Two-band Effects Research Articles

Applicability of two-band Ginzburg–Landau theory in 112-type iron-based superconductors

Experiments show that the 112-type iron-based superconductor Ca1−xLaxFeAs2 (x ∼ 0.2) has two-band superconducting properties. Previous reports have shown that the two-band and spin paramagnetic effects are responsible for the inter-plane (H//c) and in-plane (H//ab) upper critical field behaviors of the 112-type iron-based superconductor Ca1−xLaxFeAs2 (x ∼ 0.2) respectively. With the effective mass anisotropy, the present paper suggests that the two-band Ginzburg–Landau theory may be applicable to study the magnetic properties of 112-type iron-based superconductors in arbitrary direction. The theoretical calculations on the upper critical fields fit well with the experimental results over a wide range of temperature. In addition, the upper critical field increases with the increase of the directional angle, which is similar to the result for MgB2. By comparing the upper critical fields of Ca1−xLaxFeAs2 and Ca1−xLaxFe0.98Co0.02As2, we also analyze how impurities affect the physical properties of 112-type iron-based superconductors.

Correlation between non-Fermi-liquid behavior and superconductivity in (Ca, La)(Fe,Co) As2 iron arsenides: A high-pressure study

Non-Fermi-liquid (NFL) phenomena associated with correlation effects have been widely observed in the phase diagrams of unconventional superconducting families. Exploration of the correlation between the normal state NFL, regardless of its microscopic origins, and the superconductivity has been argued as a key to unveiling the mystery of the high-Tc pairing mechanism. Here we systematically investigate the pressure-dependent in-plane resistivity and Hall coefficient (RH ) of a high-quality 112-type Fe-based superconductor Ca1-xLaxFe1-yCoyAs2 (x = 0.2,y = 0.02). With increasing pressure, the normal-state resistivity of the studied sample exhibits a pronounced crossover from non-Fermi-liquid to Fermi-liquid behaviors. Accompanied with this crossover, Tc is gradually suppressed. In parallel, the extremum in the Hall coefficient RH (T ) curve, possibly due to anisotropic scattering induced by spin fluctuations, is also gradually suppressed. The symbiosis of NFL and superconductivity implies that these two phenomena are intimately related. Further study on the pressure-dependent upper critical field reveals that the two-band effects are also gradually weakened with increasing pressure and reduced to the one-band Werthamer-Helfand-Hohenberg limit in the low-Tc regime. Overall, our paper supports the picture that NFL, multigap, and extreme RH (T ) are all of the same magnetic origin, i.e., the spin fluctuations in the 112 iron arsenide superconductors.

Two-band and pauli-limiting effects on the upper critical field of 112-type iron pnictide superconductors

The temperature dependence of upper critical field μ0Hc2 of Ca0.83La0.17FeAs2 and Ca0.8La0.2Fe0.98Co0.02As2 single crystals are investigated by measuring the resistivity for the inter-plane (H//c) and in-plane (H//ab) directions in magnetic fields up to 60 T. It is found that μ0Hc2(T) of both crystals for H//c presents a sublinear temperature dependence with decreasing temperature, whereas the curve of μ0Hc2(T) for H//ab has a convex curvature and gradually tends to saturate at low temperatures. μ0Hc2(T) in both crystals deviates from the conventional Werthamer-Helfand-Hohenberg (WHH) theoretical model without considering spin paramagnetic effect for H//c and H//ab directions. Detailed analyses show that the behavior of μ0Hc2(T) in 112-type Iron-based superconductors (IBSs) is similar to that of most IBSs. Two-band model is required to fully reproduce the behavior of μ0Hc2(T) for H//c, while the effect of spin paramagnetic effect is responsible for the behavior of μ0Hc2(T) for H//ab.

