Chemical Potential Shift Research Articles

The shift in chemical potential of electrons upon intercalation of ZrSe2 by copper

The effective valence of copper in the region of its concentration up to x = 0.25 in CuxZrSe2 has been determined based on an analysis of binding energies of the Zr3d and Se3d core levels and the electromotive forces from an electrochemical experiment for CuxZrSe2, as a function of Cu concentration x. It has been shown that the domination of covalent bonding between copper and the ZrSe2 host lattice within this concentration range results in a notable reduction in the effective valence of copper in comparison to its nominal value.

High-Q resonances based on BIC in graphene-dielectric hybrid double-T metasurfaces for terahertz modulation and sensing

In recent years, there has been a growing interest in bound states in the continuum (BIC) in metasurfaces. One particular area of focus is achieving high-quality (Q) factor resonance, as this is crucial for enhancing the performance of refractive index sensors. In this study, a graphene-dielectric hybrid metasurface that supports the bound states in the continuum is proposed. By varying the width of the dielectric rectangle, quasi-BIC resonances with a high Q factor can be excited, and the Q factor can reach 752724.95 and 272004.759 respectively. The analysis of multipole decomposition reveals that the two quasi-BIC resonances are predominantly influenced by the electric quadrupole and magnetic dipole, respectively. Moreover, the transmittance of the resonance point can be changed rapidly with the change of the chemical potential of graphene, so the function of modulation can be realized by changing the chemical potential of graphene. Based on these findings, we have designed a terahertz wave modulator, which exhibits modulation depths of 98.1% and 99.9% at the two resonance peaks, respectively. The corresponding chemical potential shifts are 50 meV and 0.5 eV. Additionally, we have investigated the sensing performance of the metasurface. By analyzing the magnitude of the frequency shifts of the quasi-BIC resonance peaks at different gas refractive indexes, we have determined sensitivities of 740 GHz RIU−1 and 630 GHz RIU−1 at the two resonance peaks. The maximum figure of merit (FOM) values are 132911.39 RIU−1 and 45000 RIU−1, respectively. This research serves as a valuable reference for the design of dynamic optical modulators and sensors operating in the terahertz band.

Outstanding Room‐Temperature Thermoelectric Performance of n‐type Mg3Bi2‐Based Compounds Through Synergistically Combined Band Engineering Approaches

AbstractThermoelectric cooling materials based on Bi2Te3 have a long history of unsurpassed performance near room temperature. Recently, research into price‐competitive Mg3(Bi, Sb)2‐based materials are focused on replacing traditional cooling materials. Here, the thermoelectric properties of Mg3.2Bi1.998−xSbxTe0.002Cu0.005 (x = 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5) polycrystalline compounds are investigated. In all temperature regions, electrical resistivity and Seebeck coefficient are increased with Sb concentration. The electronic transport properties of Sb‐alloyed compounds are maximized by synergistically combined band engineering approaches such as band structure change caused by lattice strain, increased electronic density of states, and chemical potential shift, leading to exceptionally high‐power factor values of over 3.0 mW m−1 K−2 at room temperature. Furthermore, with increasing Sb content, thermal conductivity values are systematically reduced due to the promotion of alloy scattering of phonons and suppression of the bipolar contribution. Consequently, these multiple approaches significantly enhance thermoelectric performance, resulting in an enhancement of thermoelectric figure‐of‐merit zT above 1.1 at 348–423 K. Additionally, a zTavg of 1.1 is recorded at 300–450 K, making it an unrivaled value among the reported n‐type Mg3Bi2‐based thermoelectric materials. Overall, this work demonstrates that Mg3Bi2‐based materials are more promising for thermoelectric cooling applications compared to Bi2Te3‐based materials.

