Electron-like Fermi Surface Research Articles

Seebeck Coefficient in a Cuprate Superconductor: Particle-Hole Asymmetry in the Strange Metal Phase and Fermi Surface Transformation in the Pseudogap Phase

We report measurements of the Seebeck effect in both the $ab$ plane ($S_{\rm a}$) and along the $c$ axis ($S_{\rm c}$) of the cuprate superconductor La$_{1.6-x}$Nd$_{0.4}$Sr$_{x}$CuO$_4$ (Nd-LSCO), performed in magnetic fields large enough to suppress superconductivity down to low temperature. We use the Seebeck coefficient as a probe of the particle-hole asymmetry of the electronic structure across the pseudogap critical doping $p^{\star} = 0.23$. Outside the pseudogap phase, at $p = 0.24 > p^{\star}$, we observe a positive and essentially isotropic Seebeck coefficient as $T \rightarrow 0$. That $S > 0$ at $p = 0.24$ is at odds with expectations given the electronic band structure of Nd-LSCO above $p^{\star}$ and its known electron-like Fermi surface. We can reconcile this observation by invoking an energy-dependent scattering rate with a particle-hole asymmetry, possibly rooted in the non-Fermi liquid nature of cuprates just above $p^{\star}$. Inside the pseudogap phase, for $ p < p^{\star}$, $S_{\rm a}$ is seen to rise at low temperature as previously reported, consistent with the drop in carrier density $n$ from $n \simeq 1 + p$ to $n \simeq p$ across $p^{\star}$ as inferred from other transport properties. In stark contrast, $S_{\rm c}$ at low temperature becomes negative below $p^{\star}$, a novel signature of the pseudogap phase. The sudden drop in $S_{\rm c}$ reveals a change in the electronic structure of Nd-LSCO upon crossing $p^{\star}$. We can exclude a profound change of the scattering across $p^{\star}$ and conclude that the change in the out-of-plane Seebeck coefficient originates from a transformation of the Fermi surface.

Ferromagnetic fluctuations in the Rashba-Hubbard model

We study the occurrence and the origin of ferromagnetic fluctuations in the longitudinal spin susceptibility of the $t$-$t'$-Rashba-Hubbard model on the square lattice. The combined effect of the second-neighbor hopping $t'$ and the spin-orbit coupling leads to ferromagnetic fluctuations in a broad filling region. The spin-orbit coupling splits the energy bands, leading to two van Hove fillings, where the sheets of the Fermi surface change their topology. Between these two van Hove fillings the model shows ferromagnetic fluctuations. We find that these ferromagnetic fluctuations originate from interband contributions to the spin susceptibility. These interband contributions only arise if there is one holelike and one electronlike Fermi surface, which is the case for fillings in between the two van Hove fillings. We discuss implications for experimental systems and propose a test on how to identify these types of ferromagnetic fluctuations in experiments.

Effects of a k⃗-dependent Hybridisation on the Fermi surface of an extended d–p Hubbard model

ABSTRACTThe topology of the Fermi surface of an extended d–p Hubbard model is investigated using Green's function technique in a n-pole approximation. The effects of the d–p hybridisation on the Fermi surface are the main focus in the present work. Nevertheless, the effects of doping, Coulomb interaction and hopping to second-nearest-neighbours on the Fermi surface, are also studied. Particularly, it is shown that the crossover from hole-like to electron-like Fermi surface (Lifshitz transition) is deeply affected by the d–p hybridisation. Moreover, the pseudogap present in the low doping regime is also affected by the hybridisation. The results show that both the doping and the hybridisation act in the sense of suppresses the pseudogap. Therefore, the systematic investigation of the Fermi surface topology, shows that not only the doping but also the hybridisation can be considered as a control parameter for both the pseudogap and the Lifshitz transition. Assuming that the hybridisation is sensitive to external pressure, the present results agree qualitatively with recent experimental data for the cuprate Nd-LSCO.