Testing V3Si for two-band superconductivity

Is V3Si a two-band or a single-band superconductor? Everyone who searches the literature for this question will find conflicting answers, for V3Si was claimed to be a perfect example of two-band and claimed to be a perfect example of single-band superconductivity. In this paper we intend to clarify the situation by presenting new experimental facts acquired from the magnetic properties of a V3Si single crystal. We probe for field-dependent two-band effects by analyzing the reversible magnetization at different temperatures, and we probe for temperature-dependent two-band effects by analyzing the superfluid density obtained by two different methods at different magnetic fields. All our results are reliably described within the single-band models and thus support the single-band scenario for V3Si but do not completely rule out the presence of a very small second gap.

Carrier transport in magnesium diboride: Role of nano-inclusions

Anisotropic-gap and two-band effects smear out the superconducting transition (Tc) in literature reported thermal conductivity of MgB2, where large electronic contributions also suppress anomaly-manifestation in their negligible phononic-parts. Present thermal transport results on scarcely explored specimens featuring nano-inclusions exhibit a small but clear Tc-signature, traced to relatively appreciable phononic conduction, and its dominant electronic-scattering. The self-formed MgO as extended defects strongly scatter the charge carriers and minutely the phonons with their longer-mean-free-path near Tc. Conversely, near room temperature, the shorter-dominant-wavelength phonon's transport is hugely affected by these nanoparticles, undergoing ballistic to diffusive crossover and eventually entering the Ioffe-Regel mobility threshold regime.

Coexistence of the antiferromagnetic and superconductingorder and its effect on spin dynamics in electron-doped high-Tc cuprates

In the framework of the slave-boson approach to thet–t′–t′′–J model, it is found that for electron-doped high-Tc cuprates, the staggered antiferromagnetic (AF) order coexists with the superconducting(SC) order in a wide doping level ranged from underdoped to nearly optimally doped at themean-field level. In the coexisting phase, it is revealed that the spin responseis commensurate in a substantial frequency range below a crossover frequencyωc forall dopings considered, and it switches to the incommensurate structure when the frequency is higherthan ωc. This result is in agreement with the experimental measurements. Comparison ofthe spin response between the coexisting phase and the pure SC phase with adx2−y2-wave pairing plus a higher harmonics term (DP+HH) suggests that the inclusion of the two-band effect is important to consistently account forboth the dispersion of the spin response and the non-monotonic gap behavior in theelectron-doped cuprates.

Two-band effect on the temperature and the angle dependences of the ratio of the surface to the bulk superconductivity in MgB2

We investigated the ratio of the surface superconducting field (Hc3) to the bulk superconducting field (Hc2) in MgB2 single crystals at different temperatures and at different angles between the field and the c-axis of the crystal by using electrical transport measurements. The temperature dependence of the ratio Hc3/Hc2 at different angles was qualitatively well described by a recent theory based on the two-gap nature of MgB2. The angular dependence of the ratio shows that it can be both enhanced and reduced, which can be explained by the two-band theory. If slight deviations of Hc3/Hc2 from the value for the half-infinite geometry used in the theory are to be accommodated, effects due to the finite geometry and to the orientation of the applied field relative to the crystal plane need to be considered.

Density-wave and antiferromagnetic states of fermionic atoms in optical lattices

We study two-band effects on ultracold fermionic atoms in optical lattices by means of dynamical mean-field theory. We find that at half filling an atomic-density-wave (ADW) state emerges owing to the two-band effects in the attractive interaction region, while an antiferromagnetic state appears in the repulsive interaction region. As the orbital splitting is increased, quantum phase transitions from the ADW state to the superfluid state and from the antiferromagnetic state to the metallic state occur in the corresponding regions. By systematically changing the orbital splitting and the interaction, we obtain the phase diagram at half filling. The results are discussed using the effective boson model derived for strong attractive interaction.

