Charmonium states in quark-gluon plasma

We discuss how the spectral changes of quarkonia at T _c can reflect the ‘critical’ behaviour of QCD phase transition. Starting from the temperature dependencies of the energy density and pressure from lattice QCD calculation, we extract the temperature dependencies of the scalar and spin-2 gluon condensates near T _c. We also parametrize these changes into the electric and magnetic condensate near T _c. While the magnetic condensate hardly changes across T _c, we find that the electric condensate increases abruptly above T _c. Similar abrupt change is also seen in the scalar condensate. Using the QCD second-order Stark effect and QCD sum rules, we show that these sudden changes induce equally abrupt changes in the mass and width of J/ψ, both of which are larger than 100 MeV at slightly above T _c.      

Changes in vibrational modes near the magnetic phase transition in magnetostrictive materials

Mössbauer studies of ⁵⁷Fe in RFe₂ (R = Y, Tb, Dy, Ho) at temperatures 300 to 800 K and in R(Co_0.98Fe_0.02)₂ (R = Tb, Dy) at temperatures 85 to 300 K have been performed. The spectra yield the temperature dependence of the easy axis of magnetization, the magnetic hyperfine fields, electric field gradients and isomer shift. From the total Mössbauer absorption area the recoil free fraction was determined. The relative intensities of the various Mössbauer absorption lines yield the anisotropy in the recoil free fraction. In all compounds, except in YFe₂, all measured quantities show large variations near the magnetic phase transition temperature (T_c). The variation in the recoil free fraction and in its anisotropy and the variation in the electric field gradient are all consistent, quantitatively, with anisotropic softening in the vibration modes of the Fe nucleus near T_c In RFe₂ the modes in which the iron vibrates along the local C_3h symmetry axes are softened less than the perpendicular modes. In RCo₂ it is the other way around. It seems, that these critical softening phenomena are due to the strong magnon-phonon coupling present in these materials and absent in YFe₂. This conclusion is confirmed by a theoretical analysis given in an adjacent paper. In it we use the Green function method to calculate the average of the square of vibrational amplitudes in a Debye solid with a strong magnon-phonon coupling at temperatures close to the magnetic phase transition. We show that these amplitudes may increase or decrease near T_c depending on the details of the coupled system.     

Magneto-optical properties of the iron garnet Tb3Fe5O12 near the magnetic compensation temperature

The magneto-optical susceptibility and magnetic hysteresis loops of the Faraday effect, which accompany the technical magnetization of the iron garnet Tb₃Fe₅O₁₂, have been investigated experimentally in the temperature region near the magnetic compensation point of this ferrimagnet T _c = 249 K. It has been found that, during the technical magnetization as the temperature approaches the magnetic compensation point T _c , the velocity of domain walls increases, whereas the magneto-optical susceptibility has local maxima to the right and to the left from T _c . Mentioned features of magneto-optical properties of the iron garnet Tb₃Fe₅O₁₂ are associated with the resonance natural frequency of the domain wall oscillations in the crystal with the frequency of the alternating magnetic field. It has been shown that the used theoretical model of the magnetic resonance of domain walls makes it possible to consistently describe (at the qualitative level) the revealed regularities of the variation of the Faraday effect in the iron garnet Tb₃Fe₅O₁₂ near T _c .     

Manifestation of a series of phase transitions in IR spectra of a multiferroic TbMnO3

We have studied IR transmittance spectra of a multiferroic TbMnO₃. Three phase transitions (T _N = 43 K, T _FE = 28 K, and T _Tb = 7 K), which manifest themselves in a shift of the absorption band edge of manganese and in changes in the spectral parameters of lines of f-f transitions of terbium, have been detected. An incommensurate character of the magnetic structure leads to nonequivalence of terbium positions and to an additional inhomogeneous broadening. The spontaneous electric polarization, which arises at T < T _FE, significantly affects the crystal field that acts on terbium and noticeably shifts its energy levels. The phase transition with ordering of terbium (7 K) is accompanied by a lowering of the energy of the ground state of the Tb³⁺ ion.     

Low-frequency electrical resistance of iron,cobalt, and nickel in the vicinity of their Curie temperatures

Alternating-current electrical resistance measurements between 17 Hz and 100 kHz were made on high purity Fe, Co, and Ni wires in the vicinity of their Curie temperatures (T _c). The electrical resistance was independent of frequency for temperatures (T) aboveT _c. As the temperature was lowered, however, there was an abrupt jump in the electrical resistance atT _c followed by a gradual decrease toward its dc value. The magnitude of the electrical resistance jump atT _c increased as the square root of the frequency. The enhancement of the electrical resistance forT≦T _c is produced by an abrupt decrease of the skin depth atT _c which, in turn, is due to the sudden increase in the initial magnetic permeability atT _c. Measurements of the ac electrical resistance in the vicinity of the Curie temperature of certain ferromagnetic metals can be utilized to 1) accurately determine the Curie temperature using frequencies as low as 17 Hz, and 2) quantitatively determine the initial magnetic permeability as a function of temperature and heat treatment.     

