Materials and mechanisms of hole superconductivity

The theory of hole superconductivity proposes that there is a single mechanism of superconductivity that applies to all superconducting materials. This paper discusses several material families where superconductivity occurs and how they can be understood within this theory. Materials discussed include the elements, transition metal alloys, high T_c cuprates both hole-doped and electron-doped, MgB₂, iron pnictides and iron chalcogenides, doped semiconductors, and elements under high pressure.     

Scanning tunneling microscopy and spectroscopy on iron-pnictides

Tremendous excitement has followed the recent discovery of superconductivity up to T_c = 56 K in iron–arsenic based materials (pnictides). This discovery breaks the monopoly on high-T_c superconductivity held by copper-oxides (cuprates) for over two decades and renews hope that high-T_c superconductivity may finally be theoretically understood and widely applied.Since scanning tunneling microscopy (STM) and spectroscopy (STS) have been key tools in the investigation and understanding of both conventional and unconventional superconductivity, these techniques are also applied to the pnictides. While the field is still in its early stages, several important achievements by STM and STS have been reported on the pnictides. In this paper, we will review their contribution towards an understanding of superconductivity in this new class of materials.     

A new (Hg,V)-based 1212-type cuprate (Hg,V) Sr2(Y,Ca) Cu2Oz with Tc(onset) up to 110 K

We have successfully synthesized a new (Hg, V)-based 1212-type cuprate (Hg_1?x V _x )Sr₂(Y_1?y Ca _y )Cu₂O _z . The electrical resistance measurements showed that some of the as-synthesized materials with proper Hg/V and Y/Ca ratios exhibit weak superconductivity. Oxygen-annealing significantly improved their superconducting behavior, andT _c(onset) up to 110 K was observed. The lattice parameters of (Hg,V)-1212 were found to be in the order ofa=3.8415(1) Å andc=11.8514(6) Å. Substitu ting mercury atoms by vanadium ones results in an important increase of thea parameter and an important decrease of thec parameter compared to the known Hg-based Sr-bearing compounds. The crystal structure of (Hg, V)-1212 was refined by Rietveld refinement against X-ray powder data using the tetragonal symmetry of space groupP4/mmm. The V and Cu valence states and superconductivity in this new (Hg, V)-1212 cuprate are briefly discussed.     

Synthesis and superconductivity of a new 1222-type T1-based layered cuprate (T1, Nb) Sr2(Nd,Ce)2Cu2O z

Synthesis and superconductivity of a new 1222-type layered cuprate (Tl_1–x Nb _x ) Sr₂(Nd_1–y Ce _y )₂Cu₂O _z have been studied. The structure of this cuprate is directly related to that of Nb-1222 NbSr₂(Nd, Ce)₂Cu₂O _z with tetragonal body-center lattice. Partial substitution of Tl for Nb in Nb-1222 phase improves its superconductivity. (Tl_1–x Nb _x ) Sr₂(Nd_0.75Ce_0.25)₂Cu₂O _z samples prepared by the typical procedure exhibit superconductivity withT _c of 30–40 K. Effects of Tl and Nb on superconductivity of this cuprate are briefly discussed.     

Doping effect on the carrier scattering in iron-pnictide superconductors studied by charge transport

In order to reveal the role of “carrier doping” in the iron-based superconductors, we investigated the transport properties of the oxygen-deficient iron-arsenides LnFeAsO_1−y (Ln=La, Ce, Pr and Nd) over a wide doping range. We found that the effect of “doping” in this system is mainly on the carrier scattering rather than carrier density, quite distinct from that in high-T_c cuprates. In the case of La system with lower T_c, the low temperature resistivity is dominated by T² term and fairly large magnetoresistance is observed. On the other hand, in the Nd system with higher T_c, carriers are subject to stronger scattering showing nearly T-linear resistivity and small magnetoresistance. Such strong scattering appears intimately correlated with high-T_c superconductivity in the iron-based system.     

