arXiv subjects
Publications and source records attributed to I. I. Mazin.
Experiments that have been interpreted as providing evidence that the pseudogap phase in cuprates is an electronic nematic are discussed from the point of view of lattice structure. We conclude that existing experiments are not sufficient to prove a nematic origin of the pseudogap in underdoped cuprates.
Hanaguri et al (Science, 8th of January 2010, p. 181) have reported STM measurements on an Fe-based superconductor Fe(Se,Te). After Fourier-transforming their data they found three sets of sharp peaks in the reciprocal space. They interpreted one as a Bragg peak (an umklapp scattering) and two as resulting from quasiparticle interference (QPI) We point out that at least one of these two peaks is also a Bragg peak, corresponding to another reciprocal lattice vector, and therefore is not a QPI peak, and the third peak has similar structure, too sharp for a QPI peak in a nodeless superconductor with an isotropic gap, proposed by Hanaguri et al. We argue that this peak must also be a Bragg peak related to a surface reconstruction doubling the unit cell. We also calculate the QPI pattern, assuming an isotropic gap, and find that indeed it is quite different from the experiment. Finally, we speculate about possible origin of the magnetic field dependence of the Bragg peaks in this compound.
Chuang et al (Reports, 8th of January 2010, p. 181)1 report quasiparticle interference (QPI) imaging that shows pronounced C2 asymmetry. They interpreted this result as indication of an electronic nematic phase with an electron band dispersive only along the b-axis of this orthorhombic material. We argue that this asymmetry is consistent with the underlying long range magnetic order and that LDA electronic structure provides a better description of the QPI images than the 1D band structure conjectured by Chuang et al.
One of the most popular scenarios for the superconductivity in Fe-based superconductors (FeBSC) posits that the bosons responsible for electronic pairing are spin-fluctuations with a wave vector spanning the hole Fermi surfaces (FSs) near $Γ$ and the electron FSs near M points. So far all FeBSC for which neutron data are available do demonstrate such excitations, and the band structure calculations so far were finding quasi-nested FSs in all FeBSC, providing for a peak in the spin susceptibility at the desired wave vectors. However, the newest addition to the family, Sr$_{2}$VO$_{3}$FeAs, has been calculated to have a very complex FS with no visible quasi-nesting features. It was argued therefore that this material does not fall under the existing paradigm and calls for revisiting our current ideas about what is the likely cause of superconductivity in FeBSC. In this paper, I show that the visible complexity of the FS is entirely due to the V-derived electronic states. Assuming that superconductivity in Sr$_{2}$VO$_{3}$FeAs, as in the other FeBSC, originates in the FeAs layers, and the superconducting electrons are sensitive to the susceptibility of the FeAs electronic subsystem, I recalculate the bare susceptibility, weighting the electronic states with their Fe character, and obtain a susceptibility that fully supports the existing quasi-nesting model.
BiOCu_{1-y}S was recently reported to superconduct at T=5.8 K at y~0.15. Band structure calculations indicate that stoichiometric BiOCuS is a band insulator. In this paper, I show that hole-doped (whether in the virtual crystal approximation or with actual Cu vacancies) BiOCuS is on the verge of a ferromagnetic instability (cf. Pd metal), and therefore conventional superconductivity with Tc~6 K is quite unlikely. Presumably, hole-doped BiOCuS is another example of superconductivity mediated by spin-fluctuations
Using first principles calculations, we analyze structural and magnetic trends as a function of charge doping and pressure in BaFe$_2$As$_2$, and compare to experimentally established facts. We find that density functional theory, while accurately reproducing the structural and magnetic ordering at ambient pressure, fails to reproduce some structural trends as pressure is increased. Most notably, the Fe-As bondlength which is a gauge of the magnitude of the magnetic moment, $μ$, is rigid in experiment, but soft in calculation, indicating residual local Coulomb interactions. By calculating the magnitude of the magnetic ordering energy, we show that the disruption of magnetic order as a function of pressure or doping can be qualitatively reproduced, but that in calculation, it is achieved through diminishment of $|μ|$, and therefore likely does not reflect the same physics as detected in experiment. We also find that the strength of the stripe order as a function of doping is strongly site-dependent: magnetism decreases monotonically with the number of electrons doped at the Fe site, but increases monotonically with the number of electrons doped at the Ba site. Intra-planar magnetic ordering energy (the difference between checkerboard and stripe orderings) and interplanar coupling both follow a similar trend. We also investigate the evolution of the orthorhombic distortion, $e=(a-b)/(a+b),$ as a function of $μ$, and find that in the regime where experiment finds a linear relationship, our calculations are impossible to converge, indicating that in density functional theory, the transition is first order, signalling anomalously large higher order terms in the Landau functional.
