High-field Transition Research Articles

Field-induced magnetic order in bis-tetramethyltetraselenafulvalenium perchlorate(TMTSF)2ClO4

A microscopic study of the organic superconductor bis-tetramethyltetraselenafulvalenium perchlorate [${(\mathrm{TMTSF})}_{2}$Cl${\mathrm{O}}_{4}$] has been undertaken with the use of nuclear magnetic resonance techniques. Proton and selenium resonances were recorded over the field range 60-120 kOe at a temperature of 1.5 K. The proton spin-lattice relaxation-rate measurements clearly reflect both the high-field phase transition and magnetotransport anomalies. The selenium resonance is strongly attenuated above the high-field transition which indicates the onset of field-induced magnetic order. These results and their bearing on the high-field electronic properties of the Bechgaard salts are discussed.

Metamagnetism of palladium

Abstract Self-consistent spin-polarized energy band studies with the applied magnetic field included, show Pd metal to undergo a metamagnetic transition in high magnetic fields. The calculated moments and susceptibilities are found to be greatly enhanced for tetragonally (“stretched”) Pd as in the Au/Pd/Au sandwiches studied by Brodsky and Freeman and to indicate the possibility of ferromagnetism in tet. Pd.

Neutron diffraction study of UAs in high magnetic fields

Magnetization experiments have shown that at low temperature and in an applied magnetic field of ∼90 kOe antiferromagnetic (AF) uranium arsenide transforms to a new ferrimagnetic state. The high-field transition produces a state with magnetic components parallel to the applied field. A small ferromagnetic component (0.4 μ B) is seen in magnetization experiments but with neutron diffraction we have observed directly the large AF component (∼ 1.8 μ B) which has a q value of 0.58 ± 0.01 c ∗ in contrast to the zero field type IA structure with q = (0,0,0.50) . This transition is quite different from that occuring in the conventional spin-flop transition.

Frequency dependence of the surface cyclotron resonance of tellurium

Using submillimeter and microwave radiation up to 3-mm wavelength, we investigated the frequency dependence of the surface cyclotron resonance of both electrons and holes on ($1\overline{1}00$) tellurium surfaces in a metal-insulator-semiconductor arrangement. Special effort was made to observe the high-field transition of the surface hole in accumulation corresponding to the ${H}_{7}$ line in the bulk resonance spectra. The interpretation of the experimental results reveals an important increase of the cyclotron mass at low photon energies.

Rotational selection rules for doublet-quartet transitions in asymmetric top molecules

Selection rules on the rotational structure of doublet-quartet transitions in asymmetric top molecules are discussed on the basis of the spin-orbit coupling mechanism, rather than by symmetry arguments, and some additional rules are found with respect to a previous work. In the case that the combining doublet and quartet states are appreciably mixed only with levels directly connected to them by the spin-orbit operator, it turns out that, except for the very low values of the rotational quantum numbers, branches with |Δ N − Δ J| > 1 are essentially forbidden. Thus, branches with Δ N = ± 3 ( T and N branches) are not expected to be observable. In this case, it is found also that in a high magnetic field transitions with Δ M s = ±2 are forbidden, and only Δ M s = 0, ±1 bands are expected to occur. All these restrictions are removed if the combining states are also appreciably contaminated by states indirectly connected to them by the spin-orbit operator, through intermediate levels.

ESR study of exchange coupled pairs in copper diethyldithiocarbamate—explanation of the low temperature hyperfine anomaly

A study of the hyperfine interaction in the ESR of Cu-Cu pairs in single crystals of copper diethyldithiocarbamate as a function of temperature has shown distinct differences in the hyperfine structure in the two fine structure transitions at 20 K, the spectrum not having the same hyperfine intensity pattern in the low field fine structure transition in contrast to that of the high field transition. The details of the structure of both the fine structure transitions in the 20 K spectrum have now been explained by recognizing the fact that the mixing of the nuclear spin states caused by the anisotropic hyperfine interaction affects the electron spin states | + 1 > and | −> differently. This has incidentally led to a determination of the sign ofD confirming the earlier model. The anomalous hyperfine structure is found to become symmetric at 77 K and 300 K. It is proposed that the reason for this lies in the dynamics of spin-lattice interaction which limits the lifetime of the spin states in each of the electronic levels | − 1 >, | 0 > and | + 1 > The estimate of spin-lattice relaxation time agrees with those indicated from other studies. The model proposed here for the hyperfine interaction of pairs in the electronic triplet state is of general validity.

