arXiv subjects

Hiroyuki Yamase

Publications and source records attributed to Hiroyuki Yamase.

At least 55 records · Page 3Linked to original sources

arXiv2016

Coexistence of incommensurate magnetism and superconductivity in the two-dimensional Hubbard model

We analyze the competition of magnetism and superconductivity in the two-dimensional Hubbard model with a moderate interaction strength, including the possibility of incommensurate spiral magnetic order. Using an unbiased renormalization group approach, we compute magnetic and superconducting order parameters in the ground state. In addition to previously established regions of Neel order coexisting with d-wave superconductivity, the calculations reveal further coexistence regions where superconductivity is accompanied by incommensurate magnetic order.

cond-mat.str-el↗

arXiv2015

$d$-wave bond-order charge excitations in electron-doped cuprates

We study charge excitation spectra in the two-dimensional $t$-$J$ model on a square lattice to explore a charge-order tendency recently found in electron-doped cuprates around the carrier density 0.15. The static susceptibility of $d$-wave charge density, which corresponds to the nematic susceptibility at the momentum transfer ${\bf q}=(0,0)$, shows two characteristic peaks at momenta of the form ${\bf q}_{1}=(q',q')$ and ${\bf q}_{2}=(q,0)$. These two peaks originate from the so-called $2k_{F}$ scattering processes enhanced by the $d$-wave character of the bond-charge density. The peak at ${\bf q}_{1}$ is much broader, but develop to be very sharp in the vicinity of its instability, whereas the peak at ${\bf q}_{2}$ becomes sharper with decreasing temperature, but does not diverge. The equal-time correlation function, which is measured by resonant x-ray scattering, exhibits a momentum dependence similar to the static susceptibility. We also present energy-resolved charge excitation spectra. The spectra show a V-shaped structure around ${\bf q}=(0,0)$ and bend back toward close to zero energy due to the charge-order tendency at ${\bf q}_{1}$ and ${\bf q}_{2}$. The resulting spectra form gap-like features with a maximal gap at ${\bf q} \approx {\bf q}_{1}/2$ and ${\bf q}_{2}/2$. We discuss implications for the recent experiments in electron-doped cuprates.

cond-mat.str-el↗

arXiv2015

Spin nematic fluctuations near a spin-density-wave phase

We study an interacting electronic system exhibiting a spin nematic instability. Using a phenomenological form for the spin fluctuation spectrum near the spin-density-wave (SDW) phase, we compute the spin nematic susceptibility in energy and momentum space as a function of temperature and the magnetic correlation length xi. The spin nematic instability occurs when xi reaches a critical value xi_{cr}, i.e., its transition temperature T_{SN} is always higher than the SDW critical temperature T_{SDW}. In particular, xi_{cr} decreases monotonically with increasing T_{SN}. Concomitantly, low-energy nematic fluctuations are present in a wider temperature region as T_{SN} becomes higher. Approaching the spin nematic instability, the nematic spectral function at zero momentum exhibits a central peak as a function of energy for a finite temperature and a soft mode at zero temperature. These properties originate from the general feature that the imaginary part of the spin-fluctuation bubble has a term linear in energy and its coefficient is proportional to the square of temperature. Furthermore we find that the nematic spectral function exhibits a diffusive peak around zero momentum and zero energy without clear dispersive features. A possible phase diagram for the spin nematic and SDW transitions is also discussed.

cond-mat.str-el↗

arXiv2014

Strong particle-hole asymmetry of charge instabilities in doped Mott insulators

We study possible charge instabilities in doped Mott insulators by employing the two-dimensional t-J model with a positive value of the next nearest-neighbor hopping integral t' on a square lattice, which is applicable to electron-doped cuprates. Although the d-wave charge density wave (flux phase) and d-wave Pomeranchuk instability (nematic order) are dominant instabilities for a negative t' that corresponds to hole-doped cuprates, we find that those instabilities are strongly suppressed and become relevant only rather close to half filling. Instead, various types of bond orders with modulation vectors close to (pi,pi) are dominant in a moderate doping region. Phase separation is also enhanced, but it can be suppressed substantially by the nearest-neighbor Coulomb repulsion without affecting the aforementioned charge instabilities.

