Large Pseudogap Research Articles

Observation and quantification of the pseudogap in unitary Fermi gases.

The microscopic origin of high-temperature superconductivity in cuprates remains unknown. It is widely believed that substantial progress could be achieved by better understanding of the pseudogap phase, a normal non-superconducting state of cuprates1,2. In particular, a central issue is whether the pseudogap could originate from strong pairing fluctuations3. Unitary Fermi gases4,5, in which the pseudogap-if it exists-necessarily arises from many-body pairing, offer ideal quantum simulators to address this question. Here we report the observation of a pair-fluctuation-driven pseudogap in homogeneous unitary Fermi gases of lithium-6 atoms, by precisely measuring the fermion spectral function through momentum-resolved microwave spectroscopy and without spurious effects from final-state interactions. The temperature dependence of the pairing gap, inverse pair lifetime and single-particle scattering rate are quantitatively determined by analysing the spectra. We find a large pseudogap above the superfluid transition temperature. The inverse pair lifetime exhibits a thermally activated exponential behaviour, uncovering the microscopic virtual pair breaking and recombination mechanism. The obtained large, temperature-independent single-particle scattering rate is comparable with that set by the Planckian limit6. Our findings quantitatively characterize the pseudogap in strongly interacting Fermi gases and they lend support for the role of preformed pairing as a precursor to superfluidity.

Superconducting gap and pseudogap in the surface states of the iron-based superconductor PrFeAsO1−y studied by angle-resolved photoemission spectroscopy

In order to study the possible superconductivity at the polar surfaces of 1111-type iron-based superconductors, which is doped with a large amount of holes in spite of the electron doping in bulk materials, we have performed angle-resolved photoemission spectroscopy (ARPES) studies on superconducting ${\mathrm{PrFeAsO}}_{1\ensuremath{-}y}$ crystals. We have indeed observed the opening of a superconducting gap on surface-derived hole pockets as well as on a bulk-derived hole pocket. The superconducting gap is found to open on the surface-derived hole pockets below the bulk ${T}_{\text{c}}$, which suggests that the surface superconductivity is possibly induced by proximity effect from the bulk. We have also observed the opening of a large pseudogap on the surface-derived hole pockets, which is similar to the pseudogap in 122-type bulk superconductors doped with a smaller amount of holes. This suggests that the opening of a large pseudogap is a characteristic property of hole-doped iron-based superconductors.

Incoherent Cooper Pairing and Pseudogap Behavior in Single-Layer FeSe/SrTiO3

In many unconventional superconductors, the presence of a pseudogap - a suppression in the electronic density of states extending above the critical temperature - has been a long-standing mystery. Here, we employ combined \textit{in situ} electrical transport and angle-resolved photoemission spectroscopy (ARPES) measurements to reveal an unprecedentedly large pseudogap regime in single-layer FeSe/SrTiO$_3$, an interfacial superconductor where incoherent Cooper pairs are initially formed above $T_{\Delta}$ $\approx$ 60 K, but where a zero resistance state is only achieved below $T_{0}$ $<$ 30 K. We show that this behavior is accompanied by distinct transport signatures of two-dimensional phase fluctuating superconductivity, suggesting a mixed vortex state hosting incoherent Cooper pairs which persist well above the maximum clean limit $T_{c}$ of $\approx$ 40 K. Our work establishes the critical role of reduced dimensionality in driving the complex interplay between Cooper pairing and phase coherence in two-dimensional high-$T_c$ superconductors, providing a paradigm for understanding and engineering higher-$T_{c}$ interfacial superconductors.

