arXiv subjects

Rafael M. Fernandes

Publications and source records attributed to Rafael M. Fernandes.

At least 163 records · Page 9Linked to original sources

arXiv2017

Superconductivity mediated by quantum critical antiferromagnetic fluctuations: The rise and fall of hot spots

In several unconventional superconductors, the highest superconducting transition temperature $T_{c}$ is found in a region of the phase diagram where the antiferromagnetic transition temperature extrapolates to zero, signaling a putative quantum critical point. The elucidation of the interplay between these two phenomena - high-$T_{c}$ superconductivity and magnetic quantum criticality - remains an important piece of the complex puzzle of unconventional superconductivity. In this paper, we combine sign-problem-free Quantum Monte Carlo simulations and field-theoretical analytical calculations to unveil the microscopic mechanism responsible for the superconducting instability of a general low-energy model, called spin-fermion model. In this approach, low-energy electronic states interact with each other via the exchange of quantum critical magnetic fluctuations. We find that even in the regime of moderately strong interactions, both the superconducting transition temperature and the pairing susceptibility are governed not by the properties of the entire Fermi surface, but instead by the properties of small portions of the Fermi surface called hot spots. Moreover, $T_{c}$ increases with increasing interaction strength, until it starts to saturate at the crossover from hot-spots dominated to Fermi-surface dominated pairing. Our work provides not only invaluable insights into the system parameters that most strongly affect $T_{c}$, but also important benchmarks to assess the origin of superconductivity in both microscopic models and actual materials.

cond-mat.supr-con↗

arXiv2017

Induced spin-triplet pairing in the coexistence state of antiferromagnetism and singlet superconductivity: collective modes and microscopic properties

The close interplay between superconductivity and antiferromagnetism in several quantum materials can lead to the appearance of an unusual thermodynamic state in which both orders coexist microscopically, despite their competing nature. A hallmark of this coexistence state is the emergence of a spin-triplet superconducting gap component, called $π$-triplet, which is spatially modulated by the antiferromagnetic wave-vector, reminiscent of a pair-density wave. In this paper, we investigate the impact of these $π$-triplet degrees of freedom on the phase diagram of a system with competing antiferromagnetic and superconducting orders. Although we focus on a microscopic two-band model that has been widely employed in studies of iron pnictides, most of our results follow from a Ginzburg-Landau analysis, and as such should be applicable to other systems of interest, such as cuprates and heavy fermions. The Ginzburg-Landau functional reveals not only that the $π$-triplet gap amplitude couples tri-linearly with the singlet gap amplitude and the staggered magnetization magnitude, but also that the $π$-triplet $d$-vector couples linearly with the magnetization direction. While in the mean field level this coupling forces the $d$-vector to align parallel or anti-parallel to the magnetization, in the fluctuation regime it promotes two additional collective modes - a Goldstone mode related to the precession of the $d$-vector around the magnetization and a massive mode, related to the relative angle between the two vectors, which is nearly degenerate with a Leggett-like mode associated with the phase difference between the singlet and triplet gaps. We also investigate the impact of magnetic fluctuations on the superconducting-antiferromagnetic phase diagram, showing that due to their coupling with the $π$-triplet order parameter, the coexistence region is enhanced.

cond-mat.supr-con↗

arXiv2017

Transverse fields to tune an Ising-nematic quantum critical transition

The paradigmatic example of a continuous quantum phase transition is the transverse field Ising ferromagnet. In contrast to classical critical systems, whose properties depend only on symmetry and the dimension of space, the nature of a quantum phase transition also depends on the dynamics. In the transverse field Ising model, the order parameter is not conserved and increasing the transverse field enhances quantum fluctuations until they become strong enough to restore the symmetry of the ground state. Ising pseudo-spins can represent the order parameter of any system with a two-fold degenerate broken-symmetry phase, including electronic nematic order associated with spontaneous point-group symmetry breaking. Here, we show for the representative example of orbital-nematic ordering of a non-Kramers doublet that an orthogonal strain or a perpendicular magnetic field plays the role of the transverse field, thereby providing a practical route for tuning appropriate materials to a quantum critical point. While the transverse fields are conjugate to seemingly unrelated order parameters, their non-trivial commutation relations with the nematic order parameter, which can be represented by a Berry-phase term in an effective field theory, intrinsically intertwines the different order parameters.

