1,854 research outputs found
Spin Berry phase in anisotropic topological insulators
Three-dimensional topological insulators are characterized by the presence of
protected gapless spin helical surface states. In realistic samples these
surface states are extended from one surface to another, covering the entire
sample. Generally, on a curved surface of a topological insulator an electron
in a surface state acquires a spin Berry phase as an expression of the
constraint that the effective surface spin must follow the tangential surface
of real space geometry. Such a Berry phase adds up to pi when the electron
encircles, e.g., once around a cylinder. Realistic topological insulators
compounds are also often layered, i.e., are anisotropic. We demonstrate
explicitly the existence of such a pi Berry phase in the presence and absence
(due to crystal anisotropy) of cylindrical symmetry, that is, regardless of
fulfilling the spin-to-surface locking condition. The robustness of the spin
Berry phase pi against cylindrical symmetry breaking is confirmed numerically
using a tight-binding model implementation of a topological insulator nanowire
penetrated by a pi-flux tube.Comment: 9 pages, 4 figures (6 panels
Weak topological insulator with protected gapless helical states
A workable model for describing dislocation lines introduced into a
three-dimensional topological insulator is proposed. We show how fragile
surface Dirac cones of a weak topological insulator evolve into protected
gapless helical modes confined to the vicinity of dislocation line. It is
demonstrated that surface Dirac cones of a topological insulator (either strong
or weak) acquire a finite-size energy gap, when the surface is deformed into a
cylinder penetrating the otherwise surface-less system. We show that when a
dislocation with a non-trivial Burgers vector is introduced, the finite-size
energy gap play the role of stabilizing the one-dimensional gapless states.Comment: 8 pages, 17 figure
Quantization of Conductance Minimum and Index Theorem
We discuss the minimum value of the zero-bias differential conductance
in a junction consisting of a normal metal and a nodal
superconductor preserving time-reversal symmetry. Using the quasiclassical
Green function method, we show that is quantized at in the limit of strong impurity scatterings in the
normal metal. The integer represents the number of perfect
transmission channels through the junction. An analysis of the chiral symmetry
of the Hamiltonian indicates that corresponds to the
Atiyah-Singer index in mathematics.Comment: 5 pages, 1 figur
Scaling Rule for Very Shallow Trench IGBT toward CMOS Process Compatibility
2012 24th International Symposium on Power Semiconductor Devices and ICs (ISPSD 2012), June 3-7, 2012, Bruges, BelgiumDeep trench gate is used for latest IGBT to improve device performance. By large difference from deep submicron CMOS structure, there is no process compatibility among CMOS device and trench gate IGBT. We propose IGBT scaling rule for shrinking IGBT cell structure both horizontally and vertically. The scaling rule is theoretically delivered by structure based equations. Device performance improvement was also predicted by TCAD simulations even with very shallow trench gate. The rule enables to produce trench gate IGBT on large diameter wafer in CMOS factory with superior productivity
IGBT Scaling Principle Toward CMOS Compatible Wafer Processes
A scaling principle for trench gate IGBT is proposed. CMOS technology on large diameter wafer enables to produce various digital circuits with higher performance and lower cost. The transistor cell structure becomes laterally smaller and smaller and vertically shallower and shallower. In contrast, latest IGBTs have rather deeper trench structure to obtain lower on-state voltage drop and turn-off loss. In the aspect of the process uniformity and wafer warpage, manufacturing such structure in the CMOS factory is difficult. In this paper, we show the scaling principle toward shallower structure and better performance. The principle is theoretically explained by our previously proposed “Structure Oriented” analytical model. The principle represents a possibility of technology direction and roadmap for future IGBT for improving the device performance consistent with lower cost and high volume productivity with CMOS compatible large diameter wafer technologies
Effects of the phase coherence on the local density of states in superconducting proximity structures
We theoretically study the local density of states in superconducting
proximity structure where two superconducting terminals are attached to a side
surface of a normal-metal wire. Using the quasiclassical Green's function
method, the energy spectrum is obtained for both of spin-singlet -wave and
spin-triplet -wave junctions. In both of the cases, the decay length of the
proximity effect at the zero temperature is limited by a depairing effect due
to inelastic scatterings. In addition to the depairing effect, in -wave
junctions, the decay length depends sensitively on the transparency at the
junction interfaces, which is a unique property to odd-parity superconductors
where the anomalous proximity effect occurs.Comment: 11 pages, 9 figure
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