Tunneling spectroscopy of novel layered superconductors: MgB2, Li0.48(THF)xHfNCl and related substances (original) (raw)
Related papers
2012
The discovery of novel high-T c superconductivity in MgB 2 (T c = 39.5 K) and Li0.48(THF)yHfNCl (Tc = 25.5 K) initiated substantial progress in the field of superconductivity physics and its applications despite the fact that competing high-Tc cuprates remain the world leaders in almost all practically important superconducting parameters. This article describes electron tunneling and point-contact experimental studies of the indicated two materials and related substances, being crucial to elucidate the character of the quasiparticle energy spectrum both in superconducting and normal state. The account is based mostly on our own experiments, although works carried out in other laboratories are taken into account as well.
Tunneling spectroscopy of MgB2 and Li0.5(THF)yHfNCl
Physica B: Condensed Matter, 2003
Break-junction tunneling spectra have been measured on MgB 2 and Li 0.5 (THF) y HfNCl to investigate the nature of the superconducting gap. The observed largest gap values at 4 K are 2D ¼ 18220 and 11-12 meV for MgB 2 (T c ¼ 39 K) and Li 0.5 (THF) y HfNCl (26 K), respectively. These values lead to the similar ratio 2D=k B T c ¼ 526; which is extremely large in comparison with that of the conventional strong-coupling superconductors. r
European Physical Journal B, 2010
Tunneling measurements have been carried out on layered superconductors of the β(SmSI)-type -Li0.48(THF)xHfNCl (THF = C4H8O) and HfNCl0.7 -by means of break-junction and scanning tunneling spectroscopy. Break-junction technique reveals Bardeen-Cooper-Schrieffer (BCS) -like gap structures with typical gap values of 2Δ(4.2 K) = 11−12 meV for Li0.48(THF)xHfNCl with the highest Tc = 25.5 K. Some of our measurements revealed multiple gaps and dip-hump structures, the largest gap 2Δ (4.2 K) ≈ 17-20 meV closing at Tc. This was shown both by break-junction and scanning-tunneling spectroscopy. From these experiments it stems that the highest obtained gap ratio 2Δ/kBTc ∼ 8 substantially exceeds the BCS weak-coupling limiting values: ≈3.5 and ≈4.3 for s-wave and d-wave order parameter symmetry, respectively. Such large 2Δ/kBTc ratios are rather unusual for conventional superconductors but quite common to high-Tc cuprates, as well as to organic superconductors. Our studies allowed to collect much more evidence concerning the huge pairing energy in those materials and to investigate in detail the complexity of their superconducting gap spectra. An origin of the observed phenomena still remains to be clarified.
Physica C-superconductivity and Its Applications, 2005
Controversial tunnel and point-contact experimental data on superconducting gap values in MgB2 are analyzed and interpreted in terms of the spatial inhomogeneity and the proximity effect appropriate to this layered compound. Existing specific heat measurements are explained in the same way. Break-junction tunnel studies of polycrystalline MgB2 samples are carried out and a clear-cut multiplicity of gaps is observed, although the shapes of the I–V characteristics vary substantially.
Low temperature physics, 2019
The systematic study of the nanoscale local electronic states on the MgB 2 surface was performed using the low-temperature scanning tunnel microscopy/spectroscopy (STM/STS). The STM topography shows the atomic image of the hexagonal lattice with the constant parameter a′ = 0.31 nm, which is identified as mainly the Mg site occupancy. The temperature-dependent STS measurements were analyzed assuming the existence of two energy gaps. As a result, the fitting gap amplitudes ∆ fit 10.2 meV and 4.8 meV were found at T = 4.9 K. The scanned conductance (dI / dV) maps in the area of 4×2 nm 2 show homogenous distributions of the gaps associated with the π-band. In addition, the conductance peaks at zero-bias voltage were observed through defined lines with lengths about ∼0.8 nm, which is much smaller than the superconducting coherence length ξ ab ~ 40 nm of MgB 2. The form of the zero-bias peaks looks like that in the case of the Andreev-Saint-James reflection at the tip-sample contact.