cond-mat.str-el↗

arXiv2016

Spin anisotropy due to spin-orbit coupling in optimally hole-doped Ba$_{0.67}$K$_{0.33}$Fe$_{2}$As$_{2}$

We use polarized inelastic neutron scattering to study the temperature and energy dependence of spin space anisotropies in the optimally hole-doped iron pnictide Ba$_{0.67}$K$_{0.33}$Fe$_{2}$As$_{2}$ ($T_{\rm c}=38$ K). In the superconducting state, while the high-energy part of the magnetic spectrum is nearly isotropic, the low-energy part displays a pronouced anisotropy, manifested by a $c$-axis polarized resonance. We also observe that the spin anisotropy in superconducting Ba$_{0.67}$K$_{0.33}$Fe$_{2}$As$_{2}$ extends to higher energies compared to electron-doped BaFe$_{2-x}TM_{x}$As$_{2}$ ($TM=$Co, Ni) and isovalent-doped BaFe$_{2}$As$_{1.4}$P$_{0.6}$, suggesting a connection between $T_{\rm c}$ and the energy scale of the spin anisotropy. In the normal state, the low-energy spin anisotropy for optimally hole- and electron-doped iron pnictides onset at temperatures similar to the temperatures at which the elastoresistance deviate from Curie-Weiss behavior, pointing to a possible connection between the two phenomena. Our results highlight the relevance of the spin-orbit coupling to the superconductivity of the iron pnictides.

cond-mat.supr-con↗

arXiv2016

Superconductivity in FeSe thin films driven by the interplay between nematic fluctuations and spin-orbit coupling

The origin of the high-temperature superconducting state observed in FeSe thin films, whose phase diagram displays no sign of magnetic order, remains a hotly debated topic. Here we investigate whether fluctuations arising due to the proximity to a nematic phase, which is observed in the phase diagram of this material, can promote superconductivity. We find that nematic fluctuations alone promote a highly degenerate pairing state, in which both $s$-wave and $d$-wave symmetries are equally favored, and $T_{c}$ is consequently suppressed. However, the presence of a sizable spin-orbit coupling or inversion symmetry-breaking at the film interface lifts this harmful degeneracy and selects the $s$-wave state, in agreement with recent experimental proposals. The resulting gap function displays a weak anisotropy, which agrees with experiments in monolayer FeSe and intercalated Li$_{1-x}$(OH)$_{x}$FeSe.

cond-mat.supr-con↗

arXiv2016

Displacement and annihilation of Dirac gap-nodes in d-wave iron-based superconductors

Several experimental and theoretical arguments have been made in favor of a $d-$wave symmetry for the superconducting state in some Fe-based materials. It is a common belief that a $d-$wave gap in the Fe-based superconductors must have nodes on the Fermi surfaces centered at the $Γ$ point of the Brillouin zone. Here we show that, while this is the case for a single Fermi surface made out of a single orbital, the situation is more complex if there is an even number of Fermi surfaces made out of different orbitals. In particular, we show that for the two $Γ$-centered hole Fermi surfaces made out of $d_{xz}$ and $d_{yz}$ orbitals, the nodal points still exist near $T_{c}$ along the symmetry-imposed directions, but are are displaced to momenta between the two Fermi surfaces. If the two hole pockets are close enough, pairs of nodal points can merge and annihilate at some $T<T_{c}$, making the $d-$wave state completely nodeless. These results imply that photoemission evidence for a nodeless gap on the $d_{xz}/d_{yz}$ Fermi surfaces of KFe$_{2}$As$_{2}$ does not rule out $d-$wave gap symmetry in this material, while a nodeless gap observed on the $d_{xy}$ pocket in K$_{x}$Fe$_{2-y}$Se$_{2}$ is truly inconsistent with the $d-$wave gap symmetry.