Angular dependence of field-induced phase transitions in αFe 2O 3

The angular dependence of the field-induced phase transitions in single crystal αFe 2O 3 are measured below T M(260°K) with a torquemeter. There sults are in good agreement with calculations which include a fourth-order anisotropy. For magnetic fields above the high field transitions, no anisotropy is detected between susceptibilities along the principal axes.

Thermal Phase Transitions in Superconducting Nb-Zr Alloys

Experimental data are presented for the thermal propagation of a normal-superconducting boundary along wires of Nb-25%Zr, Nb-37%Zr, and Nb-51%Zr. The thermal propagation velocity was measured as a function of current at 4.2°K in constant longitudinal and transverse magnetic fields up to the upper critical field Hc2 as well as in a zero magnetic field for temperatures between 2.5° and 11°K. Previous models which describe thermal propagation phenomena neglected the temperature dependences of the specific heat, thermal conductivity, and electrical resistivity. A more exact model is proposed here which takes into account the temperature dependences of these quantities, and all the present experimental data are in good agreement with this model. From a measurement at 4.2°K of the dependence of the thermal velocity on magnetic field, the high-field transition temperatures as a function of magnetic field are calculated for temperatures above 4.2°K. In conjunction with the theory, the linear dependence of the thermal velocity on current implies a weak current dependence for both the transition temperature and superconducting-state specific heat in Nb-Zr alloys up to 3×105 A/cm2. A method is discussed for using thermal propagation techniques to determine the superconducting-state specific heat in the presence of known transport currents and magnetic fields.

High-Magnetic-Field Specific Heat of a Low-Dislocation-Density Alloy Superconductor

The Specific heat $C$ of a well-annealed alloy V-5 at.% Ta, measured at $1.4\ensuremath{\le}T\ensuremath{\le}5$ \ifmmode^\circ\else\textdegree\fi{}K in steady magnetic fields, displays sharp, bulk, superconducting transitions at upper critical fields ${H}_{c2}$ a factor $\ensuremath{\approx}10$ larger than the calorimetrically derived thermodynamic critical fields ${H}_{c}$. The transitions are similar to those observed earlier by Morin ${\mathit{et}\mathit{al}.}^{1}$ in ${\mathrm{V}}_{3}$Ga, but in the present case it is unlikely that the bulk nature of the high-field transitions can be attributed to a nearly complete occupation of the specimen volume by dislocation-centered high-field superconducting filaments of diameter comparable to the penetration depth, since electron transmission microscopy studies on an identically prepared specimen indicate that in at least 95% of the specimen volume the mean separation between dislocations is greater than 1.4\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ cm. However, the present data are explicable on the basis of the Ginzburg-Landau-Abrikosov-Gor'kov theory with a parameter $\ensuremath{\kappa}\ensuremath{\approx}\frac{{H}_{c2}}{\sqrt{2}{H}_{c}}\ensuremath{\approx}7$. The transition specific heat jumps $\frac{\ensuremath{\Delta}C({T}_{s})}{\ensuremath{\gamma}{T}_{s}}=1.44,1.15,1.10,0.94$ occur at ${T}_{s}=4.30,4.09,3.85,3.37$ \ifmmode^\circ\else\textdegree\fi{}K in fields $H=0,1,2,4$ kG, respectively, where $\ensuremath{\gamma}\ensuremath{\equiv}$ normal state electronic specific heat coefficient = 9.20 mJ/mole ${(\mathrm{K}\mathrm{\ifmmode^\circ\else\textdegree\fi{}})}^{2}$. The $\ensuremath{\Delta}C({T}_{s})$ values are in fair agreement with those calculated via Ehrenfest's equation for second-order phase transitions using Abrikosov's theoretical value of ${(\frac{\ensuremath{\partial}I}{\ensuremath{\partial}H})}_{T}$ at ${T}_{s}$ for $\ensuremath{\kappa}=7$, where $I\ensuremath{\equiv}$ magnetization. For $(\frac{{T}_{s}}{T})\ensuremath{\ge}1.8$, $\frac{{C}_{\mathrm{es}}}{\ensuremath{\gamma}{T}_{s}}=a \mathrm{exp}(\ensuremath{-}\frac{b{T}_{s}}{T})$ with $a=8.95,6.24,5.01,4.7;b=1.48,1.28,1.17,1.1$; for $H=0,1,2,4$ kG, respectively, where ${C}_{\mathrm{es}}$ is the electronic contribution to the specific heat. The exponential temperature dependence of ${C}_{\mathrm{es}}$ down to 1.4\ifmmode^\circ\else\textdegree\fi{}K suggests an essentially everywhere finite, field-dependent, high-field energy gap in accord with Abrikosov's vortex model.