Electron Pockets Research Articles

High-order harmonic generations in tilted Weyl semimetals

We investigate high-order harmonic generations (HHGs) under comparison of Weyl cones in two types. Due to the hyperboloidal electron pocket structure, strong noncentrosymmetrical generations in high orders are observed around a single type-II Weyl point, especially at zero frequency. Such a remarkable DC signal is proved to have attributions from the intraband transition after spectral decomposition. Under weak pulse electric field, the linear optical response of a non-tilted Weyl cone is consistent with the Kubo theory. With extensive numerical simulations, we conclude that the non-zero chemical potential can enhance the even-order generations, from the slightly tilted system to the over-tilted systems. In consideration of dynamical symmetries, type-I and type-II Weyl cones also show different selective responses under the circularly polarized light. Finally, using a more realistic model containing two pairs of Weyl points, we demonstrate that paired Weyl points with opposite chirality can suppress the overall even-order generations.

Correlations tune the electronic structure of pentalayer nickelates into the superconducting regime

Motivated by the recent discovery of superconductivity in the pentalayer nickelate Nd$_6$Ni$_5$O$_{12}$ [Nature Materials 10.1038], we calculate its electronic structure and superconducting critical temperature. We find that electronic correlations are essential for pushing Nd$_6$Ni$_5$O$_{12}$ into the superconducting doping range as they shift the electron pockets above the Fermi energy. As a consequence, Nd$_6$Ni$_5$O$_{12}$ can be described with a single $d_{x^2-y^2}$ orbital per Ni. Instead, for the bilayer nickelate Nd$_3$Ni$_2$O$_6$ we find correlations to drive the system into a three-orbital regime also involving the Ni $d_{xz,yz}$ states. We suggest, however, that single-orbital physics with optimal doping can be restored by substituting 60% of the trivalent Nd or La by tetravalent Zr.

Anomalous Thermal Transport Driven by Electron–Phonon Coupling in 2D Semiconductor h‐BP

AbstractThe electron–phonon coupling (EPC) in semiconductors is typically much weaker than phonon–phonon scattering and its effect on lattice thermal conductivity κl has long been considered negligible. Herein, using first‐principle calculations, it is discovered that the EPC can be significant or even dominant over the intrinsic phonon–phonon scattering via doping in 2D semiconductor hexagonal boron phosphorus (h‐BP). Filling electron pocket till van Hove singularity gradually strengthens the EPC and consequently diminishes the room‐temperature κl by 25% and 80% for monolayer and bilayer h‐BP, respectively. Strikingly, at high doping levels, the EPC drives phonon transport in bilayer h‐BP to an anomalous regime where κl becomes nearly temperature (T) independent deviated from the intrinsic 1/T trend. This distinctive behavior is governed by the joint effects of horizontal mirror symmetry breaking, and weak phonon–phonon scattering stemming from the predominance of normal processes. Further considering electronic contributions, the abnormal T‐independent thermal conductivity is still reserved, thereby facilitating the experimental exploration of EPC effect on κl. This study unveils the exotic thermal transport phenomenon in one‐atom‐thick 2D semiconductors and offers a unique avenue to manipulate heat flow by externally controlling the EPC, which calls for future experimental verification.

Temperature-driven spin-zero effect in TaAs2

The electrical and thermo-electrical transport properties of the semimetal TaAs2 have been measured in a magnetic field applied along the [−2 0 1] direction. The resulting field dependences of the resistivity as well as the Hall, Seebeck, and Nernst coefficients below T ≈ 100 K can be satisfactorily described within the two-band model consisting of electron and hole pockets. At low temperature, all of the measured coefficients exhibit significant contributions from quantum oscillations. The fast Fourier-transform (FFT) of the oscillatory Nernst signal shows two fundamental frequencies, Fα = 105 T and Fβ = 221 T, and the second harmonic of the latter (F2β = 442 T). The ratio between the FFT amplitudes of Fβ and F2β changes with temperature in an unusual way, indicating the observation of a spin-zero effect caused by temperature change. This is likely related to the substantial temperature dependence of the Landé g-factor, which in turn may stem from the non-parabolic energy dispersion or temperature evolution of the spin-orbit coupling.

