Magnetic field induced chiral particle-hole condensates

We demonstrate that a chiral particle-hole condensate is always induced by a number-conserving ground state of non-zero angular momentum in the presence of a magnetic field. The magnetic interaction originates from the coupling with the intrinsic orbital moment of the chiral state when the field is applied perpendicularly to the plane. According to our numerical results the induction mechanism is practically temperature independent providing robustness to these states up to high temperatures. This opens the door for manipulating the anomalous Hall response resulting from this intricate class of states for technological applications while it also suggests that chiral particle-hole condensates may be hidden in various complex materials.Comment: This is the final version accepted by Phys. Rev. B (Brief Reports

Small-q Phonon Mediated Unconventional Superconductivity in the Iron Pnictides

We report self-consistent calculations of the gap symmetry for the iron-based high-temperature superconductors using realistic small-q phonon mediated pairing potentials and four-band energy dispersions. When both electron and hole Fermi surface pockets are present, we obtain the nodeless $s_\pm$ state that was first encountered in a spin-fluctuations mechanism picture. Nodal gap structures such as $d_{x^2-y^2}$ and $s_\pm+d_{x^2-y^2}$ and even a p-wave triplet state, are accessible upon doping within our phononic mechanism. Our results resolve the conflict between phase sensitive experiments reporting a gap changing sign attributed previously only to a non-phononic mechanism and isotope effect measurements proving the involvement of phonons in the pairing.Comment: Final version. Corrected typos. Reference adde

Nematicity from mixed S_{+-} + d_{x^2-y^2} states in iron-based superconductors

We demonstrate that in iron-based superconductors, the extended S_{+-} SC state coexists with the d_{x^2-y^2} state under generic conditions. The mixed S_{+-} + d_{x^2-y^2} SC is a natural nematic state in which the tetragonal symmetry C_4 is broken to C_2 explaining puzzling findings of nematic SC in FeSe films [Science 332, 1410 (2011)]. Moreover, we report the possibility of a first order transition at low-T from the nematic S_{+-} + d_{x^2-y^2} state to the pure d_{x^2-y^2} state induced by the Zeeman magnetic field proposing an original experimental strategy for identifying our mixed nematic state in FeSe films. Extrapolating our findings, we argue that nematicity in non superconducting states of underdoped and undoped pnictides may reflect mixed S_{+-} + d_{x^2-y^2} Density Wave states.Comment: Improvements and corrections in the texte, references adde

Magnetic-field-induced chiral hidden order in URu2Si2

Two of the most striking and yet unresolved manifestations of the hidden order (HO) in URu2Si2, are associated on one hand with the double-step metamagnetic transitions and on the other with the giant anomalous Nernst signal. Both are observed when a magnetic field is applied along the c-axis. Here we provide for the first time a unified understanding of these puzzling phenomena and the related field-temperature (B-T) phase diagram. We demonstrate that the HO phase at finite fields can be explained with a chiral dxy+idx2-y2 spin density wave, assuming that the zero field HO contains only the time-reversal symmetry preserving idx2-y2 component. We argue that the presence of the field-induced chiral HO can be reflected in a distinctive non-linear B-dependence of the Kerr angle, when a Kerr experiment is conducted for finite fields. This fingerprint can be conclusive for the possible emergence of chirality in the HO.Comment: 8 pages and 9 figures main text + 6 pages supplementary material. Philosophical Magazine: Special Issue: Focused Issue on Hidden Order in URu2Si2 (May 2014

Patterns of coexisting superconducting and particle-hole condensates

We have studied systematically the influence of particle-hole symmetric and asymmetric kinetic terms on the ordered phases that we may observe competing or coexisting in a tetragonal system. We show that there are precise patterns of triplets of ordered phases that are accessible (i.e. it is impossible to observe two of them without the third one). We found a systematic way to predict these patterns of states and tested it by identifying at least 16 different patterns of three order parameters that necessarily coexist in the presence of the kinetic terms. We show that there are two types of general equations governing the competition of all these triplets of order parameters and we provide them.Comment: Published versio

Meissner effect without superconductivity from a chiral d-density wave

We demonstrate that the formation of a chiral d-density wave (CDDW) state generates a Topological Meissner effect (TME) in the absence of any kind of superconductivity. The TME is identical to the usual superconducting Meissner effect but it appears only for magnetic fields perpendicular to the plane while it is absent for in plane fields. The observed enhanced diamagnetic signals in the non-superconducting pseudogap regime of the cuprates may find an alternative interpretation in terms of a TME, originating from a chiral d-density wave pseudogap.Comment: 4 pages, 3 figure

Spontaneous Quantum Hall Effect in chiral d-density waves

We study the electromagnetic response of a chiral ${\rm d_{xy}+id_{x^2-y^2}}$ charge density wave state. Due to parity (${\cal P}$) and time reversal (${\cal T}$) violation, Chern-Simons terms emerge in the effective action of the U(1) gauge field. As a consequence electric and magnetic fields are coupled providing the possibility of observing the Spontaneous Quantum Hall Effect i.e. generation of Hall voltage via the sole application of an electric field. We show how the Chern-Simons terms are induced and discuss the topological origin of the quantization of Hall conductance.Comment: Published versio

Magnetic Response in Quantized Spin Hall Phase of Correlated Electrons

We investigate the magnetic response in the quantized spin Hall (SH) phase of layered-honeycomb lattice system with intrinsic spin-orbit coupling lambda_SO and on-site Hubbard U. The response is characterized by a parameter g= 4 U a^2 d / 3, where a and d are the lattice constant and interlayer distance, respectively. When g< (sigma_{xy}^{s2} mu)^{-1}, where sigma_{xy}^{s} is the quantized spin Hall conductivity and mu is the magnetic permeability, the magnetic field inside the sample oscillates spatially. The oscillation vanishes in the non-interacting limit U -> 0. When g > (sigma_{xy}^{s2} mu)^{-1}, the system shows perfect diamagnetism, i.e., the Meissner effect occurs. We find that superlattice structure with large lattice constant is favorable to see these phenomena. We also point out that, as a result of Zeeman coupling, the topologically-protected helical edge states shows weak diamagnetism which is independent of the parameter g.Comment: 7 pages, the final version will be published in J. Phys. Soc. Jp

The Meissner effect in a strongly underdoped cuprate above its critical temperature

The Meissner effect and the associated perfect "bulk" diamagnetism together with zero resistance and gap opening are characteristic features of the superconducting state. In the pseudogap state of cuprates unusual diamagnetic signals as well as anomalous proximity effects have been detected but a Meissner effect has never been observed. Here we have probed the local diamagnetic response in the normal state of an underdoped La1.94Sr0.06CuO4 layer (up to 46 nm thick, critical temperature Tc' < 5 K) which was brought into close contact with two nearly optimally doped La1.84Sr0.16CuO4 layers (Tc \approx 32 K). We show that the entire 'barrier' layer of thickness much larger than the typical c axis coherence lengths of cuprates exhibits a Meissner effect at temperatures well above Tc' but below Tc. The temperature dependence of the effective penetration depth and superfluid density in different layers indicates that superfluidity with long-range phase coherence is induced in the underdoped layer by the proximity to optimally doped layers; however, this induced order is very sensitive to thermal excitation.Comment: 7 pages, 7 figures + Erratu