Search results for "MAGNETIC FIELD"
showing 10 items of 1488 documents
Simultaneous time-optimal control of the inversion of two spin-12particles
2010
We analyze the simultaneous time-optimal control of two-spin systems. The two noncoupled spins, which differ in the value of their chemical offsets, are controlled by the same magnetic fields. Using an appropriate rotating frame, we restrict the study to the case of opposite shifts. We then show that the optimal solution of the inversion problem in a rotating frame is composed of a pulse sequence of maximum intensity and is similar to the optimal solution for inverting only one spin by using a nonresonant control field in the laboratory frame. An example is implemented experimentally using nuclear magnetic resonance techniques.
Excitation of singlet–triplet coherences in pairs of nearly-equivalent spins
2019
We present approaches for an efficient excitation of singlet–triplet coherences in pairs of nearly-equivalent spins. Standard Nuclear Magnetic Resonance (NMR) pulse sequences do not excite these coherences at all or with very low efficiency. The single quantum singlet–triplet coherences, here termed the outer singlet–triplet coherences, correspond to lines of low intensity in the NMR spectrum of a strongly-coupled spin pair (they are sometimes referred to as “forbidden transitions”), whereas the zero-quantum coherences, here termed the inner singlet–triplet coherences, do not have a direct spectral manifestation. In the present study, we investigated singlet–triplet coherences in a pair of …
High-resolution observations of SN 2001gd in NGC 5033
2005
We report on 8.4 GHz VLBI observations of SN2001gd in the spiral galaxy NGC5033 made on 26 June 2002 and 8 April 2003. Our data nominally suggests a relatively strong deceleration for the expansion of SN2001gd, but we cannot dismiss the possibility of a free supernova expansion. From our VLBI observations on 8 April 2003, we inferred a minimum total energy in relativistic particles and magnetic fields in the supernova shell of E_min =(0.3-14) 10^{47} ergs, and a corresponding equipartition average magnetic field of B_min = (50--350) mG. We also present multiwavelength VLA measurements of SN2001gd, which are well fit by an optically thin, synchrotron spectrum, partially absorbed by thermal p…
Thermal conduction and modeling of static stellar coronal loops
1993
We have modeled stellar coronal loops in static conditions for a wide range of loop length, plasma pressure at the base of the loop and stellar surface gravity, so as to describe physical conditions that can occur in coronae of stars ranging from low mass dwarfs to giants as well as on a significant fraction of the Main-Sequence stars.
DYNAMICAL BAR-MODE INSTABILITY IN DIFFERENTIALLY ROTATING MAGNETIZED NEUTRON STARS
2009
This paper presents a numerical study over a wide parameter space of the likelihood of the dynamical bar-mode instability in differentially rotating magnetized neutron stars. The innovative aspect of this study is the incorporation of magnetic fields in such a context, which have thus far been neglected in the purely hydrodynamical simulations available in the literature. The investigation uses the Cosmos++ code which allows us to perform three dimensional simulations on a cylindrical grid at high resolution. A sample of Newtonian magneto-hydrodynamical simulations starting from a set of models previously analyzed by other authors without magnetic fields has been performed, providing estima…
Type-II Supernovae and Neutrino Magnetic Moment
1998
The present solar and atmospheric neutrino data together with the LSND results and the presence of hot dark matter (HDM) suggest the existence of a sterile neutrino at the eV scale. We have reanalysed the effect of resonant {\sl sterile} neutrino conversions induced by neutrino magnetic moments in a type-II supernova. We analyse the implications of $\nu_e-\nu_s$ and $\bar{\nu}_e-\bar{\nu}_s$ ($\nu_s$ denotes sterile neutrino) conversions for the supernova shock re-heating, the detected $\bar\nu_e$ signal from SN1987A and the $r$-process nucleosynthesis hypothesis. Using reasonable magnetic field profiles we determine the sensitivity of these three arguments to the relevant neutrino paramete…
Heavy fermion spin liquid in herbertsmithite
2015
We analyze recent heat capacity measurements in herbertsmithite $\rm ZnCu_3(OH)_6Cl_2$ single crystal samples subjected to strong magnetic fields. We show that the temperature dependence of specific heat $C_{mag}$ formed by quantum spin liquid at different magnetic fields $B$ resembles the electronic heat capacity $C_{el}$ of the HF metal $\rm YbRh_2Si_2$. We demonstrate that the spinon effective mass $M^*_{mag}\propto C_{mag}/T$ exhibits a scaling behavior like that of $C_{el}/T$. We also show that the recent measurements of $C_{mag}$ are compatible with those obtained on powder samples. These observations allow us to conclude that $\rm ZnCu_3(OH)_6Cl_2$ holds a stable strongly correlated …
Chaotic Antiferromagnetic Nano-Oscillator driven by Spin-Torque
2021
We theoretically describe the behavior of a terahertz nano-oscillator based on an anisotropic antiferromagnetic dynamical element driven by spin torque. We consider the situation when the polarization of the spin-current is perpendicular to the external magnetic field applied along the anisotropy easy-axis. We determine the domain of the parametric space (field, current) where the oscillator demonstrates chaotic dynamics. Characteristics of the chaotic regimes are analyzed using conventional techniques such as spectra of the Lyapunov exponents. We show that the threshold current of the chaos appearance is particularly low in the vicinity of the spin-flop transition. In this regime, we consi…
Correlation energy of two-dimensional systems: Toward non-empirical and universal modeling
2009
The capability of density-functional theory to deal with the ground-state of strongly correlated low-dimensional systems, such as semiconductor quantum dots, depends on the accuracy of functionals developed for the exchange and correlation energies. Here we extend a successful approximation for the correlation energy of the three dimensional inhomogeneous electron gas, originally introduced by Becke [J. Chem. Phys. {\bf 88}, 1053 (1988)], to the two-dimensional case. The approach aims to non-empirical modeling of the correlation-hole functions satisfying a set of exact properties. Furthermore, the electron current and spin are explicitly taken into account. As a result, good performance is …
Magnetic properties of a strongly correlated system on the Bethe lattice
2010
We study the influence of an external magnetic field h on the phase diagram of a system of Fermi particles living on the sites of a Bethe lattice with coordination number z and interacting through on-site U and nearest-neighbor V interactions. This is a physical realization of the extended Hubbard model in the narrow-band limit. Our results establish that the magnetic field may dramatically affect the critical temperature below which a long-range charge ordered phase is observed, as well as the behavior of physical quantities, inducing, for instance, magnetization plateaus in the magnetization curves. Relevant thermodynamic quantities - such as the specific heat and the susceptibility - are…