Fermi Surface and Mass Renormalization in the Iron-Based Superconductor YFe_{2}Ge_{2}.

Interaction-enhanced carrier masses are central to the phenomenology of iron-based superconductors. Quantum oscillation measurements in the new unconventional superconductor YFe_{2}Ge_{2} resolve all four Fermi surface pockets expected from band structure calculations, which predict an electron pocket in the Brillouin zone corner and three hole pockets enveloping the centers of the top and bottom of the Brillouin zone. Carrier masses reach up to 20 times the bare electron mass and are among the highest ever observed in any iron-based material, accounting for the enhanced heat capacity Sommerfeld coefficient ≃100 mJ/mol K^{2}. Mass renormalization is uniform across reciprocal space, suggesting predominantly local correlations, as in the Hund's metal scenario.

Rhombic Fermi surfaces in a ferromagnetic MnGa thin film with perpendicular magnetic anisotropy

Mn$_{1-x}$Ga$_x$ (MnGa) with the $L1_0$ structure is a ferromagnetic material with strong perpendicular magneto-crystalline anisotropy. Although MnGa thin films have been successfully grown epitaxially and studied for various spintronics devices, fundamental understandings of its electronic structure are still lacking. To address this issue, we have investigated $L1_0$-MnGa thin films using angle-resolved photoemission spectroscopy (ARPES). We have observed a large Fermi surface with a rhombic shape in the $k_x$-$k_y$ plane overlapping neighboring Fermi surfaces. The $k_z$ dependence of the band structure suggests that the band dispersion observed by ARPES comes from the three-dimensional band structure of MnGa folded by a $\sqrt{2} \times \sqrt{2}$ reconstruction. The band dispersion across the corner of the rhombic Fermi surface forms an electron pocket with a weak $k_z$ dependence. The effective mass and the mobility of the bands crossing the Fermi level near the corner are estimated from the ARPES images. Based on the experimental findings, the relationship between the observed band structure and the spin-dependent properties in MnGa-based heterostructures is discussed.

Severe violation of the Wiedemann-Franz law in quantum oscillations of NbP

The thermal conductivity (\ensuremath{\kappa}) of the Weyl semimetal NbP was studied with the thermal gradient and magnetic field applied parallel to the [0 0 1] direction. At low temperatures \ensuremath{\kappa}($B$) exhibits large quantum oscillations with frequencies matching two of several determined from the Shubnikov--de Haas effect measured on the same sample with analogous electrical current and magnetic field orientation. Both frequencies found in \ensuremath{\kappa}($B$) originate from the electron pocket enclosing a pair of Weyl nodes. The amplitude of the oscillatory component of the thermal conductivity turns out to be two orders of magnitude larger than the corresponding value calculated from the electrical conductivity using the Wiedemann-Franz law. Analysis of possible sources of this discrepancy indicates the chiral zero sound effect as a potential cause of its appearance.

Factors related to the skin thicknessof cardiovascular implantable electronic device pockets.

The skin overlying cardiovascular implantable electronic devices (CIEDs) sometimes becomes very thin after implantations, which could cause a device erosion. The factors related to the skin thickness of device pockets have not been elucidated. This study aimed to evaluate the skin thickness of CIED pockets and search for the factors associated with the thickness. Seventeen skin thickness points around the CIED pocket were measured through ultrasonography in each patient. A total of 101 patients (76 ± 11 years, 26 female) were enrolled. The median duration from the implantation to the examination was 95 months (quartile: 52.5-147.5). The median skin thickness overlying the device was 4.1 mm (3.3-5.9). Patients with heart failure and malignancy had thinner skin overlying the CIED than those without. A significant correlation existed between skin thickness and body mass index (BMI), hemoglobin, serum creatinine, estimated glomerular filtration rate (eGFR), and left ventricular ejection fraction. In contrast, age, gender, and device size did not exhibit a significant correlation with skin thickness. A multivariate logistic regression analysis revealed that chronic heart failure and a decrease in the eGFR and BMI were independent predictive factors of "very thin (≦3.3 mm)" skin of the CIED pocket late after an implantation. Aside from a low BMI, the comorbidities (low hemoglobin, heart failure, and renal dysfunction) had a stronger impact on the skin thickness overlying the device than the device size. A careful observation of the device pocket should be performed in patients with those risk factors.