cond-mat.supr-con↗

arXiv2014

Electronic nematic phase transition in the presence of anisotropy

We study the phase diagram of electronic nematic instability in the presence of xy anisotropy. While a second order transition cannot occur in this case, mean-field theory predicts that a first order transition occurs near van Hove filling and its phase boundary forms a wing structure, which we term a Griffiths wing, referring to his original work of He3-He4 mixtures. When crossing the wing, the anisotropy of the electronic system exhibits a discontinuous change, leading to a meta-nematic transition, i.e., the analog to a meta-magnetic transition in a magnetic system. The upper edge of the wing corresponds to a critical end line, which shows a non-monotonic temperature dependence as a function of the external anisotropy and vanishes at a quantum critical end point for a strong anisotropy. The mean-field phase diagram is, however, very sensitive to fluctuations of the nematic order parameter, yielding a topologically different phase diagram. The Griffiths wing is broken into two pieces. A tiny wing appears close to zero anisotropy and the other is realized for a strong anisotropy. Consequently three quantum critical end points are realized. We discuss that these results can be related to various materials including a cold atom system.

cond-mat.str-el↗

arXiv2013

Superconductivity from orbital nematic fluctuations

Recent experiments suggest that besides of antiferromagnetic fluctuations nematic fluctuations may contribute to the occurrence of superconductivity in iron pnictides. Motivated by this observation we study superconductivity from nematic fluctuations in a minimal two-band model. The employed band parameters are appropriate for iron pnictides and lead to four pockets for the Fermi line. It is shown that low-energy, long-wavelength nematic fluctuations within the pockets give rise to strong-coupling superconductivity whereas the large momenta density fluctuations between pockets are rather irrelevant. The obtained transition temperatures are similar to those typically found in the pnictides and are rather robust against repulsive Coulomb interactions. The superconducting and nematic states coexist in a large region of the phase diagram.

cond-mat.supr-con↗

arXiv2013

Electronic Raman scattering from orbital nematic fluctuations

We compute Raman scattering intensities via the lowest-order coupling to the bosonic propagator associated with orbital nematic fluctuations in a minimal model for iron pnictides. The model consists of two bands on a square lattice exhibiting four Fermi pockets and a transition from the normal to a nematic state. It is shown that the orbital fluctuations produce in the B1g channel strong quasi-elastic light scattering around the nematic critical temperature Tn, both above and below Tn. This holds for the A1g symmetry only below Tn whereas no low-energy scattering from orbital fluctuations is found in the B2g symmetry. Due to the nematic distortion the electron pocket at the X-point may disappear at low temperatures. Such a Lifshitz transition causes in the B2g spectrum a large upward shift of spectral weight in the high energy region whereas no effect is seen in the other symmetries.

cond-mat.str-el↗

arXiv2012

Mean-field theory on a coupled system of ferromagnetism and electronic nematic order

We analyze an effective model on a square lattice with two types of forward scattering interactions, which, respectively, drive ferromagnetism (FM) and electronic nematic order via a d-wave Pomeranchuk instability (dPI). The FM and dPI in general compete with each other and they are typically separated by a first order phase boundary in the plane of the chemical potential and temperature. Nevertheless there is a parameter region where the dPI occurs inside the FM phase, leading to their coexistence. We also study the effect of a magnetic field by choosing a chemical potential where the ground state is paramagnetic without a field. In this case, instead of FM, the dPI competes with a metamagnetic instability. The latter occurs above a threshold strength of the FM interaction and otherwise the dPI is stabilized with a dome-shaped phase diagram in the plane of a magnetic field and temperature. The FM interaction shifts the center of the dome to a lower field, accompanied by a substantial reduction of the field range where the dPI is stabilized and by an extension of the first order part of the transition line, although the maximal critical temperature does not change. The experimental phase diagram of the bilayer ruthenate Sr3Ru2O7 can be well captured by the present theory.