Electronic Structure of In3-x Se4 Electron Transport Layer for Chalcogenide/p-Si Heterojunction Solar Cells.

In this article, we perform density functional theory calculation to investigate the electronic and optical properties of newly reported In3–xSe4 compound using CAmbridge Serial Total Energy Package (CASTEP). Structural parameters obtained from the calculations agree well with the available experimental data, indicating their stability. In the band structure of In3–xSe4 (x = 0, 0.11, and, 0.22), the Fermi level (EF) crossed over several bands in the conduction bands, which is an indication of the n-type metal-like behavior of In3–xSe4 compounds. On the other hand, the band structure of In3–xSe4 (x = 1/3) exhibits semiconducting nature with a band gap of ∼0.2 eV. A strong hybridization among Se 4s, Se 4p and In 5s, In 5p orbitals for In3Se4 and that between Se 4p and In 5p orbitals were seen for β-In2Se3 compound. The dispersion of In 5s, In 5p and Se 4s, Se 4p orbitals is responsible for the electrical conductivity of In3Se4 that is confirmed from DOS calculations as well. Moreover, the bonding natures of In3–xSe4 materials have been discussed based on the electronic charge density map. Electron-like Fermi surface in In3Se4 ensures the single-band nature of the compound. The efficiency of the In3–xSe4/p-Si heterojunction solar cells has been calculated by Solar Cell Capacitance Simulator (SCAPS)-1D software using experimental data of In3–xSe4 thin films. The effect of various physical parameters on the photovoltaic performance of In3–xSe4/p-Si solar cells has been investigated to obtain the highest efficiency of the solar cells. The optimized power conversion efficiency of the solar cell is found to be 22.63% with VOC = 0.703 V, JSC = 38.53 mA/cm2, and FF = 83.48%. These entire theoretical predictions indicate the promising applications of In3–xSe4 two-dimensional compound to harness solar energy in near future.

Disappearance of Superconductivity and a Concomitant Lifshitz Transition in Heavily Overdoped Bi2Sr2CuO6 Superconductor Revealed by Angle-Resolved Photoemission Spectroscopy**Supported by the National Key Research and Development Program of China under Grant Nos 2016YFA0300300 and 2017YFA0302900, the Strategic Priority Research Program (B) of Chinese Academy of Sciences under Grant Nos XDB07020300 and XDB25000000, the National Basic Research Program of China under Grant

By partially doping Pb to effectively suppress the superstructure in single-layered cuprate Bi2Sr2CuO6+δ (Pb-Bi2201) and annealing them in vacuum or in high pressure oxygen atmosphere, a series of high quality Pb-Bi2201 single crystals are obtained with Tc covering from 17 K to non-superconducting in the overdoped region. High resolution angle resolved photoemission spectroscopy measurements are carried out on these samples to investigate the evolution of the Fermi surface topology with doping in the normal state. Clear and complete Fermi surfaces are observed and quantitatively analyzed in all of these overdoped Pb-Bi2201 samples. A Lifshitz transition from hole-like Fermi surface to electron-like Fermi surface with increasing doping is observed at a doping level of ∼0.35. This transition coincides with the change that the sample undergoes superconducting-to-non-superconducting states. Our results reveal the emergence of an electron-like Fermi surface and the existence of a Lifshitz transition in heavily overdoped Bi2201 samples. This provides important information in understanding the connection between the disappearance of superconductivity and the Lifshitz transition in the overdoped region.

Externally Controlled Magnetism and Band Gap in Twisted Bilayer Graphene.