(Invited) Unusual Superconducting Behavior in an Ultrathin Weyl Semimetal

Two dimensional materials are layered materials that may be readily exfoliated down to a single atomic layer, presenting an opportunity to noninvasively, and efficiently, control electrical and magnetic ordering, as well as topology. In this talk, I discuss how topology emerges from the bulk, down to the monolayer and how the influence of an electrostatic gate allows us to identify a unique superconducting state in few-layer Td-MoTe2. In the bulk, MoTe2 is a type II Weyl semimetal with a superconducting transition temperature of 120 mK. First, I will discuss how in the clean limit the superconducting transition temperature is enhanced by a factor of 60x, as compared to bulk, in monolayer Td-MoTe2, while still retaining a low carrier density (~1013/cm2), and a density of states that is comparable to the bulk. I will show how the crystal structure factors into the behavior of the superconducting state under external in-plane magnetic fields and how this can be used to quantify of the spin-orbit coupling, an important factor for determining topology and realizing topological superconducting states. After discussion of the monolayer, I will show that this enhancement remains in bilayer MoTe2, despite the change in symmetry. Strangely, our results indicate that superconductivity in bilayer MoTe2 is much more tunable by an electrostatic gate than that of monolayer MoTe2. In addition, for monolayer the response to high in-plane magnetic fields is distinct from that of other 2D superconductors and reflects the canted spin texture of the electron pockets. In bilayer, there is a strong deviation from this behavior, and instead an anomalous anisotropic behavior is observed with in-plane magnetic fields. These findings have profound implications on 2D superconductivity, where previously strong spin-orbit effects were only considered in the case of out-of-plane spin textures and offer an avenue for future exploration in similarly structured materials.

Cu doping effects on the electronic structure of Fe1−xCuxSe

Using angle-resolved photoemission spectroscopy, we investigated the evolution of the electronic structure of ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{x}\mathrm{Se}$ from $x=0$ to 0.10. We found that the substitution of Fe by Cu introduces extra electron carriers. The hole bands near the $\mathrm{\ensuremath{\Gamma}}$ point were observed to shift downward with increasing doping $x$ and completely sank down below the Fermi level (${E}_{F}$) for $x\ensuremath{\ge}0.05$. Meanwhile, the electron pockets near the M point became larger but lost the spectral weight near ${E}_{F}$. Concomitantly, the effective mass of the electron bands increased with doping. Our results show how a metal-insulator transition behavior occurs upon Cu doping in view of the electronic structure and provide a platform to further investigation on the origin of emergent magnetic fluctuation in ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{x}\mathrm{Se}$.

Enhancing thermoelectric properties in TiNiSi structure-type semimetal ZrNiSi by doping

