11,274 research outputs found
Mathematical modeling of high and low temperature heat pipes
Following a review of heat and mass transfer theory relevant to heat pipe performance, math models are developed for calculating heat-transfer limitations of high-temperature heat pipes and heat-transfer limitations and temperature gradient of low temperature heat pipes. Calculated results are compared with the available experimental data from various sources to increase confidence in the present math models. Complete listings of two computer programs for high- and low-temperature heat pipes respectively are included. These programs enable the performance to be predicted of heat pipes with wrapped-screen, rectangular-groove, or screen-covered rectangular-groove wick
Robust pinning of magnetic moments in pyrochlore iridates
Pyrochlore iridates A2Ir2O7 (A = rare earth elements, Y or Bi) hold great
promise for realizing novel electronic and magnetic states owing to the
interplay of spin-orbit coupling, electron correlation and geometrical
frustration. A prominent example is the formation of all-in/all-out
(AIAO)antiferromagnetic order in the Ir4+ sublattice that comprises of
corner-sharing tetrahedra. Here we report on an unusual magnetic phenomenon,
namely a cooling-field induced shift of magnetic hysteresis loop along
magnetization axis, and its possible origin in pyrochlore iridates with
non-magnetic Ir defects (e.g. Ir3+). In a simple model, we attribute the
magnetic hysteresis loop to the formation of ferromagnetic droplets in the AIAO
antiferromagnetic background. The weak ferromagnetism originates from canted
antiferromagnetic order of the Ir4+ moments surrounding each non-magnetic Ir
defect. The shift of hysteresis loop can be understood quantitatively based on
an exchange-bias like effect in which the moments at the shell of the FM
droplets are pinned by the AIAO AFM background via mainly the Heisenberg (J)
and Dzyaloshinsky-Moriya (D) interactions. The magnetic pinning is stable and
robust against the sweeping cycle and sweeping field up to 35 T, which is
possibly related to the magnetic octupolar nature of the AIAO order.Comment: 16 pages, 4 figure
Unusual Coupling Between Field-induced Spin Fluctuations and Spin Density Wave in Intermetallic CeAg2Ge2
We report on the experimental evidences for an unusual coupling between the
magnetic field- induced fluctuations of correlated Ce-ions coinciding with the
discontinuous movement of the underlying spin density wave in the intermetallic
rare earth compound CeAg2Ge2. The measurements performed using neutron
scattering and magnetic Gruneisen ratio methods suggest that the coupling
onsets at H= 2.7 T, T < 3.8 K and persists to the lowest measurement
temperature T ~ 0.05 K. These measurements suggest a new mechanism behind the
spin fluctuations which can affect the intrinsic properties of the system.Comment: 4 pages, 4 figures, Strongly correlated electrons syste
Pulsed THz radiation due to phonon-polariton effect in [110] ZnTe crystal
Pulsed terahertz (THz) radiation, generated through optical rectification
(OR) by exciting [110] ZnTe crystal with ultrafast optical pulses, typically
consists of only a few cycles of electromagnetic field oscillations with a
duration about a couple of picoseconds. However, it is possible, under
appropriate conditions, to generate a long damped oscillation tail (LDOT)
following the main cycles. The LDOT can last tens of picoseconds and its
Fourier transform shows a higher and narrower frequency peak than that of the
main pulse. We have demonstrated that the generation of the LDOT depends on
both the duration of the optical pulse and its central wavelength. Furthermore,
we have also performed theoretical calculations based upon the OR effect
coupled with the phonon-polariton mode of ZnTe and obtained theoretical THz
waveforms in good agreement with our experimental observation.Comment: 9 pages, 5 figure
Scanning tunneling spectroscopy of superconducting LiFeAs single crystals: Evidence for two nodeless energy gaps and coupling to a bosonic mode
The superconducting compound, LiFeAs, is studied by scanning tunneling
microscopy and spectroscopy. A gap map of the unreconstructed surface indicates
a high degree of homogeneity in this system. Spectra at 2 K show two nodeless
superconducting gaps with meV and
meV. The gaps close as the temperature is increased to the bulk
indicating that the surface accurately represents the bulk. A dip-hump
structure is observed below with an energy scale consistent with a
magnetic resonance recently reported by inelastic neutron scattering
Time-dependent density-functional theory for open systems
By introducing the self-energy density functionals for the dissipative
interactions between the reduced system and its environment, we develop a
time-dependent density-functional theory formalism based on an equation of
motion for the Kohn-Sham reduced single-electron density matrix of the reduced
system. Two approximate schemes are proposed for the self-energy density
functionals, the complete second order approximation and the wide-band limit
approximation. A numerical method based on the wide-band limit approximation is
subsequently developed and implemented to simulate the steady and transient
current through various realistic molecular devices. Simulation results are
presented and discussed.Comment: 16 pages, 12 figure
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