Effects of Zn on the grain boundary properties of La2−xSrxCu1−yZnyO4 superconductors

The properties of the grain boundaries (GBs) are of significant importance in high-T_c cuprates. Most large scale applications of cuprate superconductors involve usage of sintered compounds. The critical current density and the ability to trap high magnetic flux inside the sample depend largely on the quality of the GBs. Zn has the ability to pin vortices but it also degrades superconductivity. In this study we have investigated the effect of Zn impurity on the intergrain coupling properties in high-quality La_2−xSr_xCu_1−yZn_yO₄ sintered samples with different hole concentrations, p (≡x), over a wide range of Zn contents (y) using field-dependent AC susceptibility (ACS) measurements. The ACS results enabled us to determine the superconducting transition temperature T_c, and the temperature T_gcp, at which the randomly oriented superconducting grains become coupled as a function of hole and disorder contents. We have analyzed the behavior of the GBs from the systematic evolution of the values of T_gcp(p, y), T_c(p, y), and from the contribution to the field-dependent ACS signal coming from the intergrain shielding current. Zn suppresses both T_c and T_gcp in a similar fashion. The hole content and the carrier localization due to Zn substitution seem to have significant effect on the coupling properties of the GBs. We have discussed the possible implications of these findings in detail in this article.     

Theory of dual fermion superconductivity in hole-doped cuprates

Since the discovery of the cuprate high-temperature superconductivity in 1986, a universal phase diagram has been constructed experimentally and numerous theoretical models have been proposed. However, there remains no consensus on the underlying physics thus far. Here, we theoretically investigate the phase diagram of hole-doped cuprates based on an itinerant-localized dual fermion model, with the charge carriers doped on the oxygen sites and localized holes on the copper d _{x2 ? y2} orbitals. We analytically demonstrate that the puzzling anomalous normal state or the strange metal could simply stem from a free Fermi gas of carriers bathing in copper antiferromagnetic spin fluctuations. The short-range high-energy spin excitations also act as the “magnetic glue” of carrier Cooper pairs and induce d-wave superconductivity from the underdoped to overdoped regime, distinctly different from the conventional low-frequency magnetic fluctuation mechanism. We further sketch out the characteristic dome-shaped critical temperature T _c versus doping level. The emergence of the pseudogap is ascribed to the localization of partial carriers coupled to the local copper moments or a crossover from the strange metal to a nodal Kondo-like insulator. Our work provides a consistent theoretical framework to understand the typical phase diagram of hole-doped cuprates and paves a distinct way to the studies of both non-Fermi liquid and unconventional superconductivity in strongly correlated systems.     

Soft x-ray absorption and high-resolution powder x-ray diffraction study of superconducting CaxLa1−xBa1.75−xLa0.25+xCu3Oy system

We have studied the electronic structure of unoccupied states measured by O K-edge and Cu L-edge x-ray absorption spectroscopy (XAS), combined with crystal structure studied by high resolution powder x-ray diffraction (HRPXRD), of charge-compensated layered superconducting Ca_xLa_1−xBa_1.75−xLa_0.25+xCu₃O_y (0≤x≤0.4 and 6.4≤y≤7.3) cuprate. A detailed analysis shows that, apart from hole doping, chemical pressure on the electronically active CuO₂ plane due to the lattice mismatch with the spacer layers greatly influences the superconducting properties of this system. The results suggest chemical pressure to be the most plausible parameter to control the maximum critical temperatures (T_c^max) in different cuprate families at optimum hole density.     