The orthorhombic TiNiSi structure-type compounds show interesting electronic structures comprising in most cases a pseudogap in the density of states and several small electron and hole pockets at the Fermi energy. These features are promising and can be exploited to test their potential as thermoelectric materials for waste heat conversion. Here, we investigate the effect of electron doping in the semimetallic member $\mathrm{ZrNiSi}$ of this family. We show that by doping with Sb for Si in $\mathrm{ZrNiSi}, S$ and $\ensuremath{\sigma}$ can both be increased simultaneously for initial Sb doping defying the oppositely directed trend commonly observed in most materials. In the doped samples, $\ensuremath{\sigma}$ at $300\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ increases from $1000\phantom{\rule{0.28em}{0ex}}\mathrm{S}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ to as high as $2500\phantom{\rule{0.28em}{0ex}}\mathrm{S}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$; at the same time, the peak value of $S$, which is $\ensuremath{-}20\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{V}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ in $\mathrm{ZrNiSi}$, increases by more than a factor of two. The simultaneous enhancement of $\ensuremath{\sigma}$ and $S$ has been explained using the first-principles density functional theory based band structure calculations. The as-cast (i.e., unannealed) ${\mathrm{ZrNiSi}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}$ samples show phase segregation due to a spinodal-type decomposition with two coexisting TiNiSi structure-type phases with different Sb/Si ratios. The thermal conductivity ($\ensuremath{\kappa}$) in the doped samples drops significantly from $12\phantom{\rule{0.28em}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ ($x=0$) to nearly $2\phantom{\rule{0.28em}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ ($x=0.2$) at 300 K. As a result, the peak thermoelectric figure of merit ($zT$) increases from 0.005 $(x=0)$ to 0.023 $(x=0.2)$. Further enhancement in $zT$ is obtained by codoping of Hf (Zr site) and Sb (Si site), which improves the phase stability and chemical homogeneity while keeping the thermal conductivity still very low due to Zr-Hf point mass fluctuation, resulting in a peak $zT$ value of 0.055, i.e., almost an order of magnitude higher value than for the pristine $\mathrm{ZrNiSi}$. We show that the thermoelectric properties of TiNiSi structure-type semimetals can be enhanced by aliovalent doping. This principle can be employed on other members of the TiNiSi to improve the $zT$ in this family of compounds further.

Resonant torque differential magnetometry with high frequency quartz oscillators.

Sensitive magnetometry has been a powerful probe for investigating quantum materials. Extreme conditions, such as sub-kelvin cryogenic temperatures and ultrahigh magnetic fields, demand further durability for sensitive magnetometry. However, significant mechanical vibrations and rapid magnetic field changes give enormous challenges to conventional magnetometry. This article presents a possible solution to this problem by developing a new magnetometry technique using high-frequency quartz oscillators. The technique takes advantage of the symmetry and geometry of mechanical vibration configurations of standard commercially available MHz quartz oscillators, and the setup keeps the high quality factor resonance with the sample mounted on the oscillator. We further demonstrate the sensitivity of the technique using bismuth single crystals and a Fe0.25TaS2 ferromagnetic material. Quantum oscillations are observed in the magnetometry response below 1T, and the detected oscillation frequency is shown to come from the electron pockets of the bismuth.

Nontrivial Topological States in BaSn5 Superconductor Probed by de Haas–van Alphen Quantum Oscillations

We report the nontrivial topological states in an intrinsic type-II superconductor BaSn 5 (T c ∼ 4.4 K) probed by measuring the magnetization, specific heat, de Haas–van Alphen (dHvA) effect, and by performing first-principles calculations. The first-principles calculations reveal a topological nodal ring structure centered at the H point in the k z = π plane of the Brillouin zone, which could be gapped by spin-orbit coupling (SOC), yielding relatively small gaps below and above the Fermi level of about 0.04 eV and 0.14 eV, respectively. The SOC also results in a pair of Dirac points along the Γ–A direction, located at ∼ 0.2 eV above the Fermi level. The analysis of the dHvA quantum oscillations supports the calculations by revealing a nontrivial Berry phase originating from the hole and electron pockets related to the bands forming the Dirac cones. Thus, our study provides an excellent avenue for investigating the interplay between superconductivity and nontrivial topological states.

Enhanced s⁎-wave component of the superconducting gap in overdoped HTSC cuprates with orthorhombic distortion

In this work, the concentration and temperature dependence of the superconducting gap of the HTSC cuprate in the orthorhombic phase are obtained within the framework of the Hubbard model taking into account the exchange mechanism of pairing. To elucidate the reasons for the unexpectedly large s⁎-wave component of the superconducting gap in the overdoped regime, the electronic structure of low-energy excitations and the k-dependent contributions of pair states to the gap components were investigated. It is shown that the s⁎-wave component growth results from the shallowing of the electron pocket in the region of the s⁎-wave component maximum.