Pseudogap in BaPb x Bi1‒x O3 (x = 0.7, 0.75 and 1.0)

In this work, we have investigated the crystal and electronic structure of the orthorhombic phase of BaPb x BiO3 (BPBO) for x = 0.7 (BPBO70), 0.75 (BPBO75) and 1.0 (BPO), using temperature dependent x-ray diffraction measurements, photoemission spectroscopy, and electronic structure calculations. Our results show the importance of particle size and strain in governing superconductivity. Interestingly, the temperature evolution of the structural parameters in the case of BPBO70 is similar to that of BPBO75 but the magnitude of the change is diminished. The BPBO75 and BPO compounds exhibit metallic nature, which is corroborated by the core level studies. The electronic structure calculations in conjunction with the core level studies suggest that oxygen vacancies play an important role for metallicity observed in the end compound. The exponent to the spectral line shape close to the Fermi level suggests the origin of pseudogap to be due to other contributions in addition to disorder in the case of BPBO70 and BPO. The core level studies also show that as one goes from x = 0.70 to 1.0, there occurs chemical potential shift towards the valence band suggesting hole doping. Our results open the venue to further study these compounds as a function of particle size, nature of carriers for its transport behaviour, electronic structure below TC , composition at the grain boundaries and microscopic origin of pseudogap in the non-superconducting phase. We believe that our results call for a revision of the temperature-doping phase diagram of BPBO to include the pseudogap phase.

Experimental electronic structure of the electrically switchable antiferromagnet CuMnAs

Tetragonal CuMnAs is a room temperature antiferromagnet with an electrically reorientable Néel vector and a Dirac semimetal candidate. Direct measurements of the electronic structure of single-crystalline thin films of tetragonal CuMnAs using angle-resolved photoemission spectroscopy (ARPES) are reported, including Fermi surfaces (FS) and energy-wavevector dispersions. After correcting for a chemical potential shift of ≈− 390 meV (hole doping), there is excellent agreement of FS, orbital character of bands, and Fermi velocities between the experiment and density functional theory calculations. In addition, 2×1 surface reconstructions are found in the low energy electron diffraction (LEED) and ARPES. This work underscores the need to control the chemical potential in tetragonal CuMnAs to enable the exploration and exploitation of the Dirac fermions with tunable masses, which are predicted to be above the chemical potential in the present samples.

Effect of carrier doping on the electronic states of earth-abundant Fe–Al–Si thermoelectric materials

Fe–Al–Si-based thermoelectric (FAST) materials are non-toxic and low-cost materials that can be used for autonomous power supplies to drive internet-of-things wireless sensor devices. The conduction type can be controlled by changing the Al/Si ratio, which is suitable for fabricating reliable thermoelectric power-generation modules consisting of materials with similar thermal expansion coefficients. In this work, we evaluated the electronic structures of p- and n-type FAST materials with relatively large absolute values of the Seebeck coefficient by photoemission spectroscopy to obtain deeper insight into controlling the p-n characteristics of FAST materials. The core-level spectra suggested that the FAST materials have a covalent bonding nature. The chemical-potential shift should be the dominant factor of the core-level shift, which is consistent with the expected behavior of carrier doping of thermoelectric semiconductors, that is, rigid-band-like behavior. The size of the core-level shift of ∼0.15 eV is close to the band gap of ∼0.18 eV obtained from transport measurements. The observed electronic structure can qualitatively explain the experimental results.

Terahertz optical modulator and highly sensitive terahertz sensor governed by bound states in the continuum in graphene-dielectric hybrid metamaterial

We propose a graphene-dielectric hybrid metamaterial (GDHM) that supports the quasi-bound state in the continuum (BIC). By rotating one of the LiTaO 3 bars, the quasi-BIC resonance with high quality (Q) factor can be excited. The analysis of multipole decomposition confirms that electric quadrupole dominates this resonance. Moreover, the Q factor of quasi-BIC significantly decreases as the chemical potential of graphene increases, and the tuning transmission is achieved by changing the chemical potential of graphene. A low-energy THz-wave modulator is designed based on this scheme, and the designed modulator shows 97% modulation depth with a chemical potential shift of only 16 meV. In addition, the sensing performance of GDHM is investigated, and it is found that the sensitivity can be controlled by chemical potential. The minimum value of the sensing sensitivity can reach up to 308.91 GHz/RIU, and the maximum figure of merit (FOM) is 257.43 RIU−1. When the losses of LiTaO 3 are considered. The modulation depth, sensitivity, and maximum FOM achieve 90%, 309.54 GHz/RIU, and 121.39 RIU−1, respectively. This work provides a valuable reference for designing dynamic optical modulators and sensors in the THz region.