A new (Hg,V)-based 1212-type cuprate (Hg,V) Sr2(Y,Ca) Cu2Oz with Tc(onset) up to 110 K

We have successfully synthesized a new (Hg, V)-based 1212-type cuprate (Hg_1?x V _x )Sr₂(Y_1?y Ca _y )Cu₂O _z . The electrical resistance measurements showed that some of the as-synthesized materials with proper Hg/V and Y/Ca ratios exhibit weak superconductivity. Oxygen-annealing significantly improved their superconducting behavior, andT _c(onset) up to 110 K was observed. The lattice parameters of (Hg,V)-1212 were found to be in the order ofa=3.8415(1) Å andc=11.8514(6) Å. Substitu ting mercury atoms by vanadium ones results in an important increase of thea parameter and an important decrease of thec parameter compared to the known Hg-based Sr-bearing compounds. The crystal structure of (Hg, V)-1212 was refined by Rietveld refinement against X-ray powder data using the tetragonal symmetry of space groupP4/mmm. The V and Cu valence states and superconductivity in this new (Hg, V)-1212 cuprate are briefly discussed.     

An ARPES view on the high-<Emphasis Type="Italic">T</Emphasis><Subscript>c</Subscript> problem: Phonons <Emphasis Type="Italic">vs.</Emphasis> spin-fluctuations

We review the search for a mediator of high-T _c superconductivity focusing on ARPES experiment. In case of HTSC cuprates, we summarize and discuss a consistent view of electronic interactions that provides natural explanation of both the origin of the pseudogap state and the mechanism for high temperature superconductivity. Within this scenario, the spin-fluctuations play a decisive role in formation of the fermionic excitation spectrum in the normal state and are sufficient to explain the high transition temperatures to the superconducting state while the pseudogap phenomenon is a consequence of a Peierls-type intrinsic instability of electronic system to formation of an incommensurate density wave. On the other hand, a similar analysis being applied to the iron pnictides reveals especially strong electron-phonon coupling that suggests important role of phonons for high-T _c superconductivity in pnictides.     

NMR and NQR studies of spin dynamics and superconductivity in High-T c oxides

The magnetic and superconducting properties in the high-T _c cuprates have been investigated over a wide hole doping range by⁶³Cu,¹⁷O and²⁰⁵Tl NMR and NQR in the lightly-doped La_2?xSr_xCuO₄ (LSCO), the heavily-doped Tl₂Ba₂CuO_6+y (TBCO) and the Zn-doped YBa₂Cu₃O₇ (YBCO₇). In low doping region, the large antiferromagnetic (AF) spin correlation around the zone boundary (q=Q) causes the Curie-Weiss behavior of⁶³(1/T ₁ T) associated with that of the staggered susceptibility χ_O(T) in LSCO. In the vicinity of the hole content whereT _c has a peak, the AF spin correlation still survives, although the magnetic coherence length ξ_M is considerably short being presumably (ξ_M/a) ～ 1. The further doping destroys progressively the AF spin correlation, which is no longer present is non-superconducting TBCO compounds. These NMR evidences signify that there is an intimate relation between the presence of the AF spin correlation and the onset of the superconductivity. The local collapse of AF spin correlation is a primary cause for the unexpected strong reduction ofT _c in case of the substitution of Zn impurities into the CuO₂ plane. The superconducting properties clarified by NMR experiments cannot be accounted for by the conventional BCS model or other isotropic s-wave models. A d-wave model is applicable in interpreting consistently most of the NMR results, if the finite density of states at the Fermi level is taken into consideration and is associated with the pair breaking effect. There are increasing evidences that the magnetic mechanism for the superconductivity is promising in high-T _c cuprates.     

Effect of 3d metal ion doping on the structure and superconductivity of (Tl0.5Pb0.5)Sr2CaCu2O7

(Tl_0.5Pb_0.5)Sr₂Ca(Cu_2−xM_x)O₇ (M=Co, Ni and Zn) have been synthesized and investigated by means of X-ray diffraction, scanning electron microscope, electrical resistivity and magnetic susceptibility measurements. X-ray diffraction patterns show that all studied samples contain the nearly single ‘1212’ phase. They crystallize in a tetragonal unit cell with a=3.8028-3.8040 Å and c=12.0748-12.1558 Å. In (Tl_0.5Pb_0.5)Sr₂Ca(Cu_2−xM_x)O₇ system (M=Co or Ni), the superconducting critical temperature T_c decreases linearly with both Co and Ni concentrations and the rate of T_c decrease is around −6.5 and −7.0 K/at%, respectively. For (Tl_0.5Pb_0.5)Sr₂Ca (Cu_2−xZn_x)O₇ system, the dependence of T_c on the Zn dopant concentration deviates from a linear behavior and the Zn substitution suppresses T_c much less (−2.5 K/at%) than the Co and Ni substitutions. The suppression in T_c in Co and Ni doped samples are attributed to the magnetic pair-breaking mechanism and the reduction in the carrier concentration. The suppression of T_c in Zn doped samples is not caused by the reduction in carrier concentration which should remain constant, but rather due to nonmagnetic pair-breaking mechanism induced by disorder as well as the filling of the local Cu d_x²−y² state due to the full d band of Zn ions.     