cond-mat.str-el↗

arXiv2012

Possible charge instabilities in two-dimensional doped Mott insulators

Motivated by the growing evidence of the importance of charge fluctuations in the pseudogap phase in high-temperature cuprate superconductors, we apply a large-N expansion formulated in a path integral representation of the two-dimensional t-J model on a square lattice. We study all possible charge instabilities of the paramagnetic state in leading order of the 1/N expansion. While the d-wave charge density wave (flux phase) becomes the leading instability for various choices of model parameters, we find that a d-wave Pomeranchuk (electronic nematic phase) instability occurs as a next leading one. In particular, the nematic state has a strong tendency to become inhomogeneous. In the presence of a large second nearest-neighbor hopping integral, the flux phase is suppressed and the electronic nematic instability becomes leading in a high doping region. Besides these two major instabilities, bond-order phases occur as weaker instabilities close to half-filling. Phase separation is also detected in a finite temperature region near half-filling.

cond-mat.str-el↗

arXiv2012

Fermi surface truncation from thermal nematic fluctuations

We analyze how thermal fluctuations near a finite temperature nematic phase transition affect the spectral function $A({\bf k},ω)$ for single-electron excitations in a two-dimensional metal. Perturbation theory yields a splitting of the quasi-particle peak with a d-wave form factor, reminiscent of a pseudogap. We present a resummation of contributions to all orders in the Gaussian fluctuation regime. Instead of a splitting, the resulting spectral function exhibits a pronounced broadening of the quasi-particle peak, which varies strongly around the Fermi surface and vanishes upon approaching the Brillouin zone diagonal. The Fermi surface obtained from a Brillouin zone plot of $A({\bf k},0)$ seems truncated to Fermi arcs.

cond-mat.str-el↗

arXiv2011

Multilayer cuprate superconductors as possible systems described by resonating-valence-bond and antiferromagnetic orders

Coexistence of antiferromagnetism and d-wave superconductivity within a CuO_2 plane was recently observed in a wide doping region for multilayer high-temperature cuprate superconductors. We find that the experimental phase diagram is well reproduced in the slave-boson mean-field scheme of the two-dimensional t-J model by including antiferromagnetic order. We argue that weak three dimensionality coming from a multilayer structure is sufficient to stabilize antiferromagnetic order and its coexistence with superconductivity.

cond-mat.supr-con↗

arXiv2011

Nematic Quantum Criticality Without Order

We consider a two-dimensional interacting Fermi system which displays a nematic phase within mean-field theory. The system is analyzed using a non-perturbative renormalization-group scheme. We find that order-parameter fluctuations can suppress the nematic order obtained in mean-field theory even at zero temperature. For a suitable choice of parameters a quantum critical point surrounded by a disordered phase can be realized, giving rise to quantum critical behavior in the absence of an ordered regime in the phase diagram.

cond-mat.str-el↗

arXiv2010

Raman scattering near a d-wave Pomeranchuk instability

Motivated by recent transport and neutron scattering experiments suggesting an orientational symmetry breaking in underdoped cuprates we present a theoretical study of Raman scattering near a d-wave Pomeranchuk instability (PI). The d-wave component of Raman scattering from electrons and phonons allows to study directly order parameter fluctuations associated with the PI. Approaching the PI from the normal state by lowering the temperature a central peak emerges both in electronic and, as an additional low-frequency feature, in phononic scattering. Approaching the PI in the superconducting state at low temperature by decreasing the doping concentration the central peak is replaced by a soft mode with strongly decreasing width and energy and increasing spectral weight. These predicted low-energy features in Raman scattering could confirm in a rather direct way the presence of a PI in high-temperature cuprate superconductors and in Sr3Ru2O7.

cond-mat.str-el↗

arXiv2010

Singular non-ordering susceptibility at a Pomeranchuk instability

We study magnetic susceptibilities of two-dimensional itinerant electron systems exhibiting symmetry-breaking Fermi surface distortions, the so-called d-wave Pomeranchuk instability, in a magnetic field. In a pure forward scattering model, the longitudinal susceptibility chi^{zz} is found to exhibit a jump at a critical point. The magnitude of this jump diverges at a tricritical point. When scattering processes involving finite momentum transfers are allowed for, chi^{zz} is expected to diverge also at a critical point. The system displays multiple critical fluctuations. We argue that the features of chi^{zz} are general properties associated with singularities of a non-ordering susceptibility, leading to implications for a variety of materials including Sr_3Ru_2O_7.

