. But this finding suggests new directions in the study of magnetoresistivity that could ultimately lead to new uses of this effect. Magnetoresistance is the change in a material’s electrical resistance in response to an applied magnetic field. Materials with large magnetoresistance have found use as magnetic sensors1, in magnetic memory2, and in hard drives3 at room temperature, and their rarity has motivated many fundamental studies in materials physics at low temperatures4. Here we report the observation of an extremely large positive magnetoresistance at low temperatures in the non-magnetic layered transition-metal dichalcogenide WTe2: 452,700 per cent at 4.5 kelvins in a magnetic field of 14.7 teslas, and 13 million per cent at 0.53 kelvins in a magnetic field of 60 teslas. In contrast with other materials, there is no saturation of the magnetoresistance value even at very high applied fields. Determination of the origin and consequences of this effect, and the fabrication of thin films, nanostructures and devices based on the extremely large positive magnetoresistance of WTe2, will represent a significant new direction in the study of magnetoresistivity.

Large, non-saturating magnetoresistance in WTe2.

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Electronic Transport In Mesoscopic Systems

A symmetric van der Pauw disk of homogeneous nonmagnetic indium antimonide with an embedded concentric gold inhomogeneity is found to exhibit room-temperature geometric magnetoresistance as high as 100, 9100, and 750,000 percent at magnetic fields of 0.05, 0.25, and 4.0 teslas, respectively. For inhomogeneities of sufficiently large diameter relative to that of the surrounding disk, the resistance is field-independent up to an onset field above which it increases rapidly. These results can be understood in terms of the field-dependent deflection of current around the inhomogeneity.

https://science.sciencemag.org/content/sci/289/5484/1530.full.pdf

Enhanced Room-Temperature Geometric Magnetoresistance in Inhomogeneous Narrow-Gap Semiconductors.

Bulk ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ is known to be a topological insulator. We investigate surface states of ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}(111)$ thin films of one to six quintuple layers using density-functional theory including spin-orbit coupling. We construct a method to identify topologically protected surface states of thin film topological insulators. Applying this method to ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ thin films, we find that the topological nature of the surface states remains robust with the film thickness and that the films of three or more quintuple layers have topologically nontrivial surface states, which agrees with experiments.

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Robustness of Topologically Protected Surface States in Layering of Bi2Te3 Thin Films

We report the noise characteristics of quantum point contacts between 100 Hz and 100 kHz at 4.2 K. The noise consists of a 1/f component on top of a white background. The 1/f noise increases as the contact width decreases and shows peaks between the quantized resistance plateaus. The white noise background increases with current but is much lower than the full shot noise level, suggesting that shot noise is not generated in an ideal quantum point contact, where the electrons do not suffer backscattering as they enter and traverse the contact.

Low-frequency noise in transport through quantum pOint contacts

The magnetoresistance at temperatures below 20 K in an $n\text{-InSb}/{\text{In}}_{0.85}{\text{Al}}_{0.15}\text{Sb}$ two-dimensional electron system is studied and described in terms of antilocalization due to quantum interference under strong spin-orbit interaction. The spin-orbit interaction coefficients are extracted by fitting the magnetoresistance data to an antilocalization theory distinguishing the Rashba and Dresselhaus contributions. A good agreement between magnetoresistance data and theory suggests a Rashba coefficient $|\ensuremath{\alpha}|\ensuremath{\approx}0.03\text{ }\text{eV}\text{ }\text{\AA{}}$ and a Dresselhaus coefficient $\ensuremath{\gamma}\ensuremath{\approx}490\text{ }\text{eV}\text{ }{\text{\AA{}}}^{3}$. A strong contribution from the Dresselhaus term leads to pronounced anisotropy in the energy splitting induced by spin-orbit interaction in the two-dimensional electron dispersion.

/pdf/spin-orbit-interaction-determined-by-antilocalization-in-an-4r09pjmkcu.pdf

Spin-orbit interaction determined by antilocalization in an InSb quantum well

We have measured the low‐temperature mobility of high‐quality two‐dimensional hole systems confined at the (311)A GaAs/AlxGa1−xAs interface Variables were the thickness of the spacer layer separating the carriers from the Si dopants, and the carrier sheet density A large anisotropy in mobility is found between the [233] and [011] directions While the high mobility [233] direction yields results analogous to the two‐dimensional electron case, we conclude that transport along [011] is almost entirely determined by anisotropic interface roughness scattering

Jean J. Heremans

Papers

Enhanced Room-Temperature Geometric Magnetoresistance in Inhomogeneous Narrow-Gap Semiconductors.

Robustness of Topologically Protected Surface States in Layering of Bi2Te3 Thin Films

Low-frequency noise in transport through quantum pOint contacts

Spin-orbit interaction determined by antilocalization in an InSb quantum well

Mobility anisotropy of two‐dimensional hole systems in (311)A GaAs/AlxGa1−xAs heterojunctions