Radiation damaged MgB2: a comparison with A15 superconductors

In the present paper we discuss in detail experiments of radiation damage and their effects on MgB2 properties (Tc, resistivity, critical field, gap), making constant reference to similar experiments performed in the seventies and eighties on A15 superconductors (Nb3Sn, V3Si). We will show that the overall behaviour of MgB2 and A15s is surprisingly similar, pointing out that similar mechanisms (smearing of density of states) plays a role at high radiation doses, whereas, at low radiation, two-band effects mainly determine the MgB2 behaviuor.

Supersymmetry and the Superconductor-Insulator Transition

We present a theory of supersymmetric superconductivity and discuss its physical properties. We define the supercharges Q and Q † satisfying QψBCS = Q † ψBCS = 0 for the BardeenCooper-Schrieffer state ψBCS. They possess the property expressed by Q 2 =( Q † ) 2 =0 , and ψBCS is the ground state of the supersymmetric Hamiltonian H = E(QQ † + Q † Q )f or E> 0. The superpartners ψg and ψBCS are shown to be degenerate. Here ψg denotes a fermionic state within the superconducting gap that exhibits a zero-energy peak in the density of states. A supersymmetric model of superconductivity with two bands is presented. On the basis of this model we argue that the system of interest goes into a superconducting state from an insulator if an attractive interaction acts between states in the two bands. There are many unusual properties of this model due to an unconventional gap equation stemming from the two-band effect. The model exhibits an unconventional insulator-superconductor first-order phase transition. In the ground state, a first-order transition occurs at the supersymmetric point. We show that certain universal relations in the BCS theory, such as that involving the ratio ∆(0)/kBTc, do not hold in the present model.

Two-Band Effect on the Superconducting Fluctuating Diamagnetism inMgB2

The field dependence of the magnetization above the transition temperature Tc in MgB₂ is shown to evidence a diamagnetic contribution consistent with superconducting fluctuations reflecting both the σ and π bands. In particular, the upturn field Hup in the magnetization curve, related to the incipient effect of the magnetic field in quenching the fluctuating pairs, displays a double structure, in correspondence to two correlation lengths. The experimental findings are satisfactorily described by the extension to the diamagnetism of a recent theory for paraconductivity, in the framework of a zero-dimensional model for the fluctuating superconducting droplets above Tc.

Are Th-doped [formula omitted] and [formula omitted] two-band superconductors?

The superconducting sesquicarbides R 2 C 3 have noncentrosymmetric point group symmetry T d . Spin–orbit coupling lifts the spin degeneracy of electronic bands in the most of the Brillouin zone. Nevertheless, due to high symmetry, there are a few directions along which the Fermi surfaces must touch. This leads to two-band effects in the superconducting state.

Publisher's Note: Two-band effects in the angular dependence ofHc2ofMgB2single crystals [Phys. Rev. B70, 132503 (2004)

Publisher's Note: Two-band effects in the angular dependence of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi>H</mml:mi><mml:mrow><mml:mi>c</mml:mi><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:math>of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi mathvariant="normal">Mg</mml:mi><mml:msub><mml:mi mathvariant="normal">B</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:math>single crystals [Phys. Rev. B<b>70</b>, 132503 (2004)

Two-band Effects in the Critical Fields of MgB2

The influence of the two-gap/two-band superconductivity on the anisotropy properties of MgB2 is discussed. It is shown that due to the different anisotropies and different field sensitivities of both bands the lower critical field Hc1 and the upper critical one Hc2 reveal very different and temperature-dependent anisotropies. The deviations of the angular dependence of Hc2(Θ) from the classical form is discussed in the framework of the two-band model.

Two-band effects in the angular dependence ofHc2ofMgB2single crystals

The angular dependence of the upper critical field Hc2 of MgB2 single crystals is studied at various temperatures by means of specific heat and transport measurements in magnetic fields up to 17 T. Clear deviations from Ginzburg-Landau behavior are observed at all temperatures and are explained by two-band effects. The angular- and temperature dependence of the deviations are in qualitative agreement with theoretical predictions based on band-structure calculations. Quantitative agreement is obtained with an interband coupling slightly stronger than the calculated, enabling band-structure anisotropies and interband coupling strength to be experimentally estimated. This provides a new pathway to the study of disorder and doping effects in MgB2.