Paraconductivity of pseudogapped superconductors

We calculate Aslamazov-Larkin paraconductity $\sigma_{AL}(T)$ for a model of strongly disordered superconductors (dimensions $d=2,3$) with a large pseudogap whose magnitude strongly exceeds transition temperature $T_c$. We show that, within Gaussian approximation over Cooper-pair fluctuations, paraconductivity is just twice larger that the classical AL result at the same $\epsilon = (T-T_c)/T_c$. Upon decreasing $\epsilon$, Gaussian approximation is violated due to local fluctuations of pairing fields that become relevant at $\epsilon \leq \epsilon_1 \ll 1 $. Characteristic scale $\epsilon_1 $ is much larger than the width $\epsilon_2$ of the thermodynamical critical region, that is determined via the Ginzburg criterion, $\epsilon_2 \approx \epsilon_1^d$. We argue that in the intermediate region $\epsilon_2 \leq \epsilon \leq \epsilon_1$ paraconductivity follows the same AL power law, albeit with another (yet unknown) numerical prefactor. At further decrease of the temperature, all kinds of fluctuational corrections become strong at $\epsilon \leq \epsilon_2$; in particular, conductivity occurs to be strongly inhomogeneous in real space.

Atomic-scale distortions and temperature-dependent large pseudogap in thin films of the parent iron-chalcogenide superconductor Fe1+yTe

We investigate with scanning tunneling microscopy/spectroscopy (STM/STS) and density functional theory (DFT) calculations the surface structures and the electronic properties of Fe1+yTe thin films grown by pulsed laser deposition. Contrary to the regular arrangement of antiferromagnetic nanostripes previously reported on cleaved single-crystal samples, the surface of Fe1+yTe thin films displays a peculiar distribution of spatially inhomogeneous nanostripes. Both STM and DFT calculations show the bias-dependent nature of such features and support the interpretation of spin-polarized tunneling between the FeTe surface and an unintentionally magnetized tip. In addition, the spatial inhomogeneity is interpreted as a purely electronic effect related to changes in hybridization and Fe–Fe bond length driven by local variations in the concentration of excess interstitial Fe cations.Unexpectedly, the surface density of states measured by STS strongly evolves with temperature in close proximity to the antiferromagnetic–paramagnetic first-order transition, and reveals a large pseudogap of 180–250 meV at about 50–65 K. We believe that in this temperature range a phase transition takes place, and the system orders and locks into particular combinations of orbitals and spins because of the interplay between excess interstitial magnetic Fe and strongly correlated d-electrons.

Collective modes and ultrasonic attenuation in a pseudogapped superconductor

We develop a theory of ultrasound decay rate $\alpha$ in a model of a superconductor with a large pseudogap $\Delta_P$, and show that at low temperatures ($T \ll T_c$) the magnitude of the decay rate $\alpha$ is controlled by the ratio of $T/\Delta$, where $\Delta \ll \Delta_P$ is a superconducting collective gap. Thus we propose new method to measure the collective gap $\Delta$ in a situation when strong pseudogap is present.

Pseudogap Phenomena Near the BKT Transition of a Two-Dimensional Ultracold Fermi Gas in the Crossover Region

We investigate strong-coupling properties of a two-dimensional ultracold Fermi gas in the normal phase. In the three-dimensional case, it has been shown that the so-called pseudogap phenomena can be well described by a (non-self-consistent) $T$-matrix approximation (TMA). In the two-dimensional case, while this strong coupling theory can explain the pseudogap phenomenon in the strong-coupling regime, it unphysically gives large pseudogap size in the crossover region, as well as in the weak-coupling regime. We show that this difficulty can be overcome when one improve TMA to include higher order pairing fluctuations within the framework of a self-consistent $T$-matrix approximation (SCTMA). The essence of this improvement is also explained. Since the observation of the BKT transition has recently been reported in a two-dimensional $^6$Li Fermi gas, our results would be useful for the study of strong-coupling physics associated with this quasi-long-range order.