Intrinsic anomalous Hall effect and Lifshitz transition in a ferromagnetic kagome-lattice metal

Magnetic topological materials with broken time-reversal symmetry have demonstrated colossal intrinsic anomalous Hall effects, originating from large Berry curvature in momentum space. Here, we report the electrical transport study of a ferromagnetic kagome-lattice material Nd3Al, which is predicted to be a magnetic topological high symmetry line metal candidate. We observed a polarity reversal of ordinary Hall resistivity across 40 K, plainly indicating a perceptible shift in chemical potential and change of the Fermi surface, i.e., temperature-induced Lifshitz transition. More strikingly, as the shifting of Fermi level around the band (anti-)crossing points contributes to a considerable Berry curvature, the anomalous Hall conductivity ultimately stabilizes to a constant of approximately ∼427 Ω−1 cm−1 below 40 K, accompanied by a maximum anomalous Hall angle reaching 1.4%, conforming to the intrinsic dissipationless topological Berry-phase mechanism. The similar scaling behavior of anomalous Hall conductivity in Nd3Al to that of magnetic Weyl semimetal Co3Sn2S2 further signals the possible presence of nontrivial topological bands in kagome Nd3Al. In view of the kagome-lattice structure and predicted topological nature, our work unveils the significant potential of the large intrinsic anomalous Hall effect in Nd3Al for investing the interaction between ferromagnetism and topology.

Temperature dependence of band shifts induced by impurity ionization in ZrTe5

The topological phase transition in ${\mathrm{ZrTe}}_{5}$ remains a challenge for its sensitivity to the structural parameters and carrier densities. One important issue is that the temperature dependence of the band shifts has been reported controversially. Using angle-resolved photoemission spectroscopy, we study the temperature evolution of the band structures of ${\mathrm{ZrTe}}_{5}$ with samples prepared by chemical vapor transport and flux methods. We find two distinct types of band shift with temperature, one monotonically moving downward to high binding energies with the increase of temperature and another one moving downward first and then reversely upward. By assuming ${\mathrm{ZrTe}}_{5}$ is a semiconductor with a limited amount of impurities, we analyze the temperature-induced ionization of the impurities and chemical potential shift to interpret the discrepant experimental observations. In this scenario, the discrepancies in band structures and the controversial results on temperature-dependent band shifts can be reconciled in a unified picture.

Large-Gap Quantum Spin Hall State and Temperature-Induced Lifshitz Transition in Bi4Br4.

Searching for quantum spin Hall insulators with large fully opened energy gap to overcome the thermal disturbance at room temperature has attracted tremendous attention because of the robustness of one-dimensional (1D) spin-momentum locked topological edge states in the practical applications of electronic devices and spintronics. Here, we report the investigation of topological nature of monolayer Bi4Br4 by the techniques of angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy. The possible topological nontriviality of 1D edge state integrals within the large energy gap (∼0.2 eV) is revealed by the first-principle calculations. The ARPES measurements at different temperatures show a temperature-induced Lifshitz transition, corresponding to the resistivity anomaly evoked by the chemical potential shift. The connection between the emergency of superconductivity and the Lifshitz transition is discussed.

Temperature and pressure induced electronic topological transitions in titanium

The electronic topological transitions (ETT) in d-transition metals are considered using titanium as an example. It is shown that, upon a temperature- or pressure-induced transition from the ω-phase to the α-phase, the chemical potential shifts to the region of topological features of the electronic energy spectrum, characterized by a negative curvature of the density of states of d-electrons (DOS). As a result, the sign of the mode-mode interaction parameter changes. The ETT leads to an increase in paramagnon Bose excitations and to the appearance of spatial fluctuations of electron density waves. It is shown that a sharp decrease in volume during the transition from the hp ω-to hcp α-phase is the result of the ETT. In this case, the obtained numerical values are in agreement with the experimental data at normal pressure. Entropy calculations show that paramagnons and spatial fluctuations of the electron density are of the most important reasons for the thermodynamic stability of the titanium structural phases.