TThe parent compounds of iron-arsenide superconductors, $A$Fe$_{2}$As$_{2}$ ($A$=Ca, Sr, Ba), undergo a tetragonal to orthorhombic structural transition at a temperature $T_{\mathrm{TO}}$ in the range 135 to 205K depending on the alkaline earth element. Below $T_{\mathrm{TO}}$ the free standing crystals split into equally populated structural domains, which mask intrinsic, in-plane, anisotropic properties of the materials. Here we demonstrate a way of mechanically detwinning CaFe$_{2}$As$_{2}$ and BaFe$_{2}$As$_{2}$. The detwinning is nearly complete, as demonstrated by polarized light imaging and synchrotron $X$-ray measurements, and reversible, with twin pattern restored after strain release. Electrical resistivity measurements in the twinned and detwinned states show that resistivity, $ρ$, decreases along the orthorhombic $a_{o}$-axis but increases along the orthorhombic $b_{o}$-axis in both compounds. Immediately below $T_{\mathrm{TO}}$ the ratio $ρ_{bo}/ ρ_{ao}$ = 1.2 and 1.5 for Ca and Ba compounds, respectively. Contrary to CaFe$_{2}$As$_{2}$, BaFe$_{2}$As$_{2}$ reveals an anisotropy in the nominally tetragonal phase, suggesting that either fluctuations play a larger role above $T_{\mathrm{TO}}$ in BaFe$_{2}$As$_{2}$ than in CaFe$_{2}$As$_{2}$, or that there is a higher temperature crossover or phase transition.
I point out that the observed discrepancy between the measured and calculated Drude weights in LaFePO may be caused not only by mass renormalization, but also by a DFT inaccuracy in determining the relative positions of the hole and electron band.
We report a systematic investigation, together with a theoretical analysis, of the resistivity and Hall effect in single crystals of Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$, over a wide doping range. We find a surprisingly great disparity between the relaxation rates of the holes and the electrons, in excess of an order of magnitude in the low-doping, low-temperature regime. The ratio of the electron to hole mobilities diminishes with temperature and doping (away from the magnetically ordered state) and becomes more conventional. We also find a straightforward explanation of the large asymmetry (compared to cuprates) of the superconducting dome: in the underdoped regime the decisive factor is the competition between AF and superconductivity (SC), while in the overdoped regime the main role is played by degradation of the nesting that weakens the pairing interaction. Our results indicate that spin-fluctuations due to interband electron-hole scattering play a crucial role not only in the superconducting pairing, but also in the normal transport.
We investigate the charge density wave (CDW) instability in single and double layers, as well as in the bulk 2H-NbSe$_{2}$. We demonstrate that the density functional theory correctly describes the metallic CDW state in the bulk 2H-NbSe$_{2}$. We predict that both mono- and bilayer NbSe$_{2}$ undergo a CDW instability. However, while in the bulk the instability occurs at a momentum $\mathbf{q}_{CDW}\approx{2/3}\mathbf{ΓM}$, in free-standing layers it occurs at $\mathbf{q}_{CDW}\approx{1/2}\mathbf{ΓM}$. Furthermore, while in the bulk the CDW leads to a metallic state, in a monolayer the ground state becomes semimetallic, in agreement with recent experimental data. We elucidate the key role that an enhancement of the electron-phonon matrix element at $\mathbf{q}\approx\mathbf{q}_{CDW}$ plays in forming the CDW ground state.
We investigate the electronic structure of EuFe$_{2}$As$_{2}$ using optical spectroscopy and first-principles calculations. At low temperature we observe the evolution of \textit{two} gap-like features, one having a BCS mean-field behavior and another with strongly non-BCS behavior. Using band structure calculations, we identify the former with a spin-Peierls-like partial gap in $d_{yz}$ bands, and the latter with the transition across the large exchange gap in $d_{xz}/d_{xy}$ bands. Our results demonstrate that the antiferromagnetism in the ferropnictides is neither fully local nor fully itinerant, but contains elements of both.