Experimental and First‐Principles Studies of the Ternary Borides Ta3Ru5B2 and M3–xRu5+xB2 (M = Zr, Hf)

AbstractTa3Ru5B2, Zr2.86(5)Ru5.14(5)B2, and Hf2.83(2)Ru5.17(2)B2 have been successfully synthesized as polycrystalline powders as well as single crystals and characterized by powder and single‐crystal X‐ray diffraction and EDX analysis. Furthermore, their electronic structures and bonding characteristics have been analyzed by DFT calculations. The three phases are isotypic and crystallize with a Ti3Co5B2‐type structure (space group P4/mbm). Their electronic densities of states, which all possess a large pseudogap at the Fermi level, closely follow the rigid band model. Bonding analyses indicated that Ru–B and Ru–M (M = Zr, Hf, Ta) heteroatomic interactions are mainly responsible for their structural stability and that the homoatomic M–M interactions behave differently when connecting either two tetragonal or two pentagonal Ru prisms at the same distance.

Effect of the pseudogap on the infrared response in cuprate superconductors

One of the most essential aspects of cuprate superconductors is a large pseudogap coexisting with a superconducting gap, then some anomalous properties can be understood in terms of the formation of the pseudogap. Within the kinetic energy-driven superconducting mechanism, the effect of the pseudogap on the infrared response of cuprate superconductors in the superconducting state is studied. By considering the interplay between the superconducting gap and pseudogap, the electron current–current correlation function is evaluated based on the linear response approach and it then is employed to calculate finite-frequency conductivity. It is shown that in the underdoped and optimally doped regimes, the transfer of the part of the low-energy spectral weight of the conductivity spectrum to the higher energy region to form a midinfrared band is intrinsically associated with the presence of the pseudogap.

Effect of the pseudogap on the transition temperature in the cuprates and implications for its origin

One of the most intriguing aspects of cuprates is a large pseudogap coexisting with a high superconducting transition temperature. Here, we study pairing in the cuprates from electron-electron interactions by constructing the pair vertex using spectral functions derived from angle resolved photoemission data for a near optimal doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ sample that has a pronounced pseudogap. Assuming that that the pseudogap is {\it not} due to pairing, we find that the superconducting instability is strongly suppressed, in stark contrast to what is actually observed. Using an analytic approximation for the spectral functions, we can trace this suppression to the destruction of the BCS logarithmic singularity from a combination of the pseudogap and lifetime broadening. Our findings strongly support those theories of the cuprates where the pseudogap is instead due to pairing.

Nature of the Bad Metallic Behavior ofFe1.06TeInferred from Its Evolution in the Magnetic State

We investigate with angle-resolved photoelectron spectroscopy the changes of the Fermi surface and the main bands from the paramagnetic state to the antiferromagnetic (AFM) state occurring below 72 K in Fe1.06Te. The evolution is completely different from that observed in Fe pnictides, as nesting is absent. The AFM state is a rather good metal, in agreement with our magnetic band structure calculation. On the other hand, the paramagnetic state is very anomalous with a large pseudogap of ~65 meV on the electron pocket that closes in the AFM state. We discuss this behavior in connection with spin fluctuations existing above the magnetic transition and the correlations predicted in the spin-freezing regime of the incoherent metallic state.

Comment on “High-field studies of superconducting fluctuations in high-Tccuprates: Evidence for a small gap distinct from the large pseudogap”

By using high magnetic field data to estimate the background conductivity, Rullier-Albenque and coworkers have recently published [Phys.Rev.B 84, 014522 (2011)] experimental evidence that the in-plane paraconductivity in cuprates is almost independent of doping. In this Comment we also show that, in contrast with their claims, these useful data may be explained at a quantitative level in terms of the Gaussian-Ginzburg-Landau approach for layered superconductors, extended by Carballeira and coworkers to high reduced-temperatures by introducing a total-energy cutoff [Phys.Rev.B 63, 144515 (2001)]. When combined, these two conclusions further suggest that the paraconductivity in cuprates is conventional, i.e., associated with fluctuating superconducting pairs above the mean-field critical temperature.