Procalcitonin as a biomarker of cardiac implantable electronic device pocket infection: a prospective validation study

Abstract Funding Acknowledgements Type of funding sources: None. Introduction The implantation of cardiac implantable electronic devices (CIEDs) such as pacemakers and implantable cardioverter-defibrillators is increasing along with the complexitly of these devices. CIED infection is an uncommon, but severe complication associated with the presence of a device and is associated with a high mortality and morbidity. Lead-related infections and frank endocarditis are associated with a systemic inflammatory response and, in general, are readily identified. Isolated pocket infections do not produce such a systemic response and are thus more complex to diagnose. There is a reliance on clinical accumen and examination of local signs of infection. There is thus a need for a reliable biomaker to help identify cases of pocket infection. Aim Our group have previously shown procalcitonin (PCT) to be a potentially useful biomaker in the clinial situation of possible pocket infection. We aim to prospectively validate the proposed cut-off value of 0.05ng/ml for the procalcitonin (PCT) biomaker in an independent cohort, which we have previously identified as showing promise in this clinical situation. Methods In this prospective case-control validation study the PCT levels of 81 patients with confirmed pocket infections were compared to 81 controls, matched for age and renal function, presenting for elective generator replacement or lead revision unrelated to infection. Exclusion criteria included: concomitant infectious or inflammatory diseases, end-stage renal failure, active malignancy or receiving immunosuppressive therapy. Results A PCT over 0.05 ng/ml was found in 68% (n= 55) of pocket infections and 24% (n= 19) of controls. Using the pre­defined cut-off value of 0.05 ng/ml PCT had a sensitivity of 68% and a specificity of 77% for diagnosing pocket infections. ROC analysis revealed area under the curve of 0.752 (standard error 0.039, p <0.001 ) for PCT. In patients presenting with minimal infective signs the sensivity remained high (67% vs 70% with extensive inflammation) and similarly remained high in thus who had received anti-biotic therapy prior to PCT sampling (65% vs 69%). Conclusion PCT is a potentially useful biomarker to aid the diagnosis of a pocket infection when used with the prospecitvely validated cut-off value of 0.05ng/ml. The sensitivity of the PCT positive result remained high even in patients pre-treated with antibiotics or with minimal clinical signs of inflammation.

FeSe and the Missing Electron Pocket Problem

The nature and origin of electronic nematicity remains a significant challenge in our understanding of the iron-based superconductors. This is particularly evident in the iron chalcogenide, FeSe, where it is currently unclear how the experimentally determined Fermi surface near the M point evolves from having two electron pockets in the tetragonal state, to exhibiting just a single electron pocket in the nematic state. This has posed a major theoretical challenge, which has become known as the missing electron pocket problem of FeSe, and is of central importance if we wish to uncover the secrets behind nematicity and superconductivity in the wider iron-based superconductors. Here, we review the recent experimental work uncovering this nematic Fermi surface of FeSe from both ARPES and STM measurements, as well as current theoretical attempts to explain this missing electron pocket of FeSe, with a particular focus on the emerging importance of incorporating thedxyorbital into theoretical descriptions of the nematic state. Furthermore, we will discuss the consequence this missing electron pocket has on the theoretical understanding of superconductivity in this system and present several remaining open questions and avenues for future research.

Experimental and computational approaches to study the high temperature thermoelectric properties of novel topological semimetal CoSi

Here, we study the thermoelectric properties of topological semimetal CoSi in the temperature range 300–800 K by using combined experimental and density functional theory (DFT) based methods. CoSi is synthesized using arc melting technique and the Rietveld refinement gives the lattice parameters of a = b = c = 4.445 Å. The measured values of Seebeck coefficient (S) shows the non-monotonic behaviour in the studied temperature range with the value of ∼−81 μV K−1 at room temperature. The |S| first increases till 560 K (∼−93 μV K−1) and then decreases up to 800 K (∼−84 μV K−1) indicating the dominating n-type behaviour in the full temperature range. The electrical conductivity, σ (thermal conductivity, κ) shows the monotonic decreasing (increasing) behaviour with the values of (12.1 W m−1 K−1) and (14.2 W m−1 K−1) Ω−1 m−1 at 300 K and 800 K, respectively. The κ exhibits the temperature dependency as, κ ∝ T 0.16. The DFT based Boltzmann transport theory is used to understand these behaviour. The multi-band electron and hole pockets appear to be mainly responsible for deciding the temperature dependent transport behaviour. Specifically, the decrease in the |S| above 560 K and change in the slope of σ around 450 K are due to the contribution of thermally generated charge carriers from the hole pockets. The temperature dependent relaxation time (τ) is computed by comparing the experimental σ with calculated σ/τ and it shows temperature dependency of 1/T 0.35. Further this value of τ is used to calculate the temperature dependent electronic part of thermal conductivity (κ e) and it gives a fairly good match with the experiment. Present study suggests that electronic band-structure obtained from DFT provides a reasonably good estimate of the transport coefficients of CoSi in the high temperature region of 300–800 K.