Angle-resolved photoemission spectroscopy study of the electronic structure evolution in Sn4X3 (X = P, As, Sb)

The binary compounds Sn4X3 (X = P, As, Sb) have recently surfaced as a unique platform in which to study the interplay between nontrivial band topology and superconductivity with both strengthening as X varies from P to Sb. Here we perform the first systematic comparison of the electronic structure between the three member compounds. Results of our angle-resolved photoemission spectroscopy experiments on both Sn4Sb3 and Sn4P3 are compared with first-principles bulk and surface electronic structure calculations as well as with those recently reported for Sn4As3. Based on a thorough characterization of the electronic structure in three-dimensional momentum space, three salient aspects have been identified, including a monotonic chemical potential shift in a non-rigid-band fashion, a monotonic variation in the magnitude of a band splitting of surface origin, and the emergence of additional surface-related bands. We show that these observations can be reasonably captured by the evolution of the initial-state band structure given by first-principles calculations, and argue that the covalent bonding between the surface Sn monolayer and the underlying X monolayer, rather than the spin–orbit coupling, likely plays an important role in accounting for the observed differences in the electronic structure across the Sn4X3 family. Our finding depicts a comprehensive picture for the electronic structure evolution in the Sn4X3 family, and paves the way to deciphering the relationship between the nontrivial band topology and superconductivity therein.

Nonlinear electric conductivity and THz-induced charge transport in graphene

Based on the quantum master equation approach, the nonlinear electric conductivity of graphene is investigated under static electric fields for various chemical potential shifts. The simulation results show that, as the field strength increases, the effective conductivity is firstly suppressed, reflecting the depletion of effective carriers due to the large displacement in the Brillouin zone caused by the strong field. Then, as the field strength exceeds 1 MV m−1, the effective conductivity increases, overcoming the carrier depletion via the Landau–Zener tunneling process. Based on the nonlinear behavior of the conductivity, the charge transport induced by few-cycle THz pulses is studied to elucidate the ultrafast control of electric current in matter.

Magneto-vortical effect in strong magnetic field

We develop covariant chiral kinetic theory with Landau level basis. We use it to investigate a magnetized plasma with a transverse electric field and a steady vorticity as perturbations. After taking into account vacuum shift in the latter case, we find the resulting current and stress tensor in both cases can be matched consistently with constitutive equations of magnetohydrodynamics. We find the solution in the vorticity case contains both shifts in temperature and chemical potential as well as excitations of the lowest Landau level states. The solution gives rise to an vector charge density and axial current density. The vacuum parts coming from both shifts and excitations agree with previous studies and the medium parts coming entirely from excitations leads to a new contribution to vector charge and axial current density consistent with standard chiral vortical effect.

Ionic gate spectroscopy of 2D semiconductors

Reliable and precise measurements of the relative energy of band edges in semiconductors are needed to determine band gaps and band offsets, as well as to establish the band diagram of devices and heterostructures. These measurements are particularly important in the field of two-dimensional materials, in which many new semiconducting systems are becoming available through exfoliation of bulk crystals. For two-dimensional semiconductors, however, commonly employed techniques suffer from difficulties rooted either in the physics of these systems, or of technical nature. The very large exciton binding energy, for instance, prevents the band gap to be determined from a simple spectral analysis of photoluminescence, and the limited lateral size of atomically thin crystals makes the use of conventional scanning tunneling spectroscopy cumbersome. Ionic gate spectroscopy is a newly developed technique that exploits ionic gate field-effect transistors to determine quantitatively the relative alignment of band edges of two-dimensional semiconductors in a straightforward way, directly from transport measurements (i.e., from the transistor electrical characteristics). The technique relies on the extremely large geometrical capacitance of ionic gated devices that -- under suitable conditions -- enables a change in gate voltage to be directly related to a shift in chemical potential. Here we present an overview of ionic gate spectroscopy, and illustrate its relevance with applications to different two-dimensional semiconducting transition metal dichalcogenides and van der Waals heterostructures.

Spectroscopic signature of negative electronic compressibility from the Ti core-level of titanium carbonitride MXene

Two-dimensional transition metal carbides, carbonitrides, and nitrides, called MXenes, exhibit high metallic conductivity, ion intercalation capability, and reversible redox activity, prompting their applications in energy storage and conversion, electromagnetic interference (EMI) shielding, and electronics, among many other fields. It has been shown that replacement of ∼50% of carbon atoms in the most popular MXene family member, titanium carbide (Ti3C2Tx), by nitrogen atoms, forming titanium carbonitride (Ti3CNTx), leads to drastically different properties. Such properties include very high negative charge in solution and extreme EMI shielding effectiveness, exceeding all known materials, even metals at comparable thicknesses. Here, by using ultraviolet photoemission spectroscopy (UPS), the electronic structures of Ti3CNTx and Ti3C2Tx are systematically investigated and compared as a function of charge carrier density. We observe that, in contrast to Ti3C2Tx, the Ti 3p core-level of Ti3CNTx exhibits a counterintuitive shift to a lower binding energy of up to ∼250 meV upon increasing the electron density, which is a spectroscopic signature of negative electronic compressibility (NEC). These experimentally measured chemical potential shifts are well captured by the density functional theory (DFT) calculation. The DFT results also further suggest that the hybridization of titanium–nitrogen bonding in Ti3CNTx helps to promote the available states of Ti atoms for receiving more electrons above the fermi level and leads to the observed NEC. Our findings explain the differences in electronic properties between the two very important and widely studied MXenes and also suggest a new strategy to apply the NEC effect of Ti3CNTx in energy and charge storage applications.