cond-mat.supr-con↗

arXiv2016

Scanning tunneling spectroscopy as a probe of multi-Q magnetic states of itinerant magnets

The combination of electronic correlations and Fermi surfaces with multiple nesting vectors can lead to the appearance of complex multi-Q magnetic ground states, hosting unusual states such as chiral density-waves and quantum Hall insulators. Distinguishing single-Q and multi-Q magnetic phases is however a notoriously difficult experimental problem. Here we propose theoretically that the local density of states near a magnetic impurity, whose orientation may be controlled by an external magnetic field, can be used to map out the detailed magnetic configuration of an itinerant system and distinguish unambiguously between single-Q and multi-Q phases. We demonstrate this concept by computing and contrasting the LDOS near a magnetic impurity embedded in three different magnetic ground states relevant to iron-based superconductors -- one single-Q and two double-Q phases. Our results open a promising avenue to investigate complex magnetic configurations in itinerant systems via standard scanning tunneling spectroscopy, without requiring spin-resolved capability.

cond-mat.supr-con↗

arXiv2016

Low-energy microscopic models for iron-based superconductors: a review

The development of sensible microscopic models is essential to elucidate the normal-state and superconducting properties of the iron-based superconductors. Because these materials are mostly metallic, a good starting point is an effective low-energy model that captures the electronic states near the Fermi level and their interactions. However, in contrast to cuprates, iron-based high-$T_{c}$ compounds are multi-orbital systems with Hubbard and Hund interactions, resulting in a rather involved 10-orbital lattice model. Here we review different minimal models that have been proposed to unveil the universal features of these systems. We first review minimal models defined solely in the orbital basis, which focus on a particular subspace of orbitals, or solely in the band basis, which rely only on the geometry of the Fermi surface. The former, while providing important qualitative insight into the role of the orbital degrees of freedom, do not distinguish between high-energy and low-energy sectors and, for this reason, generally do not go beyond mean-field. The latter allow one to go beyond mean-field and investigate the interplay between superconducting and magnetic orders as well as Ising-nematic order. However, they cannot capture orbital-dependent features like spontaneous orbital order. We then review recent proposals for a minimal model that operates in the band basis but fully incorporates the orbital composition and symmetries of the low-energy excitations. We discuss the results of the renormalization group study of such a model, particularly of the interplay between superconductivity, magnetism, and spontaneous orbital order, and compare theoretical predictions with experiments on iron pnictides and chalcogenides. We also discuss the impact of the glide-plane symmetry on the low-energy models, highlighting the key role played by the spin-orbit coupling.

cond-mat.str-el↗

arXiv2016

Percolation via combined electrostatic and chemical doping in complex oxide films

Stimulated by experimental advances in electrolyte gating methods, we investigate theoretically percolation in thin films of inhomogenous complex oxides, such as La$_{1-x}$Sr$_{x}$CoO$_{3}$ (LSCO), induced by a combination of bulk chemical and surface electrostatic doping. Using numerical and analytical methods, we identify two mechanisms that describe how bulk dopants reduce the amount of electrostatic surface charge required to reach percolation: (i) bulk-assisted surface percolation, and (ii) surface-assisted bulk percolation. We show that the critical surface charge strongly depends on the film thickness when the film is close to the chemical percolation threshold. In particular, thin films can be driven across the percolation transition by modest surface charge densities \emph{via} surface-assisted bulk percolation. If percolation is associated with the onset of ferromagnetism, as in LSCO, we further demonstrate that the presence of critical magnetic clusters extending from the film surface into the bulk results in considerable volume enhancement of the saturation magnetization, with pronounced experimental consequences. These results should significantly guide experimental work seeking to verify gate-induced percolation transitions in such materials.

cond-mat.mes-hall↗

arXiv2016

Magnetism, superconductivity, and spontaneous orbital order in iron-based superconductors: who comes first and why?

