arXiv subjects

Andrey Chubukov

Publications and source records attributed to Andrey Chubukov.

29 records · Page 2Linked to original sources

arXiv2015

Superconducting and charge-density-wave orders in the spin-fermion model: a comparative analysis

We present comparative analysis of superconducting and charge-density-wave orders in the spin-fluctuation scenario for the cuprates. That spin-fluctuation exchange gives rise to d-wave superconductivity is well known. Several groups recently argued that the same spin-mediated interaction may also account for charge-density-wave order with momenta $(Q,0)$ or $(0,Q)$, detected in underdoped cuprates. This has been questioned on the basis that charge-density-wave channel mixes fermions from both nested and anti-nested regions on the Fermi surface, and fermions in the anti-nested region do not have a natural tendency to form a bound state, even if the interaction is attractive. We show that anti-nesting is not an obstacle for charge order, but to see this one needs to go beyond the conventional Eliashberg approximation. We show that in the prefect nesting/antinesting case, when the velocities of hot fermions are either parallel or antiparallel, the onset temperatures in superconducting and charge-density-wave channels are of comparable strength for any magnetic correlation length $ξ$. The superconducting $T_{\rm sc}$ is larger than $T_{\rm cdw}$, but only numerically. When the velocities of hot fermions are not strictly parallel/antiparallel, $T_{\rm cdw}$ progressively decreases as $ξ$ decreases and vanishes at some critical $ξ$.

cond-mat.str-el↗

arXiv2015

Coexistence of charge-density-wave and pair-density-wave orders in underdoped cuprates

We analyze incommensurate charge-density-wave (CDW) and pair-density-wave (PDW) orders with transferred momenta $(\pm Q,0)$/$(0,\pm Q)$ in underdoped cuprates within the spin-fermion model. Both orders appear due to exchange of spin fluctuations before magnetic order develops.We argue that the ordered state with the lowest energy has non-zero CDW and PDW components with the same momentum. Such a state breaks $C_4$ lattice rotational symmetry, time-reversal symmetry, and mirror symmetries. We argue that the feedback from CDW/PDW order on fermionic dispersion is consistent with ARPES data. We discuss the interplay between the CDW/PDW order and $d_{x^2-y^2}$ superconductivity and make specific predictions for experiments.

cond-mat.str-el↗

arXiv2015

Interplay between pair-density-wave and charge-density-wave orders in underdoped cuprates

We analyze the interplay between charge-density-wave (CDW) and pair-density-wave (PDW) orders within the spin-fermion model for the cuprates. We specifically consider CDW order with transferred momenta $(\pm Q,0)$/$(0,\pm Q)$, and PDW order with total momenta $(0,\pm Q)/(\pm Q,0)$. We show that both emerge in the spin-fermion model near the onset of antiferromagnetism. We further argue that the two orders are nearly degenerate due to an approximate SU(2) particle-hole symmetry of the model. The ${\rm SU}(2)$ symmetry becomes exact if one neglects the curvature of the Fermi surface in hot regions, in which case ${\rm U}(1)$ CDW and PDW order parameters become components of an SO(4)-symmetric PDW/CDW "super-vector". We develop a Ginzburg-Landau theory for PDW/CDW order parameters and find two possible ground states: a "stripe" state, and a "checkerboard" state. We show that the ${\rm SO}(4)$ symmetry between CDW and PDW is broken by two effects. One is the inclusion of Fermi surface curvature, which selects a PDW order immediately below the instability temperature. Another is the overlap between different hot regions, which favors CDW order at low temperatures. For the stripe state, we show that the competition between the two effects gives rise to a first-order transition from PDW to CDW inside the ordered state. We also argue that beyond mean-field theory, the onset temperature for CDW order is additionally enhanced due to feedback from a preemptive breaking of ${\mathbb Z}_2$ time-reversal symmetry. We discuss the ground state properties of a pure PDW state and a pure CDW state, and show that the PDW checkerboard state yields a vortex-anti-vortex lattice. For the checkerboard state, we considered a situation when both CDW and PDW orders are present at low $T$ and show that the presence of both condensates induces a long sought chiral $s+id_{xy}$ superconductivity.

cond-mat.str-el↗

arXiv2015

Reply to "A strong coupling critique of spin fluctuation driven charge order in underdoped cuprates"

We reply to the criticism from the authors of arXiv:1502.02782 of the spin-fluctuation scenario for charge order in the cuprates. The authors of of arXiv:1502.02782 argued that spin-fluctuation exchange cannot give rise to charge order with observed momentum $(Q, 0)/(0, Q)$ due to the absence of nesting for a half of fermions involved. We explicitly show the instability towards charge order exists even in the "worst-case" scenario of anti-nesting for a half of fermions.

cond-mat.str-el↗

arXiv2014

Spontaneous currents in a superconductor with s+is symmetry

We analyze $s + is$ state proposed as a candidate superconducting state for strongly hole-doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$. Such a state breaks time-reversal symmetry (TRS) but does not break any other discrete symmetry. We address the issue whether TRS breaking alone can generate spontaneous currents near impurity sites, which could be detected in, e.g., muSR experiments. We argue that there are no spontaneous currents if only TRS is broken. However, supercurrents do emerge if the system is put under external strain and C4 lattice rotation symmetry is externally broken.