We theoretically study the effects of electron-electron interaction in twisted bilayer graphene in atransverse dc electric field. When the twist angle is not very small, the electronic spectrum of the bilayer consists of four Dirac cones inherited from each graphene layer. An applied bias voltage leads to the appearance of two holelike and two electronlike Fermi surface sheets with perfect nesting among electron and hole components. Such a band structure is unstable with respect to the exciton band-gap opening due to the screened Coulomb interaction. The exciton order parameter is accompanied by spin-density-wave order. The gap depends on the twist angle and can be varied by a bias voltage. This result correlates well with recent transport measurements [J.-B. Liu etal., Sci. Rep. 5, 15285 (2015)SRCEC32045-232210.1038/srep15285]. Our proposal allows the coexistence of (i)an externally controlled semiconducting gap and (ii)a nontrivial multicomponent magnetic order. This is interesting for both fundamental research and applications.

Reduced electronic correlation effects in half substituted Ba(Fe1−xCox)2As2

We report a comprehensive study of the tridimensional nature and orbital character of the low-energy electronic structure in 50% Cobalt doped Ba(Fe1−xCox)2As2 (d6.5), by using polarization- and photon energy-dependent angle-resolved photoemission spectroscopy. An extra electron-like Fermi surface is observed around the Brillouin zone boundary compared with isoelectronic KyFe2−xSe2 (d6.5). The bands near the Fermi level (EF) are mainly derived from Fe/Co 3d t2g orbitals, revealing visible dispersions along the kz direction. In combination with the local density approximation and the dynamical mean-field theory calculations, we find that the As 4p bands are non-renormalized and the whole 3d band needs to be renormalized by a “single” factor of ∼1.6, indicating moderate electronic correlation effects. The “single” factor description of the correlation strength among the different 3d orbitals is also in sharp contrast to orbital-dependent correlation effects in BaFe2As2. Our findings indicate a remarkable reduction of correlation effects with little difference among 3d orbitals in BaFeCoAs2, due to the increased filling of the electronic 3d shell in the presence of significant Hund's coupling. The results support that the electronic correlation effects and multiple orbital physics play an important role in the superconductivity of the 122 system and in other ferropnictides.

Stable Weyl points, trivial surface states, and particle-hole compensation in WP2

A possible connection between extremely large magneto-resistance and the presence of Weyl points has garnered much attention in the study of topological semimetals. Exploration of these concepts in transition metal phosphide WP2 has been complicated by conflicting experimental reports. Here we combine angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations to disentangle surface and bulk contributions to the ARPES intensity, the superposition of which has plagued the determination of the electronic structure in WP2. Our results show that while the hole- and electron-like Fermi surface sheets originating from surface states have different areas, the bulk-band structure of WP2 is electron-hole-compensated in agreement with DFT. Furthermore, the detailed band structure is compatible with the presence of at least 4 temperature-independent Weyl points, confirming the topological nature of WP2 and its stability against lattice distortions.

Quantum oscillations in the type-II Dirac semi-metal candidate PtSe2

Three-dimensional topological semi-metals carry quasiparticle states that mimic massless relativistic Dirac fermions, elusive particles that have never been observed in nature. As they appear in the solid body, they are not bound to the usual symmetries of space-time and thus new types of fermionic excitations that explicitly violate Lorentz-invariance have been proposed, the so-called type-II Dirac fermions. We investigate the electronic spectrum of the transition-metal dichalcogenide PtSe2 by means of quantum oscillation measurements in fields up to 65 T. The observed Fermi surfaces agree well with the expectations from band structure calculations, that recently predicted a type-II Dirac node to occur in this material. A hole- and an electron-like Fermi surface dominate the semi-metal at the Fermi level. The quasiparticle mass is significantly enhanced over the bare band mass value, likely by phonon renormalization. Our work is consistent with the existence of type-II Dirac nodes in PtSe2, yet the Dirac node is too far below the Fermi level to support free Dirac–fermion excitations.