Magnetism and nematic order are the two non-superconducting orders observed in iron-based superconductors. To elucidate the interplay between them and ultimately unveil the pairing mechanism, several models have been investigated. In models with quenched orbital degrees of freedom, magnetic fluctuations promote stripe magnetism which induces orbital order. In models with quenched spin degrees of freedom, charge fluctuations promote spontaneous orbital order which induces stripe magnetism. Here we develop an unbiased approach, in which we treat magnetic and orbital fluctuations on equal footing. Key to our approach is the inclusion of the orbital character of the low-energy electronic states into renormalization group analysis. Our results show that in systems with large Fermi energies, such as BaFe2As2, LaFeAsO, and NaFeAs, orbital order is induced by stripe magnetism. However, in systems with small Fermi energies, such as FeSe, the system develops a spontaneous orbital order, while magnetic order does not develop. Our results provide a unifying description of different iron-based materials.

cond-mat.supr-con↗

arXiv2015

Disorder-promoted $C_4$-symmetric magnetic order in iron-based superconductors

In most iron-based superconductors, the transition to the magnetically ordered state is closely linked to a lowering of structural symmetry from tetragonal ($C_{4}$) to orthorhombic ($C_{2}$). However, recently, a regime of $C_{4}$-symmetric magnetic order has been reported in certain hole-doped iron-based superconductors. This novel magnetic ground state can be understood as a double-$\mathbf{Q}$ spin density wave characterized by two order parameters $\mathbf{M}_{1}$ and $\mathbf{M}_{2}$ related to each of the two $\mathbf{Q}$ vectors. Depending on the relative orientations of the order parameters, either a noncollinear spin-vortex crystal or a nonuniform charge-spin density wave could form. Experimentally, Mössbauer spectroscopy, neutron scattering, and muon spin rotation established the latter as the magnetic configuration of some of these optimally hole-doped iron-based superconductors. Theoretically, low-energy itinerant models do support a transition from single-$\mathbf{Q}$ to double-$\mathbf{Q}$ magnetic order, but with nearly-degenerate spin-vortex crystal and charge-spin density wave states. In fact, extensions of these low-energy models including additional electronic interactions tip the balance in favor of the spin-vortex crystal, in apparent contradiction with the recent experimental findings. In this paper, we revisit the phase diagram of magnetic ground states of low-energy multi-band models in the presence of weak disorder. We show that impurity scattering not only promotes the transition from $C_{2}$ to $C_{4}$-magnetic order, but it also favors the charge-spin density wave over the spin-vortex crystal phase. Additionally, in the single-$\mathbf{Q}$ phase, our analysis of the nematic coupling constant in the presence of disorder supports the experimental finding that the splitting between the structural and stripe-magnetic transition is enhanced by disorder.

cond-mat.str-el↗

arXiv2015

Origin of the DC and AC conductivity anisotropy in iron-based superconductors: scattering rate versus spectral weight effects

To shed light on the mechanism responsible for the in-plane resistivity anisotropy in the nematic phase of the iron-based superconductors, we investigate the impact of spin fluctuations on the anisotropic AC conductivity. On the one hand, the scattering of electrons off magnetic fluctuations causes an anisotropic scattering rate. On the other hand, the accompanying Fermi velocity renormalization contributes to both the plasma frequency and the scattering rate in antagonistic ways, giving rise to anisotropies in these quantities that exactly cancel each other in the DC limit. As a result, the effective scattering rate changes its sign as function of temperature, but the DC conductivity retains the same sign, in agreement with recent experiments.