cond-mat.str-el↗

arXiv2009

Spontaneous Fermi surface symmetry breaking in bilayered systems

We perform a comprehensive numerical study of d-wave Fermi surface deformations (dFSD) on a square lattice, the so-called d-wave Pomeranchuk instability, including bilayer coupling. Since the order parameter corresponding to the dFSD has Ising symmetry, there are two stacking patterns between the layeres, (+,+) and (+,-). This additional degree of freedom gives rise to a rich variety of phase diagrams. The phase diagrams are classified by means of the energy scale Lambda_{z}, which is defined as the bilayer splitting at the saddle points of the in-plane band dispersion. As long as Lambda_{z} ne 0, a major stacking pattern is usually (+,-), and (+,+) stacking is stabilized as a dominant pattern only when the temperature scale of the dFSD instability becomes much smaller than Lambda_z. For Lambda_{z}=0, the phase diagram depends on the precise form of the bilayer dispersion. We also analyze the effect of a magnetic field on the bilayer model in connection with a possible dFSD instability in the bilyared ruthenate Sr_3Ru_2O_7.

cond-mat.str-el↗

arXiv2009

Theory of reduced singlet pairing without the underlying state of charge stripes in the high-temperature superconductor YBa_2Cu_3O_6.45

Recently, a strongly enhanced xy anisotropy of magnetic excitations was observed in YBa_2Cu_3O_y (YBCO_y) with y=6.45 and Tc=35 K [Science 319, 597 (2008)]. Unlike the observation in YBCO_6.6 and YBCO_6.85, the anisotropy grows to be pronounced at lower temperature and at lower energy, and is not suppressed by the onset of superconductivity. We propose that the effect of singlet pairing is substantially reduced in YBCO_6.45. This reduction concomitantly enhances an order competing with singlet pairing, a strong tendency of the so-called d-wave Pomeranchuk instability, leading to the magnetic excitations observed experimentally.

cond-mat.str-el↗

arXiv2009

Self-masking of spontaneous symmetry breaking in layer materials

We study d-wave Fermi surface deformations (dFSD), the so-called Pomeranchuk instability, on bilayer and infinite-layer square lattices. Since the order parameter of the dFSD has Ising symmetry, there are two stacking patterns along the c axis: (+,+) and (+,-). We find that, as long as the c axis dispersion is finite at the saddle points of the in-plane band dispersion, the (+,-) stacking is usually favored independently on the details of interlayer coupling, yielding no macroscopic anisotropy. The dFSD provides unique spontaneous symmetry breaking that is self-masked in layer materials.

cond-mat.str-el↗

arXiv2007

Effect of magnetic field on spontaneous Fermi surface symmetry breaking

We study magnetic field effects on spontaneous Fermi surface symmetry breaking with d-wave symmetry, the so-called d-wave "Pomeranchuk instability''. We use a mean-field model of electrons with a pure forward scattering interaction on a square lattice. When either the majority or the minority spin band is tuned close to the van Hove filling by a magnetic field, the Fermi surface symmetry breaking occurs in both bands, but with a different magnitude of the order parameter. The transition is typically of second order at high temperature and changes to first order at low temperature; the end points of the second order line are tricritical points. This qualitative picture does not change even in the limit of a large magnetic field, although the magnetic field substantially suppresses the transition temperature at the van Hove filling. The field produces neither a quantum critical point nor a quantum critical end point in our model. In the weak coupling limit, typical quantities characterizing the phase diagram have a field-independent single energy scale while its dimensionless coefficient varies with the field. The field-induced Fermi surface symmetry breaking is a promising scenario for the bilayer ruthenate Sr3Ru2O7, and future issues are discussed to establish such a scenario.

cond-mat.str-el↗