Pseudospin Singlet Research Articles

Quantum Monte Carlo Study of Weyl Superconductivity

Using auxiliary-field quantum Monte Carlo we study superconductivity in a simple extension of the two-dimensional negative-U Hubbard model which hosts Weyl nodes near the Fermi surface. We discuss the model and our method as applied to the study of superconductivity in this system. We discuss the nature of pairing between Weyl quasiparticles which carry opposite spin and opposite topological charge leading to a spin-singlet pairing amplitude and to a mixture of pseudo-spin singlet and pseudo-spin triplet pairing.

Knight Shift and Leading Superconducting Instability from Spin Fluctuations in Sr_{2}RuO_{4}.

Recent nuclear magnetic resonance studies [A. Pustogow etal., Nature 574, 72 (2019)] have challenged the prevalent chiral triplet pairing scenario proposed for Sr_{2}RuO_{4}. To provide guidance from microscopic theory as to which other pair states might be compatible with the new data, we perform a detailed theoretical study of spin fluctuation mediated pairing for this compound. We map out the phase diagram as a function of spin-orbit coupling, interaction parameters, and band structure properties over physically reasonable ranges, comparing when possible with photoemission and inelastic neutron scattering data information. We find that even-parity pseudospin singlet solutions dominate large regions of the phase diagram, but in certain regimes spin-orbit coupling favors a near-nodal odd-parity triplet superconducting state, which is either helical or chiral depending on the proximity of the γ band to the van Hove points. A surprising near degeneracy of the nodal s^{'} and d_{x^{2}-y^{2}} wave solutions leads to the possibility of a near-nodal time-reversal symmetry broken s^{'}+id_{x^{2}-y^{2}} pair state. Predictions for the temperature dependence of the Knight shift for fields in and out of plane are presented for all states.

Numerically exact study of Weyl superconductivity

We study the interplay of interactions and topology in a pseudo-spin Weyl system, obtained from a minimally modified Hubbard model, using the numerically exact auxiliary-field quantum Monte Carlo method complemented by mean-field theory. We find that the pseudo-spin plays a key role in the pairing mechanism, and its effect is reflected in the structure of the pairing amplitude. An attractive on-site interaction leads to pairing between quasiparticles carrying opposite spin and opposite topological charge, resulting in the formation of real-spin singlet pairs that are a mixture of pseudo-spin singlet and pseudo-spin triplet. Our results provide a detailed characterization of the exotic pairing behavior in this system, and represent an important step towards a more complete understanding of superconductivity in the context of topological band structures, which will help guide searches for topological superconductivity in real materials and ultracold atoms.

Symmetry-Enforced Line Nodes in Unconventional Superconductors.

We classify line nodes in superconductors with strong spin-orbit interactions and time-reversal symmetry, where the latter may include nonprimitive translations in the magnetic Brillouin zone to account for coexistence with antiferromagnetic order. We find four possible combinations of irreducible representations of the order parameter on high-symmetry planes, two of which allow for line nodes in pseudospin-triplet pairs and two that exclude conventional fully gapped pseudospin-singlet pairs. We show that the former can only be realized in the presence of band-sticking degeneracies, and we verify their topological stability using arguments based on Clifford algebra extensions. Our classification exhausts all possible symmetry protected line nodes in the presence of spin-orbit coupling and a (generalized) time-reversal symmetry. Implications for existing nonsymmorphic and antiferromagnetic superconductors are discussed.

A novel superconductivity in Ir oxides with a large spin-orbit coupling

Motivated by novel 5d-electron systems such as Sr2IrO4 and Ba2IrO4, we study the ground state of a three-orbital Hubbard model with a spin-orbit coupling by using a variational Monte Carlo method. The obtained ground-state phase diagram reveals that an antiferromagnetic state, stable at around the electron density n = 5, is destabilized by carrier doping and that the ground state becomes superconducting. Similar to high-T c cuprates, a large asymmetry of the phase diagram between electron doping (n > 5) and hole doping (n < 5) is also found, which can be understood as the band structure effect. Due to the large spin-orbit coupling which entangles spin and orbital degrees of freedom, the pseudospin becomes a good quantum number, and the superconductivity found here is characterized by the \(d_{x^2 - y^2 }\)-wave “pseudospin singlet” pairing.

Monte Carlo Study of an Unconventional Superconducting Phase in Iridium OxideJeff=1/2Mott Insulators Induced by Carrier Doping

Based on a microscopic theoretical study, we show that novel superconductivity is induced by carrier doping in layered perovskite Ir oxides where a strong spin-orbit coupling causes an effective total angular momentum J(eff)=1/2 Mott insulator. Using a variational Monte Carlo method, we find an unconventional superconducting state in the ground state phase diagram of a t(2g) three-orbital Hubbard model on the square lattice. This superconducting state is characterized by a d(x(2)-y(2))-wave "pseudospin singlet" formed by the J(eff)=1/2 Kramers doublet, which thus contains interorbital as well as both singlet and triplet components of t(2g) electrons. The superconducting state is found stable only by electron doping, but not by hole doping, for the case of carrier doped Sr2IrO4. We also study an effective single-orbital Hubbard model to discuss the similarities to high-T(c) cuprate superconductors and the multiorbital effects.