Through a systematic high resolution angle-resolved photoemission study of the iron pnictide compounds (Ba,Sr)Fe$_2$As$_2$, we show that the electronic structures of these compounds are significantly reconstructed across the spin density wave ordering, which cannot be described by a simple folding scenario of conventional density wave ordering. Moreover, we find that LDA calculations with an incorporated suppressed magnetic moment of 0.5$μ_{\tiny{\textrm{B}}}$ can match well the details in the reconstructed electronic structure, suggesting that the nature of magnetism in the pnictides is more itinerant than local, while the origin of suppressed magnetic moment remains an important issue for future investigations.
We report the electronic and magnetic behaviour of the frustrated triangular metallic antiferromagnet 2H-AgNiO2 in high magnetic fields (54 T) using thermodynamic and transport measurements. Here localized d electrons are arranged on an antiferromagnetic triangular lattice nested inside a honeycomb lattice with itinerant d electrons. When the magnetic field is along the easy axis we observe a cascade of field-induced transitions, attributed to the competition between easy-axis anisotropy, geometrical frustration and coupling of the localized and itinerant system. The quantum oscillations data suggest that the Fermi surface is reconstructed by the magnetic order but in high fields magnetic breakdown orbits are possible. The itinerant electrons are extremely sensitive to scattering by spin fluctuations and a significant mass enhancement (~ 3) is found.
The theory of Andreev conductance is formulated for junctions involving normal metals (N) and multiband superconductors (S) and applied to the case of superconductors with nodeless extended $s_{\pm}$-wave order parameter symmetry, as possibly realized in the recently discovered ferro pnictides. We find qualitative differences from tunneling into s-wave or d-wave superconductors that may help to identify such a state. First, interband interference leads to a suppression of Andreev reflection in the case of a highly transparent N/S interface and to a current deficit in the tunneling regime. Second, surface bound states may appear, both at zero and at non-zero energies. These effects do not occur in multiband superconductors without interband sign reversal, though the interference can still strongly modify the conductance spectra.
We investigate the effect of a spin-density wave (SDW) on $s_{\pm}$ superconductivity in Fe-based superconductors. We show that, contrary to the common wisdom, no nodes open at the new, reconnected Fermi surfaces when the hole and electron pockets fold down in the SDW state, despite the fact that the $s_{\pm}$ gap changes sign between the two pockets. Instead, the order parameter preserves its sign along the newly formed Fermi surfaces. The familiar experimental signatures of an $s_{\pm}$ symmetry are still preserved, although they appear in a mathematically different way. For a regular $s$ case ($s_{++})$ the nodes do appear in the SDW state. This distinction suggests a novel simple way to experimentally separate an $s_{\pm}$ state from a regular $s$ in the pnictides. We argue that recently published thermal conductivity data in the coexisting state are consistent with the $s_{\pm},$ but not the $s_{++}$ state.
We review the main ingredients for an unconventional pairing state in the ferropnictides, with particular emphasis on interband pairing due to magnetic fluctuations. Summarizing the key experimental prerequisites for such pairing, the electronic structure and nature of magnetic excitations, we discuss the properties of the s+/- state that emerges as a likely candidate pairing state for these materials and survey experimental evidence in favor of and against this novel state of matter.
The London penetration depth $λ(T)$ has been measured in single crystals of Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ using the tunnel diode resonator technique. The measured doping levels of $x=$~0.038, 0.047, 0.058, 0.074 and 0.10 range from underdoped to overdoped concentrations. The measurements have shown that the density of carriers participating in superconductivity decreases sharply in the underdoped regime, but the penetration depth as a function of temperature exhibits a robust power law, $Δλ(T)\sim T^{n}$, for all measured dopings, with $n$ between 2 and 2.5. We discuss the implications of these results and possible interpretations of such robust behavior.
The newly discovered iron pnictide superconductors apparently present an unusual case of interband-channel pairing superconductivity. Here we show that, in the limit where the pairing occurs within the interband channel, several surprising effects occur quite naturally and generally: different density-of-states on the two bands lead to several unusual properties, including a gap ratio which behaves inversely to the ratio of density-of-states; the weak-coupling limit of the Eliashberg and the BCS theory, commonly taken as equivalent, in fact predict qualitatively different dependence of the $Δ_{1}/Δ_{2}$ and $Δ/T_{c}$ ratios on coupling constants. We show analytically that these effects follow directly from the interband character of superconductivity. Our results show that in the interband-only pairing model the maximal gap ratio is $\sqrt{N_{2}/N_{1}}$ as strong-coupling effects act only to reduce this ratio. This suggests that if the large experimentally reported gap ratios (up to a factor 2) are correct, the pairing mechanism must include more intraband interaction than is usually assumed.