Lifshitz Transition and Non-Fermi Liquid Behavior in Highly Doped Semimetals.

The classical Fermi liquid theory and Drude model have provided fundamental ways to understand the resistivity of most metals. The violation of the classical theory, known as non-Fermi liquid (NFL) transport, appears in certain metals, including topological semimetals, but quantitative understanding of the NFL behavior has not yet been established. In particular, the determination of the non-quadratic temperature exponent in the resistivity, a sign of NFL behavior, remains a puzzling issue. Here, a physical model to quantitatively explain the Lifshitz transition and NFL behavior in highly doped (a carrier density of ≈1022 cm-3 ) monoclinic Nb2 Se3 is reported. Hall and magnetoresistance measurements, the two-band Drude model, and first-principles calculations demonstrate an apparent chemical potential shift by temperature in monoclinic Nb2 Se3 , which induces a Lifshitz transition and NFL behavior in the material. Accordingly, the non-quadratic temperature exponent in the resistivity can be quantitatively determined by the chemical potential shift under the framework of Fermi liquid theory. This model provides a new experimental insight for nontrivial transport with NFL behavior or sign inversion of Seebeck coefficients in emerging materials.

Transport signatures of temperature-induced chemical potential shift and Lifshitz transition in layered type-II Weyl semimetal TaIrTe4

Temperature-induced Lifshitz transitions have been identified in several materials. Their chemical potential shows a substantial shift with changing temperature. The common feature of these materials is the coexistence of electron and hole pockets in the vicinity of the chemical potential. Here, we report the observation of temperature-induced chemical potential shift and Lifshitz transition in a layered type-II Weyl semimetal, TaIrTe4. The reversal of the polarity of the Hall resistivity and thermoelectric power (TEP) as the temperature increases clearly signal an appreciable shift of the chemical potential and change of the Fermi surface. It is corroborated by the improving agreement between the experimental TEP and the one calculated with temperature-dependent chemical potential. The complete disappearance of an electron pocket, consistent with the change of the Fermi surface when the chemical potential moves downwards, provides an evident signature of a temperature-induced Lifshitz transition in TaIrTe4.

Thermostatistics of a q -deformed relativistic ideal Fermi gas

In this paper, we formulate a q-deformed many-body theory for relativistic Fermi gas and discuss the effects of the deformation parameter q on physical properties of such systems. Since antiparticle excitations appear in the relativistic regime, a suitable treatment to the choice of deformation parameters for both fermions and antifermions must be carefully taken in order to get a consistent theory. By applying this formulation, we further study the thermostatistic properties of a q-deformed ideal relativistic Fermi gas. It can be shown that even in the noninteracting scenario, the system exhibits interesting characteristics which are significantly different from ordinary Fermi gases. Explicitly, antiparticles may become dominant due to the shift of chemical potential by the deformation parameter q. This may build a solid foundation for further studies of q-deformed relativistic interacting systems. We also apply our model to study the electron gas in a white dwarf. The effect of the deformation parameter on the Chandrasekhar limit is discussed.

Electronic properties of one-dimensional pentacene crystals

The electronic properties of an infinite row of freestanding, aligned side-by-side, pentacene molecules are derived as a function of the intermolecular overlap integral and the chemical potential shift. We use a semiclassical approximation and a first principles tight binding method to obtain conductance and mobility of this one-dimensional crystal as a function of temperature and gate voltage. For two values of the intermolecular overlap, energy bands show a metallic behavior. For all the other values, a bandgap is present and evolves with the intermolecular overlap following a typical modulation. The magnitude of the scattering parameters estimated by the observed conductivity is coherent with the existing literature values. These findings could be relevant for the implementation of organic-based sensors.