arXiv subjects

Rafael M. Fernandes

Publications and source records attributed to Rafael M. Fernandes.

At least 181 records · Page 10Linked to original sources

arXiv2014

Distinguishing spin-orbit coupling and nematic order in the electronic spectrum of iron-based superconductors

The low-energy electronic states of the iron-based superconductors are strongly affected by both spin-orbit coupling and, when present, by the nematic order. These two effects have different physical origins, yet they can lead to similar gap features in the electronic spectrum. Here we show how to disentangle them experimentally in the iron superconductors with one Fe plane per unit cell. Although the splitting of the low energy doublet at the Brillouin zone center ($Γ$-point) can be due to either the spin-orbit coupling or the nematic order, or both, the degeneracy of each of the doublet states at the zone corner ($M$-point) is protected by the space group symmetry even when spin-orbit coupling is taken into account. Therefore, any splitting at $M$ must be due to lowering of the crystal symmetry, such as due to the nematic order. We further analyze a microscopic tight-binding model with two different contributions to the nematic order: $d_{xz}/d_{yz}$ onsite energy anisotropy and the $d_{xy}$ hopping anisotropy. We find that a precise determination of the former, which has been widely used to characterize the nematic phase, requires a simultaneous measurement of the splittings of the $Γ$-point doublet and at the two low-energy $M$-point doublets. We also discuss the impact of twin domains and show how our results shed new light on ARPES measurements in the normal state of these materials.

cond-mat.supr-con↗

arXiv2014

Theory of the evolution of magnetic order in Fe$_{1+y}$Te compounds with increasing interstitial iron

We examine the influence of the excess of interstitial Fe on the magnetic properties of Fe$_{1+y}$Te compounds. Because in iron chalcogenides the correlations are stronger than in the iron arsenides, we assume in our model that some of the Fe orbitals give rise to localized magnetic moments. These moments interact with each other via exchange interactions as well as phonon-mediated biquadratic interactions that favor a collinear double-stripe state, corresponding to the ordering vectors $\left(\pmπ/2,\pmπ/2\right)$. The remaining Fe orbitals are assumed to be itinerant, giving rise to the first-principle derived Fermi surface displaying nesting features at momenta $\left(π,0\right)/\left(0,π\right)$. Increasing the amount of itinerant electrons due to excess Fe, $y$, leads to changes in the Fermi surface and to the suppression of its nesting properties. As a result, due to the Hund's coupling between the itinerant and localized moments, increasing $y$ leads to modifications in the local moments' exchange interactions via the multi-orbital generalization of the long-range Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. By numerically computing the RKKY corrections and minimizing the resulting effective exchange Hamiltonian, we find, in general, that the excess electrons introduced in the system change the classical magnetic ground state from a double-stripe state to an incommensurate spiral, consistent with the experimental observations. We show that these results can be understood as a result of the suppression of magnetic spectral weight of the itinerant electrons at momenta $\left(π,0\right)/\left(0,π\right)$, combined with the transfer of broad magnetic spectral weight from large to small momenta, promoted by the introduction of excess Fe.

cond-mat.str-el↗

arXiv2014

Time-reversal symmetry breaking superconductivity in the coexistence phase with magnetism in Fe-pnictides

We argue that superconductivity in the coexistence region with spin-density-wave (SDW) order in weakly doped Fe-pnictides differs qualitatively from the ordinary $s^{+-}$ state outside the coexistence region, as it develops an additional gap component which is a mixture of intra-pocket singlet ($s^{++}$) and inter-pocket spin-triplet pairings (the $t-$state). The coupling constant for the $t-$channel is proportional to the SDW order and involves interactions that do not contribute to superconductivity outside of the SDW region. We argue that the $s^{+-}$ and $t-$type superconducting orders coexist at low temperatures, and the relative phase between the two is in general different than $0$ or $π$, manifesting explicitly the breaking of the time-reversal symmetry promoted by long-range SDW order. We show that this exotic state emerges already in the simplest model of Fe-pnictides, with one hole pocketand two symmetry-related electron pockets. We argue that in some parameter range time-reversal gets broken even before long-range superconducting order develops.

cond-mat.supr-con↗

arXiv2014

Impact of local-moment fluctuations on the magnetic degeneracy of iron arsenide superconductors