Enhanced superconducting proximity effect in strongly correlated heterostructures

The electronic properties of a strongly correlated heterostructure consisting of $t\text{\ensuremath{-}}J$ layer and metallic layer have been investigated by using the Gutzwiller projected mean-field approximation. Considering the proximity effect due to the large pseudogap energy scale of $t\text{\ensuremath{-}}J$ layer, a large superconducting gap could be induced on the metallic layer. This enhanced superconducting gap may be even larger than that of the $t\text{\ensuremath{-}}J$ layer. Related physical quantities including spectral functions and density of states are obtained. The consequences of these results on experiments are discussed.

Andreev and Single-Particle Tunneling Spectra of Underdoped Cuprate Superconductors

We study tunneling spectroscopy between a normal metal and an underdoped cuprate superconductor modeled by a phenomenological theory in which the pseudogap is a precursor to the undoped Mott insulator. In the low barrier tunneling limit, the spectra are enhanced by Andreev reflection only within a voltage region of the small superconducting energy gap. In the high barrier tunneling limit, the spectra show a large energy pseudogap associated with single particle tunneling. Our theory semiquantitatively describes the two gap behavior observed in tunneling experiments.

A comparative scanning tunneling spectroscopy investigation of the (12110)-surface of decagonal Al–Ni–Co and the (100)-surface of its approximant Y–Al–Ni–Co

We present a low-temperature scanning tunneling spectroscopy study of the local density of states (LDOS) on the (12110)-surface of the decagonal quasicrystal d–Al–Ni–Co and the (100)-surface of its structurally closely related approximant Y–Al–Ni–Co in the range of ±1 eV around the Fermi energy. The LDOS of both surfaces are dominated by a large parabolic pseudogap centered at the Fermi energy, which can be attributed to the Hume–Rothery stabilization mechanism or to an orbital hybridization. Superimposed on this large pseudogap, a spatially varying fine structure is revealed, whose spatial distribution correlates with the structures of both of the surfaces. This fine structure shows narrow peaks and pseudogaps exhibiting typical energy widths between 50 and 150 meV. The spatial extent of these localized states is of the order of the width of the topographic features, which is about 0.45 nm. Our findings show that specific electronic states are localized on equivalent topographic features regardless of the presence of periodic or quasiperiodic long-range order. We interpret this result as an indication that the fine structure in the LDOS is dominated much more by the complex short-range order than by the presence of the quasiperiodic long-range order.

Large pseudogap and nodal superconducting gap inBi2Sr2−xLaxCuO6+δandBi2Sr2CaCu2O8+δ: Scanning tunneling microscopy and spectroscopy

In the present work, scanning tunneling microscopy/spectroscopy (STM/STS) measurements were carried out on underdoped ${\text{Bi}}_{2}{\text{Sr}}_{2\ensuremath{-}x}{\text{La}}_{x}{\text{CuO}}_{6+\ensuremath{\delta}}$ and ${\text{Bi}}_{2}{\text{Sr}}_{2}{\text{CaCu}}_{2}{\text{O}}_{8+\ensuremath{\delta}}$ to clarify the origin of the pseudogap, in particular, the inhomogeneous large pseudogap. The nodal part of a $d$-wave pairing gap, which is under no influence of the inhomogeneous large pseudogap, was also examined by relating the homogeneous bottom part of the STS gap to a nodal $d$-wave gap in momentum space. We report that the inhomogeneous large pseudogap in the antinodal region links to a two-dimensional electronic charge order, and that the gap size of the nodal $d$-wave part ${\ensuremath{\Delta}}_{\text{sc}}$ scales with the superconducting critical temperature ${T}_{\text{c}}$ in the pseudogap regime.