Pseudogap phase of cuprate superconductors confined by Fermi surface topology

The properties of cuprate high-temperature superconductors are largely shaped by competing phases whose nature is often a mystery. Chiefly among them is the pseudogap phase, which sets in at a doping p* that is material-dependent. What determines p* is currently an open question. Here we show that the pseudogap cannot open on an electron-like Fermi surface, and can only exist below the doping pFS at which the large Fermi surface goes from hole-like to electron-like, so that p* ≤ pFS. We derive this result from high-magnetic-field transport measurements in La1.6−xNd0.4SrxCuO4 under pressure, which reveal a large and unexpected shift of p* with pressure, driven by a corresponding shift in pFS. This necessary condition for pseudogap formation, imposed by details of the Fermi surface, is a strong constraint for theories of the pseudogap phase. Our finding that p* can be tuned with a modest pressure opens a new route for experimental studies of the pseudogap.

Magneto-transport and electronic structures of BaZnBi2

We report the magneto-transport properties and electronic structures of BaZnBi2. BaZnBi2 is a quasi-two-dimensional material with metallic behavior. Transverse magnetoresistance (MR) depends on magnetic field linearly and exhibits Shubnikov–de Haas (SdH) oscillation at low temperature and high field. The observed linear MR may originate from the disorder in samples or the edge conductivity in compensated two-component systems. The first-principles calculations reveal the absence of stable gapless Dirac fermion. Combined with the trivial Berry phase extracted from the SdH oscillation, BaZnBi2 is suggested as a topologically trivial semimetal. Nearly compensated electron-like Fermi surfaces (FSs) and hole-like FSs coexist in BaZnBi2.

Strain effects on electronic structures of monolayer iron sulphide and selenide

Structural and electronic properties of single atomic layers of FeS and FeSe superconductors have been investigated from first principles within the density functional theory (DFT). For both systems the mixed single-stripe and block-checkerboard type antiferromagnetic ground states are predicted. Namely, the single-stripe and block-checkerboard orders are almost degenerate. The metal-insulator transition in a hypothetical block-checkerboard phase of FeS occurs only under significant tensile strain, i.e. for in-plane lattice parameters larger than 3.8 Å. Compressive strain leads to almost a complete disappearance of magnetism in FeS system, whereas tensile strain strengthens it. The electronic structure of FeS monolayer in nonmagnetic phase is similar to that of FeSe, exhibiting the (π,π) nesting between hole- and electron-like Fermi surface sheets. The findings presented here explain different characters of superconductivity in FeS and FeSe materials and encourage further experimental studies on FeS-based thin films and heterostructures.

