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1.
The nature of the pseudogap state and its relation to the d-wave superconductivity in high-T c superconductors is still an open issue. The vortex-like excitations detected by the Nernst effect measurements exist in a certain temperature range above superconducting transition temperature T c, which strongly support that the pseudogap phase is characterized by finite pairing amplitude with strong phase fluctuations and imply that the phase transition at T c is driven by the loss of long-range phase coherence. We first briefly introduce the electronic phase diagram and pseudogap state of high-T c superconductors, and then review the results of Nernst effect for different high-T c superconductors. Related theoretical models are also discussed.  相似文献   

2.
The nature of the pseudogap state and its relation to the d-wave superconductivity in high-T c superconductors is still an open issue. The vortex-like excitations detected by the Nernst effect measurements exist in a certain temperature range above superconducting transition temperature T c, which strongly support that the pseudogap phase is characterized by finite pairing amplitude with strong phase fluctuations and imply that the phase transition at T c is driven by the loss of long-range phase coherence. We first briefly introduce the electronic phase diagram and pseudogap state of high-T c superconductors, and then review the results of Nernst effect for different high-T c superconductors. Related theoretical models are also discussed.  相似文献   

3.
Superconducting excitations—Bogoliubov quasiparticles—are the quantum mechanical mixture of negatively charged electron (−e) and positively charged hole (+e). Depending on the applied voltage bias in scanning tunneling microscope (STM) one can sample the particle and hole content of such a superconducting excitation. Recent STM experiments offer a unique insight into the inner workings of the superconducting state of superconductors. We propose a new observable quantity for STM studies that is the manifestation of the particle-hole dualism of the quasiparticles. We call it a Bogoliubov angle. This angle measures the relative weight of particle and hole amplitude in the superconducting (Bogoliubov) quasiparticle. We propose that this quantity can be measured locally by comparing the ratio of tunneling currents at positive and negative biases locally. This Bogoliubov angle allows one to measure directly the energy and position dependent particle-hole admixture and therefore visualize robustness of superconducting state locally. It may also allow one to measure the particle-hole admixture of excitations in normal state above critical temperature and thus may be used to measure superconducting correlations in pseudogap state.  相似文献   

4.
Effects of non-magnetic randomness on the critical temperature T c and diamagnetism are studied in a class of quasi-one dimensional superconductors. The energy of Josephson-coupling between wires is considered to be random, which is typical for dirty organic superconductors. We show that this randomness destroys phase coherence between the wires and T c vanishes discontinuously when the randomness reaches a critical value. The parallel and transverse components of the penetration depth are found to diverge at different critical temperatures T c (1) and T c , which correspond to pair-breaking and phase-coherence breaking. The interplay between disorder and quantum phase fluctuations results in quantum critical behavior at T = 0, manifesting itself as a superconducting-normal metal phase transition of first-order at a critical disorder strength.  相似文献   

5.
6.
在文献[1]中,我们导出了超导临界温度Tc的一个严格级数表式。本文讨论这个级数的收敛范围,以及通过解析延拓来扩展收敛范围的可能性。结论是:我们的Tc级数(指文献[1]原来的级数,或者经过延拓后的级数)在∞>λ>λ0的整个范围内,都是收敛的。这里λ0是Matsubara表象中使决定Tc的方程具有正实数解的最小的λ值。实际上,就是库仑赝势。因此,这就是说,也许除了少数非常弱耦合的超导体以外,我们的T<  相似文献   

7.
Using electron spin resonance (ESR) technique we have obtained data evidencing the existence of magnetic vortices in high-temperature superconductors at temperatures above the critical one T c. We have studied magnetic excitations in Bi2Sr2Ca2Cu3O10 single crystals above T c with the method of surface spin decoration. The surface layer of diphenylpicrylhydrazyl was used as a sensitive probe of magnetic field distortions. The temperature dependence of the ESR signal parameters has indicated that far above T c the magnetic flux of a sample is affected by the superconducting order parameter fluctuations while close to T c its changes are due to vortex-type excitations.  相似文献   

8.
To investigate the electronic states in YRuB2 and LuRuB2, we have carried out 11B NMR measurements. In the normal state, the spin-lattice relaxation rates 1/T1's in these compounds are proportional to the temperature T. 1/T1's show a small coherence peak just below the superconducting transition temperature Tc and decrease exponentially well below Tc. YRuB2 and LuRuB2 are found to be BCS superconductors with the energy gap 2Δ(0)=3.52 kBTc.  相似文献   

9.
A two-dimensional (2D) assembly of noninteracting, temperature-dependent, pre-formed Cooper pairs in chemical/thermal equilibrium with unpaired fermions is examined in a binary boson-fermion statistical model as the Bose-Einstein condensation (BEC) singularity temperature Tc is approached from above. Compared with BCS theory (which is not a BEC theory) substantially higher Tcs are obtained without any adjustable parameters, that fall roughly within the range of empirical Tcs for quasi-2D cuprate superconductors.  相似文献   

10.
A quantum mechanical model requiring only strong quantum interaction for a charged particle gas estimates the superconducting transition temperature for wide-ranging states of matter. A general equation is derived which estimates the critical temperatureT c the energy gap, and the coherence length for the classical metallic superconductors, heavy-electron superconductors, the perovskites, metallic hydrogen, and neutron stars. Estimates forT c , the coherence length, and the energy gap which are model independent for coupling mechanisms agree well with accepted values for these materials. Estimates are made for threedimensional quasi-two and quasi-one-dimensional states.  相似文献   