cond-mat.supr-con↗

arXiv2013

Quantum-critical pairing in electron-doped cuprates

We revisit the problem of spin-mediated superconducting pairing at the antiferromagnetic quantum-critical point with the ordering momentum (π,π) = 2k_F. The problem has been previously considered by one of the authors. However, it was later pointed out that that analysis neglected umklapp processes for the spin polarization operator. We incorporate umklapp terms and re-evaluate the normal state self-energy and the critical temperature of the pairing instability. We show that the self-energy has a Fermi-liquid form and obtain the renormalization of the quasiparticle residue Z, the Fermi velocity, and the curvature of the Fermi surface. We argue that the pairing is a BCS-type problem, but go one step beyond the BCS theory and calculate the critical temperature T_c with the prefactor. We apply the results to electron-doped cuprates near optimal doping and obtain T_c \geq 10 K, which matches the experimental results quite well.

cond-mat.supr-con↗

arXiv2012

Magnetism in parent Fe-chalcogenides: quantum fluctuations select a plaquette order

We analyze magnetic order in iron-chalcogenide Fe$_{1+y}$Te -- the parent compound of high-temperature superconductor Fe$_{1+y}$Te$_{1-x}$Se$_x$. Neutron scattering experiments show that magnetic order in this material contains components with momentum $Q_1=(π/2, π/2)$ and $Q_2 =(π/2, -π/2)$ in Fe-only Brillouin zone. The actual spin order depends on the interplay between these two components. Previous works argued that spin order is a single-$Q$ state (either $Q_1$ or $Q_2$). Such an order breaks rotational $C_4$ symmetry and order spins into a double diagonal stripe. We show that quantum fluctuations actually select another order -- a double $Q$ plaquette state with equal weight of $Q_1$ and $Q_2$ components, which preserves $C_4$ symmetry but breaks $Z_4$ translational symmetry. We argue that the plaquette state is consistent with recent neutron scattering experiments on Fe$_{1+y}$Te.

cond-mat.str-el↗

arXiv2011

Chiral superconductivity from repulsive interactions in doped graphene

We identify graphene as a system where chiral superconductivity can be realized. Chiral superconductivity involves a pairing gap that winds in phase around the Fermi surface, breaking time reversal symmetry. We consider a unique situation arising in graphene at a specific level of doping, where the density of states is singular, strongly enhancing the critical temperature T_c. At this doping level, the Fermi surface is nested, allowing superconductivity to emerge from repulsive electron-electron interactions. We show using a renormalization group method that superconductivity dominates over all competing orders for any choice of weak repulsive interactions. Superconductivity develops in a doubly degenerate, spin singlet channel, and a mean field calculation indicates that the superconductivity is of a chiral d+id type. We therefore predict that doped graphene can provide experimental realization of spin-singlet chiral superconductivity.

cond-mat.mes-hall↗

arXiv2008

Temperature dependence of the nodal Fermi velocity in layered cuprates

We explain recently observed linear temperature dependence of the nodal Fermi velocity $v_F (T)$ in near-optimally doped cuprates. We argue that it originates from electron-electron interaction, and is a fundamental property of an arbitrary 2D Fermi liquid. We consider a spin-fermion model with the same parameters as in earlier studies, and show that the T term is about 30% at 300K, in agreement with the data. We show that the sub-leading term in $v_F (T)$ is a regular (and small) $T^2$ correction. We also show that at a $2k_F$ quantum-critical point, temperature corrections to the dispersion are singular.

cond-mat.supr-con↗

arXiv2005

Thermodynamics of a Fermi liquid in a magnetic field

We present calculations of the non-analytic terms in the spin susceptibility chi_s(T) and the specific heat C(T) to systems in a magnetic field. Without a field, chi_s(T) and C(T)/T are linear in T in 2D, while in 3D, chi_s(T) is proportional to T^2 and C(T)/T proportional to T^2 logT. We show that in a magnetic field, the linear in T terms in 2D become scaling functions of mu_B H/T. We present explicit expressions for these functions and show that at high fields, mu_B H >> T, chi_s(T,H) scales as |H|. We also show that in 3D, chi_s(T,H) becomes non-analytic in a field and at high fields scales as H^2 log|H|.

cond-mat.str-el↗

arXiv2003

Condensation energy in phonon and spin mediated superconductors - strong coupling approach

We consider the condensation energy, $E_c$ of strongly coupled, magnetically-mediated superconductors within the context of the spin fermion model. We argue that although experimentally obtained values of $E_c$ are of the same order of magnitude as would be expected from BCS theory in optimally and overdoped cuprates, this agreement is coincidental. The actual physics behind the condensation energy is much richer. In particular, we argue that it is vital to take both the fermionic and bosonic contributions to the condensation energy into account when considering such materials, and that it is only the sum of the two contributions $E_c$ which has physical meaning. Both the experimental and our theoretically calculated condensation energies exhibit a {\it decrease} with further underdoping past optimal doping, e.g. in the mid to strong coupling regime. Below optimal doping, the physics is qualitatively different from BCS theory as the gain in the condesation energy is a reult of the feedback on spin excitations, while the fermionic contribution to $E_c$ is positive due to an "undressing" feedback on the fermions. We argue that the same feedback effect accounts for a gain in the kinetic energy at strong coupling.

cond-mat↗