Two-band effects in the transport properties of MgB2

We present resistivity and thermal conductivity measurements on bulk samples, prepared either by a standard method or by a one-step technique. The latter samples, due to their high density and purity, show residual resistivity values as low as 0.5 μΩ cm and thermal conductivity values as high as 215 W mK−1, higher than the single-crystal values. The thermal and electrical data of all the samples are analysed in the framework of the Bloch–Gruneisen equation, giving reliable parameter values. In particular, the temperature resistivity coefficient, obtained both from resistivity and thermal conductivity, comes out ten times larger in the dirty samples than in the clean samples. This result supports the hypothesis of Mazin et al (2002 Preprint cond-mat/0204013) that π and σ bands conduct in parallel, with prevailing π conduction in clean samples and σ conduction in dirty samples.

Superconductivity indications of the two-chain Hubbard model due to the two-band effect

Corroborating our assertion worked out in preceding papers that the two-chain Hubbard model has a superconducting (SC) parameter region, we report new results obtained by the Lanczos diagonalization method. The SC region indicated by SC pair correlation functions show the tendency to converge to a considerable definite region in the parameter space with increase of the system size. A certain SC correlation function reveals that the SC order parameters pertinent to the two bands have different signs. The SC pair operators at two rungs keep strong spatial correlation in our SC region in striking contrast to the non-SC region. Calculation of correlation exponent K ϱ allows us to get an SC parameter region which proves to be in a qualitative agreement with the above-mentioned one.

Superconducting transition of the two-chain Hubbard model indicated by diagonalization calculations

In the two-chain Hubbard system the on-site Coulumb interaction at each site generates a pair-transfer interaction between the two bands consisting of bonding and antibonding orbitals for the two sites located on the opposite chains and linked by the transverse transfer energy. It is shown to be as strong as the intraband repulsive interaction generated in the same way so that it can give rise to superconductivity due to the two-band effect. Verifying this reasoning, exact-and also variational-diagonalization calculations clearly indicate that a superconducting state arises in this system in an appropriate region of band and filling parameters for a wide range of Coulomb strengths. A scaling analysis with systems of electron number equal to 6, 10 and 14 supports the assertion that the superconductivity survives in the limit of infinite size. The essence of this mechanism looks relevant to a wide variety of systems including D[M(dmit) 2] 2 superconductors and high- T c oxides.

Nernst effect in epitaxial Nd 1.85Ce .15CuO 4 films

We report measurements of thermally-driven flux motion (Nernst effect) in epitaxial Nd 1.85Ce .15CuO 4 films. It is found that, because of its relatively small H C2 (6 Tesla) compared with other high T c materials such as YBCO and BSCCO, the Nernst coefficient in the mixed state of Nd 1.85Ce .15CuO 4 depends much more strongly on sample temperature and applied magnetic field. The flux-line transport entropy, extracted from Nernst effect and resistivity measurements, has been found to qualitatively agree with the Maki's expression. As the applied magnetic field exceeds H C2, however, the observed normal-state Nernst effect is much larger than expected. This unusual behavior may be a two-band effect.

Theoretical investigation of the longitudinal Hall effect in SrAs3.

The resistivity and Hall coefficients of ${\mathrm{SrAs}}_{3}$ are calculated for T=0 K using different model band structures. The magnitude of the experimentally observed low-temperature values for the first-order longitudinal Hall coefficients (LHE) cannot be reproduced within a single-band model based on a realistic Fermi surface with monoclinic symmetry. An ellipsoidal two-band model, however, yields a quantitative description of the measured LHE. This result strongly supports our earlier assertion that the LHE is essentially a two-band effect.