London penetration depth and coherence peak in ammonia-intercalated fulleride superconductors

The compound (NH₃)_xNaRb₂C₆₀ (x ～ 1.6) has been studied by¹H nuclear magnetic resonance. The proton line shows no shift at any temperature, while lineshape analysis of the low-temperature (30 K) spectra reveals the presence of a Knight shift interaction of pure anisotropic nature, whose amplitude has been estimated. Relaxation measurements show a single-exponential recovery of the magnetization at all investigated temperatures and fields. A Korringa behavior is observed below 65 K and a clear Hebel-Slichter coherence peak is detected just below T_c. The London penetration depth can be estimated from the line broadening observed below T_c, which yields λ ≈ 825 nm.     

Critical nuclear relaxation in cobalt metal

Nuclear magnetic resonance of cobalt metal was investigated in the paramagnetic and ferromagnetic states and in the critical region below T_c. The Knight shift and spin lattice relaxation times were measured in the paramagnetic phase in the solid and liquid states from 1578 K to 1825 K. The resonant frequency, spin-lattice and spin-spin relaxation times were measured in the ferromagnetic phase from room temperature to 1385 K. The main part of (T₁T)^-1 results from fluctuating orbital moments in both phases except near T_c where this process forms the background for critical spin relaxation. The critical exponents for T^-1₁ and for the magnetization in the ferromagnetic state were found to be n' = 0.96 ± 0.07 and β = 0.308 ± 0.012, respectively.     

The soft mode properties in Raman spectra of improper ferroelastics Hg2Cl2 and Hg2Br2

The temperature dependence of the frequency and intensity of the soft mode line appearing in the Raman spectra of Hg₂Cl₂, Hg₂Br₂ below the critical point T_c was studied. A proportionality between the scattering cross section of the soft mode and the square of its frequency v_sm was established. A strong stress-induced increase of the intensity and frequency of the soft mode line was observed near T_c, the relative shift of v_sm being ～30% at ～0.5 kg/mm² stress and T_c?T=5°K.     

Nucleon propagator at finite temperature

We study the nucleon propagator at finite temperature in the framework of finite energy QCD sum rules. We find that the nucleon mass is approximately constant over a wide range of temperature, increasing sharply near the critical temperature for deconfinementT _c . The coupling of the nucleon to quarks is a monotonically decreasing function ofT, vanishing atT=T _c .     

Magnetic properties of high-T c superconductors from NQR and NMR measurements

NMR and NQR spectra and spin-lattice relaxation measurements carried out in LASCO and YBCO-type crystals are presented and analyzed in order to derive insights on the correlations and spin-dynamics of the Cu²⁺ ions and on the microscopic mechanisms of high-T _c superconductivity. As an illustrative example on how the magnetic correlation length and spin dynamics properties can be extracted from the relaxation rateW, the³⁵Cl NMR data in the two-dimensional Heisenberg system Sr₂CuCl₂O₂, around the paramagnetic-antiferromagnetic (PA-AF) transition are first considered. Then the¹³⁹La NQR relaxation measurements in La_2?xSr_xCuO₄ are briefly reviewed and it is shown how a simple picture of localized Cu²⁺ magnetic moments, whose spin fluctuation times are controlled by the charge defects induced by the doping, leads in a direct way to quantitative estimates for the progressive shift, on cooling, of the spectral density of the low-frequency spin excitations towards the high frequency range. This phenomenon can be described in terms of effective spin at the Cu²⁺ ions, and its similarities with the analogous effect of progressive delocalization in Heavy Fermions systems are pointed out. Thus, the superconducting transition appears to occur in an unconventional Fermi liquid with AF correlations among itinerant pseudoparticles, possibly involving a mechanism not directly related to the magnetic correlated dynamics. In fact, a universal behavior of the relaxation rates as a function of temperature is observed, regardless of the transition temperatureT _c. The independence ofT _c from the low frequency static and dynamical spin properties is also indicated by⁸⁹Y Knight shifts and from⁶³Cu relaxation rates in systems like YBa₂Cu₄O₈ (Y124), whereT _c can be changed by atomic substitutions and by controlling the oxygen stoichiometry. The effect of an external magnetic field on the correlated spin dynamics of the AF Fermi liquid is investigated and from a comparison of Cu NQR relaxation and NMR relaxation in oriented powder of YBCO and LASCO it is shown that the external field has the small but unambiguous effect of depressing the relaxation rates aboveT _c, besides strongly enhancing them in the superconducting phase. A maximum in the ratio \({{W\left( {NQR} \right)} \mathord{\left/ {\vphantom {{W\left( {NQR} \right)} {W\left( {\vec H\left\| {\vec c} \right.} \right)}}} \right. \kern-0em} {W\left( {\vec H\left\| {\vec c} \right.} \right)}}\) is thus observed around 80 K, either in LASCO or in YBCO, again indicating that the transition could be driven by a mechanism not directly involving the spin dynamic properties. To study the role of the fluxions belowT _c ⁸⁹Y NMR shifts and spectra in oriented powders of YBCO are analyzed. Information on the spin susceptibility and on the structure of the vortex lattice is obtained. In addition, from the temperature behavior of the linewidth a motional narrowing related to flux melting is evidenced. The effective correlation time for the vortex motion is derived and it is discussed why μ⁺SR cannot detect it in view of the different rigid-lattice line broadening.     