We investigate the fate of the orthorhombic stripe-type magnetic state (ordering vectors $\left(π,0\right)$/$\left(0,π\right)$), observed in most iron-pnictide superconductors, in the presence of localized magnetic moments that tend to form a Neel state (ordering vector $\left(π,π\right)$). We show that before long-range Neel order sets in, the coupling between the conduction electrons and the fluctuations of the local moments favors an unusual magnetic state consisting of a coherent superposition of the $\left(π,0\right)$ and $\left(0,π\right)$ orders that preserves tetragonal symmetry. The magnetization of this state is non-uniform and induces a simultaneous checkerboard charge order. We discuss signatures of this magnetic configuration on the electronic spectrum and its impact on the superconducting state, showing that its phase space for coexistence with the $s^{+-}$ state is smaller than the stripe-type state. Our results shed light on recent experimental observations on $\mathrm{Ba(Fe_{1-x}Mn_{x})_{2}As_{2}}$ compounds, where the Neel-type local Mn moments interact with the Fe conduction electrons.

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arXiv2014

Antiferromagnetism in Iron-Based Superconductors: Selection of Magnetic Order and Quasiparticle Interference

The recent discovery of superconductivity in the iron-based layered pnictides with T_c ranging between 26 and 56K generated enormous interest in the physics of these materials. Here, we review some of the peculiarities of the antiferromagnetic order in the iron pnictides, including the selection of the stripe magnetic order and the formation of the Ising-nematic state in the unfolded BZ within an itinerant description. In addition we analyze the properties of the quasiparticle interference spectrum in the parent antiferromagnetic phase.

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arXiv2014

Ultrafast observation of critical nematic fluctuations and giant magnetoelastic coupling in iron pnictides

Many of the iron pnictides have strongly anisotropic normal-state characteristics, important for the exotic magnetic and superconducting behavior these materials exhibit. Yet, the origin of the observed anisotropy is unclear. Electronically driven nematicity has been suggested, but distinguishing this as an independent degree of freedom from magnetic and structural orders is difficult, as these couple together to break the same tetragonal symmetry. Here we use time-resolved polarimetry to reveal critical nematic fluctuations in unstrained Ba(Fe_(1-x)Co_x)_2As_2. The femtosecond anisotropic response, which arises from the two-fold in-plane anisotropy of the complex refractive index, displays a characteristic two-step recovery absent in the isotropic response. The fast recovery appears only in the magnetically ordered state, whereas the slow one persists in the paramagnetic phase with a critical divergence approaching the structural transition temperature. The dynamics also reveal a gigantic magnetoelastic coupling that far exceeds electron-spin and electron-phonon couplings, opposite to conventional magnetic metals.

cond-mat.str-el↗

arXiv2013

Nematicity as a probe of superconducting pairing in iron-based superconductors

In several families of iron-based superconducting materials, a d-wave pairing instability may compete with the leading s-wave instability. Here we show that when both states have comparable free energies, superconducting and nematic degrees of freedom are strongly coupled. While nematic order causes a sharp non-analytic increase in $T_{c}$, nematic fluctuations can change the character of the s-wave to d-wave transition, favoring an intermediate state that does not break time-reversal symmetry but does break tetragonal symmetry. The coupling between superconductivity and nematicity is also manifested in the strong softening of the shear modulus across the superconducting transition. Our results show that nematicity can be used as a diagnostic tool to search for unconventional pairing states in iron pnictides and chalcogenides.

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arXiv2013

How many quantum phase transitions exist inside the superconducting dome of the iron pnictides?

Recent experiments on two iron-pnictide families suggest the existence of a single quantum phase transition (QPT) inside the superconducting dome despite the fact that two separate transition lines - magnetic and nematic - cross the superconducting dome at $T_{c}$. Here we argue that these two observations are actually consistent. We show, using a microscopic model, that each order coexists with superconductivity for a wide range of parameters, and both transition lines continue into the superconducting dome below $T_{c}$. However, at some $T_{\mathrm{merge}}<T_{c}$, the two transitions merge and continue down to T=0 as a single simultaneous first-order nematic/magnetic transition. We show that superconductivity has a profound effect on the character of this first-order transition, rendering it weakly first-order and allowing strong fluctuations to exist near the QPT.