Strong energy–momentum dispersion of phonon-dressed carriers in the lightly doped band insulator SrTiO3

Much progress has been made recently in the study of the effects of electron–phonon (el–ph) coupling in doped insulators using angle-resolved photoemission (ARPES), yielding evidence for the dominant role of el–ph interactions in underdoped cuprates. As these studies have been limited to doped Mott insulators, the important question arises as to how this compares with doped band insulators where similar el–ph couplings should be at work. The archetypical case is that of perovskite SrTiO3 (STO), well known for its giant dielectric constant of 10 000 at low temperatures, exceeding that of La2CuO4 by a factor of 500. Based on this fact, it has been suggested that doped STO should be the archetypical bipolaron superconductor. Here we report an ARPES study from high-quality surfaces of lightly doped STO. In comparison to lightly doped Mott insulators, we find the signatures of only moderate el–ph coupling; a dispersion anomaly associated with the low-frequency optical phonon with a λ ′ ∼0.3 and an overall bandwidth renormalization suggesting an overall λ ′ ∼0.7 coming from the higher frequency phonons. Furthermore, we find no clear signatures of the large pseudogap or small-polaron phenomena. These findings demonstrate that a large dielectric constant itself is not a good indicator of el–ph coupling and highlight the unusually strong effects of the el–ph coupling in doped Mott insulators.

Superconducting Gap and Pseudogap Structure in LaFeAsO1-xFxProbed by STM/STS

We report the electron tunneling study on LaFeAsO 1- x F x ( x =0.07) using low temperature UHV-STM/STS. Tunneling spectra below T c clearly shows the gap structure. We found three anomalies as kinks in the tunneling differential conductance curve: 1) Superconducting gap structure Δ SC =6 meV. 2) Small pseudogap Δ PGS =20 meV. 3) Large pseudogap Δ PGL =100–150 meV. Corresponding value of 2Δ SC / k B T c =5.8 suggests strong coupling superconductivity. Tunneling spectra at T =80 K show dip structure. The kink around V =150 mV still exist.

TUNNELING STUDY ON Ba2Ca3Cu4O8(O1-xFx)2

We report the tunneling conductance on a quadruple-layered cuprate, Ba 2 Ca 3 Cu 4 O 8( O 1-x F x)2 (2x~1.4, Tc~100 K ) that includes inequivalent outer (OP) and inner (IP) CuO 2 planes in a unit cell. Typical tunneling conductance displays the spectral dip structures as well as the superconducting gap (SCG) with clear coherence peaks. However we sometimes observe a pseudogap (PG) like structure without coherence peak at 4.2K. The magnitude of PG at 4.2K is larger than that of SCG at 4.2K and its conductance curve displays a faint structure at low energy. Therefore its origin would be different from that of small-PG observed at above Tc. In addition, we find that there are two kinds of SCGs that originate from IP and OP, and this result suggests that the superconducting property at IP is different from that at OP because of the imbalance of doped carrier between IP and OP. The observation of large PG at low temperature suggests the existence of some form of hidden ordered state at T&lt;Tc and it would coexist with superconductivity.

Strong charge fluctuations manifested in the high-temperature Hall coefficient of high-Tccuprates

By measuring the Hall coefficient R_H up to 1000 K in La_2CuO_4, Pr_{1.3}La_{0.7}CuO_4, and La_{2-x}Sr_xCuO_4 (LSCO), we found that the temperature (T) dependence of R_H in LSCO for x = 0 - 0.05 at high temperature undoubtedly signifies a gap over which the charge carriers are thermally activated, which in turn indicates that in lightly-doped cuprates strong charge fluctuations are present at high temperature and the carrier number is not a constant. At higher doping (x = 0.08 - 0.21), the high-temperature R_H(T) behavior is found to be qualitatively the same, albeit with a weakened temperature dependence, and we attempt to understand its behavior in terms of a phenomenological two-carrier model where the thermal activation is considered for one of the two species. Despite the crude nature of the model, our analysis gives a reasonable account of R_H both at high temperature and at 0 K for a wide range of doping, suggesting that charge fluctuations over a gap remain important at high temperature in LSCO deep into the superconducting doping regime. Moreover, our model gives a perspective to understand the seemingly contradicting high-temperature behavior of R_H and the in-plane resistivity near optimum doping in a consistent manner. Finally, we discuss possible implications of our results on such issues as the scattering-time separation and the large pseudogap.