Quantum phase diagram for homogeneous Bose‐Einstein condensate

We calculate the quantum phase transition for a homogeneous Bose gas in the plane of s‐wave scattering length a_s and temperature T. This is done by improving a one‐loop result near the interaction‐free Bose‐Einstein critical temperature T_c⁽⁰⁾ with the help of recent high‐loop results on the shift of the critical temperature due to a weak atomic repulsion based on variational perturbation theory. The quantum phase diagram shows a nose above T_c⁽⁰⁾, so that we predict the existence of a reentrant transition above T_c⁽⁰⁾, where an increasing repulsion leads to the formation of a condensate.     

A simple theory of 40 K superconductivity in MgB2: first-principles calculations of Tc, its dependence on boron mass and pressure

We have studied the superconducting properties of MgB₂ from first-principles under isotropic, uniaxial, and biaxial compressions. We find that the in-plane boron phonons near the zone-center are very anharmonic and strongly coupled to the planar B σ bands near the Fermi level. This mode is found to be the key to quantitatively explain the observed high T_c, the total isotope effect and the pressure dependence of T_c. We propose that a stringent test on the hole and phonon based theories of the superconductivity in MgB₂ would be a measurement of the biaxial ab-compression dependence of T_c.     

Domain structure of a thin single-crystal plate of terbium iron garnet near the magnetic compensation point

The domain structure of a thin single-crystal plate of the iron garnet Tb₃Fe₅O₁₂ has been investigated using the magneto-optical method in the temperature range near the magnetic compensation point of this ferrimagnet T _c = 248.6 K. It has been shown that, when the temperature of the sample approaches the magnetic compensation point, the domain width significantly increases, but remains finite at T = T _c . The magnetic H-T phase diagram, which determines the boundary between the multidomain and domain-free (uniformly magnetized) states of the sample, has been constructed using the data on visual observations of the transformation of the domain structure with variations in the temperature and external magnetic field. The results obtained have been interpreted in terms of the thermodynamic theory of stability of different magnetic phases of a two-sublattice cubic ferrimagnet near T _c .     

Self,nitrogen and oxygen broadening of the 115-GHz line of carbon monoxide

Self nitrogen, oxygen and air-broadened half-widths of the 115-GHz line of CO have been measured at various temperatures between 293 K (room temperature) and 220 K. The temperature dependence of the broadening parameter CCO-XW is described by a power law CCO-XW (T) = CCO-XW(293 K)(T/293)-n co-x. The values of CCOW (293 K) and nCO-X are presented for each broadening gas X, X - CO, N2 and O2. The usual relation CCO-airW (T) = 0.78CCO−N2W(T) + 0.21CCO−O2 W(T) is found to be valid in the temperature and pressure ranges of the present experiments.     

μSR studies of heavy fermion systems

We have performed both zero field and high transverse field measurements at dilution refrigerator temperatures on a number of heavy electron systems, examining the superconducting and magnetic properties of these interesting materials. Among the materials studied to date are UBe₁₃, URu₂Si₂ and U₆Fe. The magnetic field penetration depth in the superconducting state of UBe₁₃ is greater than 10000 Å, as no increase in the transverse field relaxation rate is observed belowT _c . A sharp increase in the precession frequency is seen, starting atT _c . This frequency shift shows little temperature dependence at low temperature; we found no clear evidence for unconventional superconductivity in this material. Zero field measurements in URu₂Si₂ show the weak antiferromagnetic transition at 17.5 K. Finally, we we found no clear evidence for unconventional superconductivity in this material. Zero field measurements in URu₂Si₂ show the weak antiferromagnetic transition at 17.5 K. Finally, we have observed relaxation in high transverse field due to the formation of a flux lattice in U₆Fe, a material where the electron effective mass is rather lighter than in other heavy fermion systems. The relaxation exhibits a sharp onset atT _c=3.9 K, and is flat at low temperatures as expected for a conventional superconductor.     