cond-mat.str-el↗

arXiv2015

Antagonistic in-plane resistivity anisotropies from competing fluctuations in underdoped cuprates

One of the prime manifestations of an anisotropic electronic state in underdoped cuprates is the in-plane resistivity anisotropy $Δρ\equiv(ρ_{a}-ρ_{b})/ρ_{b}$. Here we use a Boltzmann-equation approach to compute the contribution to $Δρ$ arising from scattering by anisotropic charge and spin fluctuations, which have been recently observed experimentally. While the anisotropy in the charge fluctuations is manifested in the correlation length, the anisotropy in the spin fluctuations emerges only in the structure factor. As a result, we find that spin fluctuations favor $Δρ>0$, whereas charge fluctuations promote $Δρ<0$, which are both consistent with the doping dependence of $Δρ$ observed in YBa$_{2}$Cu$_{3}$O$_{7}$. We also discuss the role played by CuO chains in these materials, and propose transport experiments in strained HgBa$_{2}$CuO$_{4}$ and Nd$_{2}$CuO$_{4}$ to probe directly the different resistivity anisotropy regimes.

cond-mat.str-el↗

arXiv2015

Phenomenological theory of the superconducting state inside the hidden-order phase of URu$_{2}$Si$_{2}$

Recent experiments have unveiled important properties of the ground state of the elusive heavy fermion $\mathrm{URu_{2}Si_{2}}$. While tetragonal symmetry-breaking was reported below the hidden-order (HO) transition at $T_{HO}\approx17.5$ K, time-reversal symmetry-breaking was observed below the superconducting transition temperature $T_{c}<T_{HO}$. Although the latter results have been used to argue in favor of a chiral $d+id$ superconducting state, such an order parameter is incompatible with broken tetragonal symmetry. Here, we employ a phenomenological model to investigate the properties of a chiral superconducting state that develops inside the hidden-order phase. In this case, there are actually two superconducting transition temperatures: while $T_{c}$ marks a normal-state to superconducting transition, $T_{c}^{*}<T_{c}$ signals a superconducting-to-superconducting transition in which time-reversal symmetry is broken. In the phase $T_{c}^{*}<T<T_{c}$, the low-energy density of states $ρ\left(ω\right)$ is enhanced due to the crossing of two nodal lines, giving rise to an unusual $ω\log(ω)$ dependence of $ρ\left(ω\right)$, which is manifested in several thermodynamic properties. We also investigate the emergence of a soft amplitude gap mode near $T_{c}^{*}$. In contrast to the usual amplitude mode near a regular normal-state to superconducting transition, this mode becomes soft near a superconducting-to-superconducting transition, which in principle allows for its detection by Raman spectroscopy. Finally, we investigate the impact of twin domains on the anisotropic properties of the superconducting state, and propose experiments in mechanically strained samples to explore the interplay between hidden order and superconductivity in $\mathrm{URu_{2}Si_{2}}$.

cond-mat.supr-con↗

arXiv2015

The origin of nematic order in FeSe

The origin of the 90 K nematic transition in the chalcogenide FeSe, which displays no magnetic order down to T=0, remains a major puzzle for a unifying theory for the iron-based superconductors. We analyze this problem in light of recent experimental data which reveal very small Fermi pockets in this material. We show that the smallness of the Fermi energy leads to a near-degeneracy between magnetic fluctuations and fluctuations in the charge-current density-wave channel. While the two fluctuation modes cooperate to promote the same preemptive Ising-nematic order, they compete for primary order. We argue that this explains why in FeSe the nematic order emerges when the magnetic correlation length is smaller than in other Fe-based materials, and why no magnetism is observed. We discuss how pressure lifts this near-degeneracy, resulting in a non-monotonic dependence of the nematic transition with pressure, in agreement with experiments.

cond-mat.str-el↗

arXiv2015

Magnetic order without tetragonal-symmetry-breaking in iron arsenides: microscopic mechanism and spin-wave spectrum