3-Band theory of Fe pnictide superconductors

For the Fe pnictide superconductors, band features important for superconductivity (SC) in the neighborhood of ε_F are mimicked by tight-binding bands with bases of Fe d_xz, d_yz and d_xy orbitals. In terms of this 3-band model, effective pair transfer processes originating from Coulombic integrals are shown to be largely enhanced due to the nesting between electron and hole pockets by explicitly summing up ladder diagrams. Obtained coupled gap equations lead to s^±-type SC. This framework are argued to provide a scheme for discussing important elements working for the high T_c SC, although its validity is restricted to the case where Dirac cones are away from ε_F. Experimental results on Fe pnictides are discussed in terms of the present results. The pair transfer process explicitly due to the exchange-like integral is suggested to be considerably important for the high T_c in MFeAsO_1−xF_x to occur.     

Coexistence of superconductivity and magnetic order in YBa2Cu4O8

⁵⁷Fe Mössbauer spectroscopy, magnetic susceptibility and powder x-ray-diffraction measurements were used to study superconductivity and magnetic order in YBa₂(Cu_1?xFe_x)₄O_8+δ. T_c is decreasing with x, disappearing for x>x_c≈0.04. For xc iron substitutes Cu, predominantly in the Cu(1) site exhibiting a single quadrupole Mössbauer spectrum at 90 K. For x>x_c magnetic order is observed in the Cu(2) site, T_N=380 (5) K for x=0.1 and H_eff (Cu(2), 4.2 K)=510(2) kOe. However, the most surprising discovery is that for x=0.025, for which T_c=27(2) K, the Fe in the Cu(1) site orders magnetically at T_N=30(2) K and H_eff (Cu(1), 4.2 K)=461(2) kOe. The coexistence and competition between superconductivity and magnetic order in the Cu(1) and Cu(2) sites in YBa₂Cu₄O₈ are discussed in terms of the previously observed phase diagrams for Y_1?xPr_xBa₂(Cu_1?yFe_y)₃O_z.     

Hole concentration dependence of Mn eg orbital state in bilayer manganites studied by magnetic Compton profile measurement

Magnetic Compton profiles (MCP) have been measured in the [100], [110] and [001] directions on the single crystals of La_2−2xSr_1+2xMn₂O₇ (x=0.30, 0.35 and 0.42) at 10 K. The occupation numbers in t_2g and two e_g type orbitals (x²−y² and 3z²−r²) of Mn-3d state are evaluated from the line-shape analysis of MCP's in the [001] direction by using theoretical profiles derived from the ab initio calculations for (MnO₆)⁸⁻cluster. It has been found that the e_g state is dominated by the x²−y² type orbital at every hole concentration, x, and the 3z²−r² type orbital population decreases with increasing x. From the result, the connections of e_g orbital state with the electron correlation effect, exchange interactions, lattice distortion and electronic inhomogeneity are discussed.     