Hydrogen absorption and its effect on the magnetic properties of rare-earth iron intermetallics

X-ray diffraction studies of the hydrogen absorption in several YFe and CeFe intermetallic compounds showed that no structural changes occur upon hydrogen absorption in Y₆Fe₂₃, YFe₃, YFe₂. The lattice constants of the hydrides were found to be appreciably larger than those of the pure intermetallic compounds. The magnetic properties of the hydrides were determined and compared with the original compounds. In all cases the magnetic moment per Fe atom proved to be much larger in the hydride phases. Hydrogen absorption can lead to a decrease as well as to an increase of the magnetic ordering temperature (T_c). These changes in T_c could adequately be explained in terms of the observed increases in lattice constant and the data available for the pressure derivative of T_c of these compounds.     

Low-frequency resonance of domain walls in the iron garnet Tb3Fe5O12 near the magnetic compensation point

The vibrational motion dynamics of domain walls (DWs) in the iron garnet Tb₃Fe₅O₁₂, a low-frequency magnetic field, and the temperature range 200–295 K (which includes the magnetic compensation point of this ferrimagnet, T _c ≈ 249 K) is studied by a magnetooptical method. The temperature dependence of the DW vibration amplitude in this garnet crystal near T _c has a resonance character. A theoretical model of the magnetic resonance of DWs is proposed to interpret the obtained experimental results; according to this model, the DW mass tends to infinity and the resonance frequency tends to zero when temperature approaches the magnetic compensation point.     

Phonon properties of vanadium-substituted lanthanum niobate derived from heat-capacity measurements

We have measured the 3–400 K heat capacity of vanadium-substituted lanthanum niobate LaNb_1−x,V_xO₄ (0 < x⩽0.35) to determine how the relevant averaged properties, or moments, of the phonon spectrum 〈ωⁿ〉 relate to certain other lattice properties of these compounds, most noteworthy of which is a large decrease in the paraelastic-ferroelastic transformation temperature T_c with x. The pertinent moments, represented by their corresponding Debye temperatures, are θ_D(−3) corresponding to the lowest frequency modes; θ_D(0) associated with the geometric mean frequency of the entire spectrum; and θ_D (2) identified with the high-frequency modes that are sampled at the upper end of the temperature range. We find that θ_D(−3) falls rapidly with x and this effect can be correlated with the comparably sharp drop in T_c. There is little effect of composition on θ_D(0) and θ_D (2) because the phonons that govern soft-mode behavior represent only a small fraction of the mode population. In the vicinity of T_c the temperature dependences of the free energies of the tetragonal and monoclinic phases are so similar that the discontinuity in C_p is immeasurably small.     

A 57 Fe Mössbauer study on the FeSe and Fe(Se,Te) superconductors: discontinuities in the hyperfine parameters at T c

Polycrystalline samples of FeSe_0.82 and FeSe_0.5Te_0.5 were synthesized using a solid-state reaction route. Bulk superconductivity was confirmed using SQUID magnetometry. The onset of T _c was at 8.0 K for FeSe_0.82 and 12.5 K for FeSe_0.5Te_0.5. Paramagnetic ⁵⁷Fe Mössbauer spectra were recorded at temperatures between 5.4 and 320 K in transmission geometry. All spectra exhibited simple quadrupole splitting. For FeSe_0.5Te_0.5 a small drop in the quadrupole splitting was observed about T _c upon cooling. Additionally, for both samples the isomer shift and the total absorption started to drop around T _c , indicating a softening of the lattice. The drop is estimated to correspond to at least 60 K from the original Debye temperatures.     

Magnetic properties of amorphous Co75W25

Saturation magnetization M(T), spin wave stiffness D and Curie temperature T_c of amorphous Co₇₅W₂₅ were determined by magnetic measurements. Tungsten reduces these quantities more than metalloids or 3d-transition metals do. T_c is below room temperature. The ratio D/T_c, however, is equal to that observed on a fcc single crystal of Co₉₂Fe₈ and on amorphous Co-Ti alloys.