cond-mat.supr-con↗

arXiv2013

Uniaxial stress effects on the structural and electronic properties of BaFe2As2 and CaFe2As2

Starting from the orthorhombic magnetically ordered phase, we investigate the effects of uniaxial tensile and compressive stresses along a, b, and the diagonal a+b directions in BaFe2As2 and CaFe2As2 in the framework of ab initio density functional theory (DFT) and a phenomonological Ginzburg-Landau model. While -contrary to the application of hydrostatic or c-axis uniaxial pressure- both systems remain in the orthorhombic phase with a pressure-dependent nonzero magnetic moment, we observe a sign-changing jump in the orthorhombicity at a critical uniaxial pressure, accompanied by a reversal of the orbital occupancy and a switch between the ferromagnetic and antiferromagnetic directions. Our Ginzburg-Landau analysis reveals that this behavior is a direct consequence of the competition between the intrinsic magneto-elastic coupling of the system and the applied compressive stress, which helps the system to overcome the energy barrier between the two possible magneto-elastic ground states. Our results shed light on the mechanisms involved in the detwinning process of an orthorhombic iron-pnictide crystal and on the changes in the magnetic properties of a system under uniaxial stress.

cond-mat.supr-con↗

arXiv2013

Suppression of superconductivity by Neel-type magnetic fluctuations in the iron pnictides

Motivated by recent experimental detection of Neel-type ($(π,π)$) magnetic fluctuations in some iron pnictides, we study the impact of competing $(π,π)$ and $(π,0)$ spin fluctuations on the superconductivity of these materials. We show that, counter-intuitively, even short-range, weak Neel fluctuations strongly suppress the $s^{+-}$ state, with the main effect arising from a repulsive contribution to the $s^{+-}$ pairing interaction, complemented by low frequency inelastic scattering. Further increasing the strength of the Neel fluctuations leads to a low-$T_{c}$ d-wave state, with a possible intermediate $s+id$ phase. The results suggest that the absence of superconductivity in a series of hole-doped pnictides is due to the combination of short-range Neel fluctuations and pair-breaking impurity scattering, and also that $T_{c}$ of optimally doped pnictides could be further increased if residual $(π,π)$ fluctuations were reduced.

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arXiv2012

Electronic transport in the Coulomb phase of the pyrochlore spin ice

We investigate the transport properties of itinerant electrons interacting with a background of localized spins in a correlated paramagnetic phase of the pyrochlore lattice. We find a residual resistivity at zero temperature due to the scattering of electrons by the static dipolar spin-spin correlation that characterizes the metallic Coulomb phase. As temperature increases, thermally excited topological defects, also known as magnetic monopoles, reduce the spin correlation, hence suppressing electron scattering. Combined with the usual scattering processes in metals at higher temperatures, this mechanism yields a non-monotonic resistivity, displaying a minimum at temperature scales associated with the magnetic monopole excitation energy. Our calculations agree quantitatively with resistivity measurements in Nd2Ir2O7, shedding light on the origin of the resistivity minimum observed in metallic spin-ice compounds.

cond-mat.str-el↗

arXiv2012

Broken translational symmetry in an emergent paramagnetic phase of graphene

We show that the spin-density wave state on the partially filled honeycomb and triangular lattices is preempted by a paramagnetic phase that breaks an emergent Z_4 symmetry of the system, associated with the four inequivalent arrangements of spins in the quadrupled unit cell. Unlike other emergent paramagnetic phases in itinerant and localized-spin systems, this state preserves the rotational symmetry of the lattice but breaks its translational symmetry, giving rise to a super-lattice structure that can be detected by scanning tunneling microscopy. This emergent phase also has distinctive signatures in the magnetic spectrum that can be probed experimentally.

cond-mat.str-el↗

arXiv2012

Manifestations of nematic degrees of freedom in the magnetic, elastic, and superconducting properties of the iron pnictides