11.
秦猛  田东平  陶应娟 《物理学报》2008,57(9):5395-5399
采用纠缠的度量方法Negativity研究了匀强磁场下自旋为1的含杂质三粒子Heisenberg XXX链的热纠缠特性.通过计算系统的杂质位与其余部分间的两体纠缠N1-23及正常位与其余部分间的两体纠缠N12-3,发现纠缠存在的临界温度Tc的改变来自杂质参数J1的变化,随杂质参数J1的增加而增加,外界磁场B< 关键词: 热纠缠 XXX链')" href="#">Heisenberg XXX链 杂质  相似文献   

12.
The transition temperature of a superconductor depends on α2 F(ω), the spectral function of the effective interaction due to phonon exchange. We discuss how strongly the transition temperature is influenced by different frequency parts of α2 F(ω). For this purpose the functional derivative δT c /δα2 F(ω) is calculated. It is shown that all frequency regions of α2 F(ω) yield a positive contribution toT c and that the most effective range covers frequencies, slightly above 2πT c . The functional derivative is calculated numerically for several superconductors from their measured α2 F(ω)-spectra. Finally, we discuss the change in transition temperature due to the softening of α2 F(ω) which has been observed in amorphous superconductors.  相似文献   

13.
Although structurally the high temperature superconductors are quasi-two-dimensional, there is both theoretical and experimental evidence of a substantial range of temperatures in which ‘stripe’ correlations make the electronic structure locally quasi-one-dimensional. We consider an array of Josephson coupled, spin gapped one dimensional electron gases as a model of the high temperature superconductors. For temperatures above Tc, this system exhibits electron fractionalization, yielding a single particle spectral response which is sharp as a function of momentum, but broad as a function of energy. For temperatures below the spin gap but above Tc, there are enhanced one-dimensional superconducting fluctuations and pseudogap phenomena. Pair tunneling induces a crossover to three-dimensional physics as Tc is approached. Below Tc, solitons are confined in multiplets with quantum numbers which are simply related to the electron, and a coherent piece of the single particle spectral function appears. The weight of this coherent piece vanishes in the neighborhood of Tc in proportion to a positive power of the interchain superfluid density. This behavior is highly reminiscent of recent ARPES measurements on the high temperature superconductors.  相似文献   

14.
15.
曹效文 《物理学报》1985,34(5):706-708
本文提出一个非晶态非过渡金属超导体的Tc经验公式,Tc=Aλ<ω>1/2/(<ω>/ω0+(1+λ)/20),式中A=(1/5)(K1/2)。计算值和实验值,以及和Garland理论值的比较表明,Tc经验公式能很好地描述非晶态超导体的Tc值。 关键词:  相似文献   

16.
A theory of high-temperature superconductivity based on the combination of the fermion-condensation quantum phase transition and the conventional theory of superconductivity is presented. This theory describes maximum values of the superconducting gap, which can be as big as Δ1~0.1ε F , with ε F being the Fermi level. We show that the critical temperature 2T c 1. If the pseudogap exists above T c , then 2T*?Δ1 and T* is the temperature at which the pseudogap vanishes. A discontinuity in the specific heat at T c is calculated. The transition from conventional superconductors to high-T c ones as a function of the doping level is investigated. The single-particle excitations and their lineshape are also considered  相似文献   

17.
New magnetization results (for Hc) on a single crystal of Bi2212 elucidating the transition from a one-component to a two- component response across the 3D-to-2D transformation are reported. The two-component response is identified to emanate from the 2D planes and interplanar links. The magnetization response above the transformation temperature Ts and very close to Tc is found to be irreversible in contrast to the reversible behavior so far believed in. A new phase diagram for the mixed state of layered superconductors near Tc is proposed.  相似文献   

18.
We discuss the excess conductivity at nonzero frequencies in a superconductor above Tc within the Gaussian approximation. We focus the attention on the temperature range not too close to Tc: within a time-dependent Ginzburg-Landau formulation, we phenomenologically introduce a short wavelength cutoff (of the order of the inverse coherence length) in the fluctuational spectrum to suppress high momentum modes. We treat the general cases of thin wires, anisotropic thin films and anisotropic bulk samples. We obtain in all cases explicit expressions for the finite frequency fluctuational conductivity. The dc case directly follows. Close to Tc the cutoff has no effect, and the known results for Gaussian fluctuations are recovered. Above Tc, and already for ε = ln(T/T c) > 10-2, we find strong suppression of the paraconductivity as compared to the Gaussian prediction, in particular in the real part of the paraconductivity. At high ε the cutoff effects are dominant. We discuss our results in comparison with data on high-Tc superconductors. Received 19 March 2002 Published online 25 June 2002  相似文献   

19.
The hyperbolic metric of the dispersion law (the effective mass tensor components of carriers are opposite in sign) in the vicinity of the Fermi contour in high-T c superconducting cuprates in the case of repulsive interaction gives rise to a superconducting state characterized by the condensate of pairs with a large total momentum (hyperbolic pairing). The gain in the energy of the superconducting state over the normal state is due to the fact that a change in the kinetic energy of pairs (because of the negative light component of the effective mass) dominates over the change in the potential energy (corresponding to energy loss). The shift of the chemical potential upon the transition to the superconducting phase is substantial in this case. With increasing repulsive interaction, the superconducting gap δK increases and the resulting gain in energy changes to an energy loss at a certain critical value of the repulsive potential. The low temperature T c of the superconducting transition and the large value of δ K in this region of potential values are the reasons for the high value of the 2δK/T c ratio and for the developed quantum fluctuations that are observed in underdoped cuprate superconductors.  相似文献   

20.
The precursor diamagnetism is studied in a stack of Al films whose thickness is much smaller than the coherence length. The magnetization shows the (T ? Tc)?1 behavior in a finite range of temperature above Tc. At Tc, in low magnetic fields constant magnetization independent of the field strength is observed. The results are in agreement with the theories of the two-dimensional superconductor.  相似文献   

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