Weyl Semimetal States Generated Extraordinary Quasi‐Linear Magnetoresistance and Nernst Thermoelectric Power Factor in Polycrystalline NbP

AbstractUnsaturated and extremely large magnetoresistance (MR), as well as the giant Nernst effect, are intriguing transport phenomena in Weyl semimetals, which are technically appealing for potential applications in magneto‐electric sensors and transverse thermoelectric conversion. The prominent properties are originated from Weyl semimetal states, i.e., the coexistence of electron and hole pockets combined with linear band dispersion. However, previous studies have been focused on small‐sized single crystals, rendering the practical applications of Weyl semimetals. Here, it is reported an unsaturated, quasi‐linear MR as well as a very large Nernst power factor PFxy in the prepared centimeter‐sized and polycrystalline Weyl semimetal NbP. An extraordinary MR of ≈2 × 104% is observed below 60 K with a magnetic field up to 55 T and persists to elevated temperatures. The unusual quasi‐linear MR behavior is explained by the theory of classical linear MR arising from structural disorder. The polycrystalline NbP exhibits state‐of‐the‐art PFxy that reaches a maximum value of 74.81 μW cm–1 K–2 at 9 T and 220 K, which is 1.5 times larger than its longitudinal thermoelectric power factor PFxx. Given that polycrystalline Weyl semimetal, NbP is suitable for large‐scale production, the results pave the way for its practical applications in magneto‐electric sensors and transverse thermoelectric conversion.

Magnetic field tuning of valley population in the Weyl phase of Nd2Ir2O7

The frustrated magnet Nd$_2$Ir$_2$O$_7$, where strong correlations together with spin-orbit coupling play a crucial role, is predicted to be a Weyl semimetal and to host topological pairs of bulk Dirac-like valleys. Here we use an external magnetic field to manipulate the localized rare earth 4f moments coupled to the 5d electronic bands. Low energy optical spectroscopy reveals that a field of only a few teslas suffices to create charge compensating pockets of holes and electrons in different regions of momentum space, thus introducing a valley population shift that can be tuned with the field.

Physical properties and electronic structure of single-crystal KCo2As2

We present a method for producing high quality $\mathrm{K}{\mathrm{Co}}_{2}{\mathrm{As}}_{2}$ crystals, stable in air and suitable for a variety of measurements. X-ray diffraction, magnetic susceptibility, electrical transport, and heat capacity measurements confirm the high quality and an absence of long range magnetic order down to at least 2 K. Residual resistivity values approaching 0.25 $\ensuremath{\mu}\mathrm{\ensuremath{\Omega}}$ cm are representative of the high quality and low impurity scattering, and a Sommerfeld coefficient $\ensuremath{\gamma}=7.3 \mathrm{mJ}/\mathrm{mol}$ ${\mathrm{K}}^{2}$ signifies weaker correlations than the Fe-based counterparts. Together with Hall effect measurements, angle-resolved photoemission experiments reveal a Fermi surface consisting of electron pockets at the center and corner of the Brillouin zone, in line with theoretical predictions and in contrast to the mixed carrier types of other pnictides with the $\mathrm{Th}{\mathrm{Cr}}_{2}{\mathrm{Si}}_{2}$ structure. A large, linear magnetoresistance of 200% at 14 T, together with an observed linear and hyperbolic, rather than parabolic, band dispersions are unusual characteristics of this compound.