UNUSUAL QUANTUM HALL EFFECTS, INDEX THEOREM AND SUPERSYMMETRY IN GRAPHENE

We exploit an index theorem and a high degree of symmetry to understand unusual quantum Hall effects of the n = 0 Landau level in graphene. The high symmetry such as the pseudospin symmetry of Fermi points in a graphene sheet cannot couple to an external magnetic field. In the absence of the magnetic field, the index theorem provides a relation between the zero-energy state of the graphene sheet and the topological deformation of the compact lattice. Under the topological deformation, the zero-energy state emerges naturally without the Zeeman splitting at the Fermi points in the graphene sheet. This results in the fact that the pseudospin is an exact symmetry. In the case of nonzero energy, the up-spin and down-spin states have the exact high symmetry of spin, forming the pseudospin singlet pairing. We describe the peculiar and unconventional quantum Hall effects of the n = 0 Landau level in graphene on the basis of the index theorem and the high degree of symmetry.

µSR Studies of Pu Metal and the Pu-based Superconductor PuCoGa5

We present recent measurements of the magnetic properties of Pu metal and the superconducting properties of PuCoGa 5 using the µSR technique. Our measurements set the most stringent upper limits to date on the magnitude of the ordered moments µ ord in α-Pu and δ-stabilized Pu (alloyed with 4.3 at. % Ga) in zero applied field: µ ord ≤10 -3 µ B at T \cong4 K. Measurements of the in-plane magnetic-field penetration depth λ( T ) in PuCoGa 5 ( T c =18.5 K) for 0.06 T applied field (≈2-5 × H c1 ) show that the temperature dependence of the superfluid density, and therefore Δλ( T )=λ( T )-λ(0), are ∝ T for T / T c ≤0.5. We estimate that λ(0)=241(3) nm. We also find no evidence for a time-reversal-symmetry violating superconducting order parameter. Taken together the measurements in PuCoGa 5 are, therefore, consistent with an even-parity (pseudo-spin singlet), d-wave pairing state.

Formula omitted] studies of the superconducting order parameter in [formula omitted

We present transverse-field (TF) measurements of the in-plane magnetic-field penetration depth λ ( T ) in single-crystalline PuCoGa 5 ( T c = 18.5 K ) for 0.06 T applied field ( ≈ 2 – 5 × H c 1 ) . We find that the temperature dependence of the superfluid density, and therefore Δ λ ( T ) = λ ( T ) - λ ( 0 ) , is ∝ T for T / T c ⩽ 0.5 . We estimate that the measured λ ( 0 ) = 241 ( 3 ) nm . ZF measurements find no evidence for time-reversal symmetry violation. The ZF and TF measurements are consistent with an even-parity (pseudo-spin singlet), d-wave pairing state.

Pseudo-spin band in the odd-odd nucleus 172Lu

High-spin states in the odd-odd nucleus 172Lu have been populated in a 170Er(7Li, 5n) reaction and the emitted \(\gamma\)-radiation was detected with the GASP array. Two sequences of a new identical band have been observed with the transition energies in the favoured and unfavoured sequences being identical within \(\approx 3\) keV at low spins and \(\approx 1\) keV at high spins over the whole observed spin range. Aninterpretation as a pseudo-spin singlet band of \(\pi 1/2^- [541] \otimes \nu 1/2^- [\widetilde{420}]\) configuration is proposed. It represents the best example of a pseudo-spin singlet band in normal deformed nuclei known until now.

Alignment and pseudospin symmetry

Pseudospin symmetry has been invoked as a possible explanation for the ``quantized alignment'' observed in some superdeformed ``identical'' bands. The clearest case involves the $1/2[301]$ orbital (a pseudospin singlet, $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\Lambda}}=0$), where pseudospin alignment provides an explanation for some identical bands in the mass-150 region. Pseudospin doublets $(\ensuremath{\Omega}=\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\Lambda}}\ifmmode\pm\else\textpm\fi{}1/2)$ can also generate quantized alignments and such an explanation, with $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\Lambda}}=1,$ has recently been proposed for the band in ${}^{191}\mathrm{Au}$ identical to ${}^{192}\mathrm{Hg}.$ The present work examines whether the data on the normally deformed nuclei support such an interpretation and concludes that pseudospin alignment is plausible for states with $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\Lambda}}=0$ or 1.

Rigorous results for the one-dimensional symmetric Anderson model.

It is shown that the ground state of the symmetric one-dimensional periodic Anderson model on a bipartite lattice with periodic boundary conditions is a pseudospin singlet and has total momentum equal to zero. \textcopyright{} 1996 The American Physical Society.

E1g model of superconducting UPt3.

The phase diagram of superconducting UPt$_3$ is explained in a Ginzburg-Landau theory starting from the hypothesis that the order parameter is a pseudo-spin singlet which transforms according to the $E_{1g}$ representation of the $D_{6h}$ point group. We show how to compute the positions of the phase boundaries both when the applied field is in the basal plane and when it is along the c-axis. The experimental phase diagrams as determined by longitudinal sound velocity data can be fit using a single set of parameters. In particular the crossing of the upper critical field curves for the two field directions and the apparent isotropy of the phase diagram are reproduced. The former is a result of the magnetic properties of UPt$_3$ and their contribution to the free energy in the superconducting state. The latter is a consequence of an approximate particle-hole symmetry. Finally we extend the theory to finite pressure and show that, in contrast to other models, the $E_{1g}$ model explains the observed pressure dependence of the phase boundaries.