Most iron-based superconductors undergo a transition to a magnetically ordered state characterized by staggered stripes of parallel spins. With ordering vectors $(π,0)$ or $(0,π)$, this magnetic state breaks the high-temperature tetragonal symmetry of the system, which is manifested by a splitting of the lattice Bragg peaks. Remarkably, recent experiments in hole-doped iron arsenides reported an ordered state that displays magnetic Bragg peaks at $(π,0)$ and $(0,π)$ but remains tetragonal. Despite being inconsistent with a magnetic stripe configuration, this unusual magnetic phase can be described in terms of a double-$\mathbf{Q}$ magnetic structure consisting of an equal-weight superposition of the ordering vectors $(π,0)$ and $(0,π)$. Here we show that a non-collinear double-$\mathbf{Q}$ magnetic configuration, dubbed \emph{orthomagnetic}, arises naturally within an itinerant three-band microscopic model for the iron pnictides. In particular, we find that strong deviations from perfect nesting and residual interactions between the electron pockets favor the orthomagnetic over the stripe magnetic state. Using an effective low-energy model, we also calculate the spin-wave spectrum of the orthomagnetic state. In contrast to the stripe state, there are three Goldstone modes, manifested in all diagonal and one off-diagonal component of the spin-spin correlation function. The total magnetic structure factor displays two anisotropic spin-wave branches emerging from both $(π,0)$ and $(0,π)$ momenta, in contrast to the case of domains of stripe order, where only one spin-wave branch emerges from each momentum. We propose that these unique features of the orthomagnetic state can be used to unambiguously distinguish it from the stripe state via neutron scattering experiments, and discuss the implications of its existence to the nature of the magnetism of the iron arsenides.

cond-mat.supr-con↗

arXiv2015

Neutron-scattering measurements of the spin excitations in LaFeAsO and Ba(Fe$_{0.953}$Co$_{0.047}$)$_{2}$As$_{2}$: Evidence for a sharp enhancement of spin fluctuations by nematic order

Inelastic neutron scattering was employed to investigate the impact of electronic nematic order on the magnetic spectra of LaFeAsO and Ba(Fe$_{0.953}$Co$_{0.047}$)$_{2}$As$_{2}$. These materials are ideal to study the paramagnetic-nematic state, since the nematic order, signaled by the tetragonal-to-orthorhombic transition at $T_{\rm S}$, sets in well above the stripe antiferromagnetic ordering at $T_{\rm N}$. We find that the temperature-dependent dynamic susceptibility displays an anomaly at $T_{\rm S}$ followed by a sharp enhancement in the spin-spin correlation length, revealing a strong feedback effect of nematic order on the low-energy magnetic spectrum. Our findings can be consistently described by a model that attributes the structural/nematic transition to magnetic fluctuations, and unveils the key role played by nematic order in promoting the long-range stripe antiferromagnetic order in iron pnictides.

cond-mat.str-el↗

arXiv2014

Manipulation of gap nodes by uniaxial strain in iron-based superconductors

In the iron pnictides and chalcogenides, multiple orbitals participate in the superconducting state, enabling different gap structures to be realized in distinct materials. Here we argue that the spectral weights of these orbitals can in principle be controlled by a tetragonal symmetry-breaking uniaxial strain, due to the enhanced nematic susceptibility of many iron-based superconductors. By investigating multi-orbital microscopic models in the presence of orbital order, we show that not only $T_{c}$ can be enhanced, but pairs of accidental gap nodes can be annihilated and created in the Fermi surface by an increasing external strain. We explain our results as a mixture of nearly-degenerate superconducting states promoted by strain, and show that the annihilation and creation of nodes can be detected experimentally via anisotropic penetration depth measurements. Our results provide a promising framework to externally control the superconducting properties of iron-based materials.

cond-mat.supr-con↗