Recent progress on the high-pressure studies of FeSe single crystal

Among the iron-based superconductors, the structurally simplest FeSe and FeSe-derived materials have attracted enormous research interest recently due to its unusual electronic properties and the great tunability of the superconducting transition temperature <italic>T</italic>_c. For the bulk FeSe single crystal, it shows a relatively low <italic>T</italic>_c<x content-type=symbol>»</x>8 K within the unusual nonmagnetic nematic phase below <italic>T</italic>_s<x content-type=symbol>»</x>90 K. It has been reported that the application of high pressure can induce a long-range antiferromagnetic order below <italic>T</italic>_m<x content-type=symbol>»</x>20 K at <italic>P</italic><x content-type=symbol>»</x>1 GPa, and then achieve a high <italic>T</italic>_c<x content-type=symbol>»</x>37 K at <italic>P</italic><x content-type=symbol>»</x>8 GPa. It remains unclear how the high-<italic>T</italic>_c superconductivity is achieved in FeSe under pressure. In this regard, a detailed temperature-pressure phase diagram mapping out the exact relationship between the high-<italic>T</italic>_c superconductivity and the normal-state nematic order and the pressure-induced antiferromagnetic order is indispensable for achieving a better understanding. By taking advantage of the cubic anvil cell apparatus that can maintain an excellent hydrostatic pressure condition up to 15 GPa, we recently undertook a comprehensive high-pressure magnetotransport study on high-quality FeSe single crystals and obtained important results. In this article, we give a brief review on the recent progresses about the high-pressure studies of FeSe. By tracking the characteristic resistivity anomalies at the nematic order, the pressure-induced antiferromagnetic order, and the superconducting transition, we constructed for FeSe a comprehensive temperature-pressure phase diagram, which depicts explicitly the detailed evolution of these competing electronic orders. We observed a dome-shaped antiferromagnetic phase boundary <italic>T</italic>_m(<italic>P</italic>), and revealed the competitive relationship between these electronic orders. The superconducting <italic>T</italic>_c experiences two successive step increases at the critical pressures when the nematic order and the antiferromagnetic order are respectively suppressed, and finally the highest <italic>T</italic>_c^max=38.5 K is realized near <italic>P</italic>_c≈6 GPa when the antiferromagnetic order just collapses. We further measured the Hall effect of FeSe to extract the electronic structure information under high pressure. Our Hall data show that the Fermi surface of FeSe in the normal state undergoes a reconstruction at <italic>P</italic><x content-type=symbol>»</x>2 GPa, when the nematic order is suppressed and the antiferromagnetic order just emerges. The dominant charge carriers for the transport properties in the normal state change from the electron to the hole type. In addition, in the high-<italic>T</italic>_c phase near the optimal<italic> </italic><italic>T</italic>_c^max at <italic>P</italic>_c<x content-type=symbol>»</x>6 GPa, the Hall coefficient experiences a dramatic enhancement and the magnetoresistance exhibits anomalous scaling behaviors, signaling the presence of strongly enhanced inter-band spin fluctuations. These results suggest that the critical antiferromagnetic fluctuations play an important role for achieving the high-<italic>T</italic>_c superconductivity in FeSe. Aided by the first-principles calculations, we further confirmed that both electron-like and hole-like Fermi surfaces are present in FeSe under high pressures, in favor of the Fermi surface nesting scenario as in the FeAs-based high-<italic>T</italic>_c<italic> </italic>superconductors. These above results on FeSe suggest great similarities with other FeAs-based superconductors, and thus constitute an important step toward a unified understanding of iron-based superconductivity.

Orbital superconductivity, defects, and pinned nematic fluctuations in the doped iron chalcogenide FeSe0.45Te0.55

We demonstrate that the differential conductance, $dI/dV$, measured via spectroscopic imaging scanning tunneling microscopy in the doped iron chalcogenide FeSe$_{0.45}$Te$_{0.55}$, possesses a series of characteristic features that allow one to extract the orbital structure of the superconducting gaps. This yields nearly isotropic superconducting gaps on the two hole-like Fermi surfaces, and a strongly anisotropic gap on the electron-like Fermi surface. Moreover, we show that the pinning of nematic fluctuations by defects can give rise to a dumbbell-like spatial structure of the induced impurity bound states, and explains the related $C_2$-symmetry in the Fourier transformed differential conductance.

Electronic structure and magnetism of RbGd2Fe4As4O2

Abstract The electronic structure, magnetism, and Fermi surface (FS) nesting of new superconductor RbGd2Fe4As4O2 (12442) were studied by the first-principes calculation. Its calculated ground state is the stripe antiferromagnetic state. The hole-like FSs will overlap with the electron-like FS sheets by the electron doping. Such significant FS nesting induces a peak of the bare susceptibility χ 0 ( q → ) at the X point, suggesting the parent compound RbGd2Fe4As4O2 is a self hole-doping induced superconductor.

Electronic Structure and Magnetism of the Multiband New Superconductor CaRbFe4As4

By first-principles calculations, we investigated the electronic structure, magnetism, and Fermi surface (FS) nesting of the newly discovered superconductor CaRbFe4As4. In the nonmagnetic (NM) state, there are ten bands crossing the Fermi level, which is more complicated than other FeAs-based superconductors, showing a multiband character. The FS consists of six holelike sheets around the Γ-point and four electronlike sheets near the Brillouin zone corner M-point. The holelike FSs will overlap with the electronlike FS sheets, if they are shifted by the vector \(\boldsymbol{{q}} = (\pi ,\pi ,0)\). Such FS nesting induces the bare susceptibility peak \(\chi _{0}(\boldsymbol{{q}})\) at the M-point. Fixed spin moment calculations indicate that CaRbFe4As4 has a strong tendency towards magnetism. Total energy calculations predicted that the ground state of CaRbFe4As4 is the stripe antiferromagnetic state. Therefore, CaRbFe4As4 is strongly similar to other FeAs-based superconductors, although it crystallized wit...