Superconductivity in Sr and Co co-doped PrFeAsO

Polycrystalline Sr and Co co-doped superconducting oxypnictides Pr_1−ySr_yFe_1−xCo_xAsO were synthesized by solid state reaction method, and superconductivity was investigated by measuring resistivity, magnetic susceptibility and thermopower. In the PrFe_1−xCo_xAsO samples with only Co doping, T_c reaches a maximum of 16 K at x=0.075-0.1, and the variation of T_c with Co content (x) is dome-shaped, consistent with the other Co-doped 1 1 1 1 phase systems such as LaFe_1−xCo_xAsO and SmFe_1−xCo_xAsO. In the Co and Sr co-doped Pr_0.8Sr_0.2Fe_1−xCo_xAsO system, T_c reaches a maximum of about 16 K at x=0.15, and the T_c‐x dome shifts to higher Co content in the phase diagram of T_c versus Co content (x). Our result indicates that Sr doping effectively induces hole-type charge carriers in the PrFeAsO system and the disorder caused by Sr and Co dopants has little effect on T_c. The thermopower data confirms that Sr dopant is hole-type and Co dopant is electron-type. The shift of T_c‐x dome to higher Co content caused by Sr doping implies that the Sr dopant can compensate for the Co doping effect. The results suggest that there could be a universal dependence of T_c on charge carrier density in both hole-type and electron-type oxypnictide superconductors.     

Suppression of Tc by Zn impurity in the electron-type LaFe0.925−y Co0.075ZnyAsO system

The effect of non-magnetic Zn impurity on superconductivity in electron-type pnictide superconductor LaFe_0.925−yCo_0.075Zn_yAsO is studied systematically. The optimally doped LaFe_0.925Co_0.075AsO without Zn impurity exhibits superconductivity at T_c^mid of 13.2 K, where T_c^mid is defiend as the mid-point in the resistive transition. In the presence of Zn impurity, the superconducting transition temperature, T_c^mid, is severely suppressed. The result is consistent with the theoretic prediction on the effect of non-magnetic impurity in the scenario of s_± pairing, but it is in sharp contrast to the previous report on the effect of Zn impurity in the F-doped systems. The possible interpretation of the different effects of Zn impurity on superconductivity in different systems is discussed.     

Magnetism on Mg1−xZnxCyNi3

The magnetic phase diagram for Mg_1−xZn_xC_yNi₃ has been tentatively constructed based on magnetization and muon spin relaxation (μSR) measurements. The superconducting phase was observed to fade as x (y) increases (decreases). The low y samples show early stages of long-range ferromagnetism, or complete long-range ferromagnetism. In the phase diagram, the ferromagnetic phase exists in addition to the superconducting phase, suggesting that there is some correlation between superconductivity and ferromagnetism, even though the coexistence of ferromagnetism and superconductivity is not observed from the μSR measurements down to 20 mK for the superconducting sample (T_c=2.5 K, (x, y)=(0, 0.9)).     

Oxygen stoichiometry,doping and the role of finite Cu-oxide features in high Tc superconductors

We consider the effect of the doping parameter x and the oxygen nonstoichiometry parameter y (in La_2−xBa_xCuO_4+y and in YBa₂Cu₃O_7−y) on the short range structure in high T_c cuprates and argue that both x and y tend to create finite Cu-O structures (FCuOS) e.g., chainlets, in these materials. This is shown to be consistent with the details of their structures, optical properties, Hall effect data etc. The electronic excitations between the confined states of these FCuOS provide a plausible pairing mechanism for superconductivity in the 2-D CuO planes present in these materials.     

Effects of excess oxygen content on the hole-carrying CuO2-layers in Tl2(Ba1−xSrx)2Ca2Cu3Oy superconducting oxides

The effects of oxygen doping on the hole-carrying CuO₂-layers in Tl₂(Ba_1−xSr_x)₂Ca₂Cu₃O_y were studied by combined chemical and valence analysis, T_c measurements and neutron diffraction. The highest T_c is characterized by an optimal excess oxygen content, Δy, dichotomizing the under- and over-doped regions for each Sr concentration. While the average Tl valence is close to 3.0 and independent of Δy, the average Cu valence shows a linear dependence with Δy. An increase of the flatness of the CuO₂ plane, characterized by the O(2)-Cu(2)-O(2) bond angle of ∼176°, was observed at the optimal Δy.