Two-Band Superconductivity in MgB2
Physical Review Letters, 2002
The study of the anisotropic superconductor MgB2 using a combination of scanning tunneling microscopy and spectroscopy reveals two distinct energy gaps at ∆1=2.3 meV and ∆2=7.1 meV. Different spectral weights of the partial superconducting density of states (PDOS) are a reflection of different tunneling directions in this multi-band system. Our experimental observations are consistent with the existence of two-band superconductivity in the presence of interband superconducting pair interaction and quasiparticle scattering. Temperature evolution of the tunneling spectra follows the BCS scenario [1] with both gaps vanishing at the bulk Tc. Indeed, the study of tunneling junctions exhibiting only the small gap (c-axis tunneling) clearly and reproducibly show that this gap persists up to the bulk Tc. The data confirm the importance of Fermi-surface sheet dependent superconductivity in MgB2 proposed in the multigap model by Liu et al. .
Possible high-temperature superconductivity in hole-doped MgB 2 C 2
Europhysics Letters (EPL), 2003
Recent computational studies confirmed by experiment have established the occurrence of superconducting temperatures, Tc, near 200 K when the pressure is close to 200 GPa in the compound H 3 S. Motivated by these findings we investigate in this work the possibility of discovering high-temperature superconductivity in the material H 3 F. We performed linearized augmented plane wave(LAPW) calculations followed by the determination of the angular momentum components of the density of states, the scattering phase shifts at the Fermi level and the electron-ion matrix element known as the Hopfield parameter. Our calculated Hopfield parameters are much larger than those found in H 3 S suggesting that they may lead to large electron-phonon coupling constant and hence a large Tc similar or even larger than that of H 3 S. However, calculations of elastic constants are inconclusive regarding the stability of this material.
Tunneling spectroscopy: A probe for high-Tc superconductivity
Microelectronics Journal, 2008
After the pioneering work of Giaever [Phys. Rev. Lett. 5 (1960) 147. ] and Shapiro in the 1960s, tunneling spectroscopy has become an important method for studying different properties of superconductors, such as the superconducting gap and the density of states. At the interface with a superconductor there is a variety of tunneling processes depending on barrier properties and the type of materials in both sides of the junction. Among other phenomena, we have single electron tunneling, cooper pairs (Josephson) tunneling [B.D. Josephson, Phys. Rev. Lett. 1 (1962) 251. [17]] and mixed (Andreev-Saint-James reflections [A.F. Andreev, Zh. Eksp. Teor. Fiz. 46 (1964) 1823, Saint-James D., J. Phys 25 (1964) 899]) tunneling.
Tunneling Spectroscopy in Small Grains of SuperconductingMgB2
Physical Review Letters, 2001
We report on tunneling spectroscopy experiments in small grains of the new binary intermetallic superconductor MgB2. Experiments have been performed at 2.5 K using a low temperature scanning tunneling microscope. Good fit to the BCS model is obtained, with a gap value of 2 meV. In the framework of this model, this value should correspond to a surface critical temperature of 13.2 K. No evidence of gap anisotropy has been found.
The origin of the anomalous superconducting properties of MgB2
Nature, 2002
Magnesium diboride 1 differs from ordinary metallic superconductors in several important ways, including the failure of conventional models 2 to predict accurately its unusually high transition temperature, the effects of isotope substitution on the critical transition temperature, and its anomalous specific heat . A detailed examination of the energy associated with the formation of charge-carrying pairs, referred to as the 'superconducting energy gap', should clarify why MgB 2 is different. Some early experimental studies have indicated that MgB 2 has multiple gaps 3-9 , but past theoretical studies 10-16 have not explained from first principles the origin of these gaps and their effects. Here we report an ab initio calculation of the superconducting gaps in MgB 2 and their effects on measurable quantities. An important feature is that the electronic states dominated by orbitals in the boron plane couple strongly to specific phonon modes, making pair formation favourable. This explains the high transition temperature, the anomalous structure in the specific heat, and the existence of multiple gaps in this material. Our analysis suggests comparable or higher transition temperatures may result in layered materials based on B, C and N with partially filled planar orbitals.