Experimental Investigation of Electronic Structure of La(O,F)BiSe2**Supported by the National Basic Research Program of China under Grant Nos 2015CB921300, 2013CB921700 and 2016YFA0300404, the National Natural Science Foundation of China under Grant Nos 11474340, 11234014, U1532267 and 11674327, and the Chinese Academy of Sciences under Grant No XDB07000000.

La(O,F)BiSe2 is a layered superconductor and has the same crystal structure with La(O,F)BiS2. We investigate the electronic structure of La(O,F)BiSe2 using the angle-resolved photoemission spectroscopy. Two electron-like Fermi surfaces around X(π, 0) are observed, corresponding to the electron doping of 0.23 per Bi site. We clearly resolve anisotropic band splitting along both Γ—X and M—X due to the cooperative effects of large spin-orbit coupling and interlayer coupling. Moreover, we observe an almost non-dispersive electronic state around −0.2eV between the electron-like bands. This state vanishes after in-situ K evaporation, indicating that it could be the localized surface state caused by defects on the cleaved surface.

Self-hole-doping–induced superconductivity in KCa2Fe4As4F2

The electronic structure, magnetism, Fermi surface nesting, and bare susceptibility of the superconductor KCa2Fe4As4F2 are discussed in relation to the Fe-pnictide superconductors based on the first-principle calculations. There are ten bands crossing the Fermi level in the nonmagnetic (NM) states, resulting in six hole-like Fermi surface (FS) sheets along the line and four electron-like FS sheets along the line. It is more complicated than other FeAs-based superconductors, showing multi-bands character. Fixed spin moment calculations and the total energies of different magnetic phase indicate that KCa2Fe4As4F2 has a strong tendency towards magnetism, i.e., the stripe antiferromagnetic state. But the self-hole-doping suppressed the spin-density wave state, and then induced superconductivity in the parent compound. Such self-doping strategy may be used for future exploration of new superconductors.

Observation of a Hidden Hole-Like Band Approaching the Fermi Level in K-Doped Iron Selenide Superconductor

One of the ultimate goals of the study of iron-based superconductors is to identify the common feature that produces the high critical temperature (Tc). In the early days, based on a weak-coupling viewpoint, the nesting between hole- and electron-like Fermi surfaces (FSs) leading to the so-called $s\pm$ state was considered to be one such key feature. However, this theory has faced a serious challenge ever since the discovery of alkali-metal-doped FeSe (AFS) superconductors, in which only electron-like FSs with a nodeless superconducting gap are observed. Several theories have been proposed, but a consistent understanding is yet to be achieved. Here we show experimentally that a hole-like band exists in KxFe2-ySe2, which presumably forms a hole-like Fermi surface. The present study suggests that AFS can be categorized in the same group as iron arsenides with both hole- and electron-like FSs present. This result provides a foundation for a comprehensive understanding of the superconductivity in iron-based superconductors.

Electronic structure and magnetism of ThFeAsN

The electronic structure and magnetic properties of ThFeAsN, a newly discovered superconductor, are investigated by means of first-principles calculations. ThFeAsN shares electronic structure and magnetic properties similar to those of LaOFeAs. Its calculated ground state is the stripe antiferromagnetic state. The hole-like Fermi surfaces (FSs) along the line largely overlap with the electron-like FS along the line with the vector . Such significant FS nesting induces a peak of the bare susceptibility at the M-point.