We investigate how emergent nematic order and nematic fluctuations affect several macroscopic properties of both the normal and superconducting states of the iron pnictides. Due to its magnetic origin, long-range nematic order enhances magnetic fluctuations, leaving distinctive signatures in the spin-lattice relaxation rate, the spin-spin correlation function, and the uniform magnetic susceptibility. This enhancement of magnetic excitations is also manifested in the electronic spectral function, where a pseudogap can open at the hot spots of the Fermi surface. In the nematic phase, electrons are scattered by magnetic fluctuations that are anisotropic in momentum space, giving rise to a non-zero resistivity anisotropy whose sign changes between electron-doped and hole-doped compounds. We also show that due to the magneto-elastic coupling, nematic fluctuations soften the shear modulus in the normal state, but harden it in the superconducting state. The latter effect is an indirect consequence of the competition between magnetism and superconductivity, and also causes a suppression of the orthorhombic distortion below T_{c}. We also demonstrate that ferro-orbital fluctuations enhance the nematic susceptibility, cooperatively promoting an electronic tetragonal symmetry-breaking. Finally, we argue that T_{c} in the iron pnictides might be enhanced due to nematic fluctuations of magnetic origin.

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arXiv2011

Anisotropic in-plane resistivity in the nematic phase of the iron pnictides

We show that the interference between scattering by impurities and by critical spin fluctuations gives rise to anisotropic transport in the Ising-nematic state of the iron pnictides. The effect is closely related to the non-Fermi liquid behavior of the resistivity near an antiferromagnetic quantum critical point. Our theory not only explains the observed sign of the resistivity anisotropy $Δρ$ in electron doped systems, but also predicts a sign change of $Δρ$ upon sufficient hole doping. Furthermore, our model naturally addresses the changes in $Δρ$ upon sample annealing and alkaline-earth substitution.

cond-mat.str-el↗

arXiv2011

Scaling of nascent nodes in extended s-wave superconductors

We analyze the low-energy properties of superconductors near the onset of accidental nodes, i.e. zeroes of the gap function not enforced by symmetry. The existence of such nodes has been motivated by recent experiments suggesting a transition between nodeless and nodal superconductivity in iron-based compounds. We find that the low-temperature behavior of the penetration depth, the specific heat, and the NMR spin-lattice relaxation rate are determined by the scaling properties of a quantum critical point associated with the nascent nodes. Although the power-law exponents are insensitive to weak short-range electronic interactions, they can be significantly altered by the curvature of the Fermi surface or by the three-dimensional character of the gap. Consequently, the behavior of macroscopic quantities near the onset of nodes can be used as a criterion to determine the nodal structure of the gap function.

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arXiv2011

Complex critical exponents for percolation transitions in Josephson-junction arrays, antiferromagnets, and interacting bosons

We show that the critical behavior of quantum systems undergoing a percolation transition is dramatically affected by their topological Berry phase $2πρ$. For irrational $ρ$, we demonstrate that the low-energy excitations of diluted Josephson-junctions arrays, quantum antiferromagnets, and interacting bosons are spinless fermions with fractal spectrum. As a result, critical properties not captured by the usual Ginzburg-Landau-Wilson description of phase transitions emerge, such as complex critical exponents, log-periodic oscillations and dynamically broken scale-invariance.

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arXiv2010

Effects of nematic fluctuations on the elastic properties of iron arsenide superconductors

We demonstrate that the changes in the elastic properties of the FeAs systems, as seen in our resonant ultrasound spectroscopy data, can be naturally understood in terms of fluctuations of emerging nematic degrees of freedom. Both the softening of the lattice in the normal, tetragonal phase as well as its hardening in the superconducting phase are consistently described by our model. Our results confirm the view that structural order is induced by magnetic fluctuations.

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arXiv2010

Transfer of optical spectral weight in magnetically ordered superconductors

We show that, in antiferromagnetic superconductors, the optical spectral weight transferred to low frequencies below the superconducting transition temperature originates from energies that can be much larger than twice the superconducting gap $Δ$. This contrasts to non-magnetic superconductors, where the optical spectrum is suppressed only for frequencies below $2Δ$. In particular, we demonstrate that the superfluid condensate of the magnetically ordered superconductor is not only due to states of the magnetically reconstructed Fermi surface, but is enhanced by transfer of spectral weight from the mid infrared peak generated by the spin density wave gap. We apply our results to the iron arsenide superconductors, addressing the decrease of the zero-temperature superfluid density in the doping regime where magnetism coexists with unconventional superconductivity.

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