Chemical Potential Shift Research Articles

Chiral excitonic instability of two-dimensional tilted Dirac cones

The electron-electron Coulomb interaction in Dirac-Weyl semimetals harbours a novel paradigm of correlation effects that hybridizes diverse realms of solid-state physics with their relativistic counterpart. Driving spontaneous mass acquisition, the excitonic condensate of strongly-interacting massless Dirac fermions is one such example whose exact nature remains debated. Here, by focussing on the two-dimensional tilted Dirac cones in the organic salt $\alpha$-(BEDT-TTF)$_2$I$_3$, we show that the excitonic instability is controlled by a small chemicalpotential shift and an in-plane magnetic field. In combined analyses based on renormalization-group approaches and ladder approximation, we demonstrate that the nuclear relaxation rate is an excellent probe of excitonic-spin fluctuations in an extended parameter region. Comparative nuclear magnetic resonance (NMR) experiments show good agreements with this result, jointly revealing the importance of intervalley nesting between field-induced, spin-split Fermi pockets of opposite charge polarities. Our work provides an accurate framework to search for excitonic instability of strongly-interacting massless fermions.

Graphene-Quantum Dot Hybrid Photodetectors with Low Dark-Current Readout.

Graphene-based photodetectors have shown responsivities up to 108 A/W and photoconductive gains up to 108 electrons per photon. These photodetectors rely on a highly absorbing layer in close proximity to graphene, which induces a shift of the graphene chemical potential upon absorption, hence modifying its channel resistance. However, due to the semimetallic nature of graphene, the readout requires dark currents of hundreds of microamperes up to milliamperes, leading to high power consumption needed for the device operation. Here, we propose a different approach for highly responsive graphene-based photodetectors with orders of magnitude lower dark-current levels. A shift of the graphene chemical potential caused by light absorption in a layer of colloidal quantum dots induces a variation of the current flowing across a metal-insulator-graphene diode structure. Owing to the low density of states of graphene near the neutrality point, the light-induced shift in chemical potential can be relatively large, dramatically changing the amount of current flowing across the insulating barrier and giving rise to an alternative gain mechanism. This readout requires dark currents of hundreds of nanoamperes up to a few microamperes, orders of magnitude lower than that of other graphene-based photodetectors, while keeping responsivities of ∼70 A/W in the infrared, almost 2 orders of magnitude higher than that of established germanium on silicon and indium gallium arsenide infrared photodetectors. This makes the device appealing for applications where high responsivity and low power consumption are required.

Rethinking the Characterization of Nanoscale Field-Effect Transistors: A Universal Approach.

Standard methods for calculating transport parameters in nanoscale field-effect transistors (FETs), namely carrier concentration and mobility, require a linear connection between the gate voltage and channel conductance; however, this is often not the case. One reason often overlooked is that shifts in chemical and electric potential can partially compensate each other, commonly referred to as quantum capacitance. In nanoscale FETs, capacitance is often unmeasurable and an analytical formula is required, which assumes the conducting channel as metallic and common methods of determining threshold voltage no longer couple properly into transport equations. As present and future FET structures become smaller and have increased channel-gate coupling, this issue will render standard methods impossible to use. This work discusses the validity of common methods of characterization for nanoscale FETs, develops a universal model to determine transport properties by only measuring the threshold voltage of an FET and presents a new parameter to easily classify FETs as either quantum capacitance-limited or metallic approximated charge transport. Also considered in this work is electrical hysteresis from trap states and, in combination with the proposed universal model, novel techniques are introduced to measure and remove the errors associated with these effects often ignored in literature.

Structural and electronic effects in GdCu alloy

We have studied structural and electronic phenomena in GdCu using x-ray diffraction (xrd), photoemission spectroscopic (PES) techniques and band structure calculations. Our structural studies show that the as prepared GdCu sample does not stabilise completely in cubic CsCl phase even at room temperature (RT). The thermal hysteresis is observed in the lattice parameter that appears to be due to strain and dislocations at the surface. The behaviour of the intensity of the most intense xrd peak suggests phase coexistence and structural link with magnetic properties. After undergoing thermal cycling to RT, with an increase in polishing depth, the GdCu sample shows different surface and bulk crystal structures at RT. This behaviour is in contrast to the behaviour of the asprepared one. The surface is predominantly cubic while the bulk exposes more its hidden orthorhombic FeB phase with an increase in the depth of polishing. To understand the manifestation of phase separation on the electronic structure, we have used DFT and DFT + U calculations and PES studies. Our results show the importance of on-site Coulombic interaction in Gd 4f and Cu 3d orbitals. At the Fermi edge, in addition to the significant contribution of Gd 5d and Cu 3d, there is also a contribution of Cu 4p states. The PES studies exhibit chemical potential shift as one compares the asprepared GdCu and the GdCu that has undergone thermal cycling. We have discussed the effect of the chemical potential shift on the valence band and core- level spectra. We believe our results will be helpful in providing insight into the generic property displayed by systems that exhibit strain dominated phase separation.

Effect of nonlinear cap layer on TM-polarized surface waves in a graphene-based photonic crystal

In this paper, we investigated the effect of nonlinear cap layer and the chemical potential of the graphene on the p-polarized surface waves (SWs) in a one-dimensional graphene-based photonic crystal (GBPC). Three different cases were considered for the dielectric tensor of the nonlinear cap layer: (1) perpendicular uniaxial approximation, (2) parallel uniaxial approximation and (3) generic case. The results of the study indicated that the effect of nonlinear cap layer on TM SWs may be neglected due to the perpendicular approximation. On the other hand, in parallel approximation as well as generic form of dielectric function, the frequency of the TM SWs within the graphene-induced band gap (GIPBG) may be controlled by the nonlinear cap layer and the chemical potential of the graphene. Moreover, according to the results, in the GIPBG of GBPC with self-defocusing cap layer, enhancing chemical potential and optical nonlinearity shift the frequency of the SWs in the same direction. However, with respect to the self-focusing cap layer, enhancing chemical potential and optical nonlinearity shift the frequency of the SWs in the opposite direction. Consequently, by adjusting the chemical potential in the self-defocusing cap layer, the tunability range of the SW frequency may be extended and in the case of self-focusing, optical nonlinear effects may be eliminated.

3D Negative Electronic Compressibility as a New Emergent Phenomenon

This paper was written in honor of Prof. Ted Geballe’s 100th birthday. It concerns a new emergent phenomenon called three-dimensional negative electronic compressibility. The physical content, driving mechanism, and context of the initial discovery of this phenomenon in an iridate material are reviewed. An integrative approach based on multiple experimental methods for carrier concentration modulation and chemical potential shift measurement is proposed for the quest of this phenomenon in other materials. Its implications on the physical properties of materials, in particular terms of electrostatic screening and phase separation, are discussed along with some of its unique prospects for applications.

Controlling the Magnetic Anisotropy of the van der Waals Ferromagnet Fe3GeTe2 through Hole Doping.

Identifying material parameters affecting properties of ferromagnets is key to optimized materials that are better suited for spintronics. Magnetic anisotropy is of particular importance in van der Waals magnets, since it not only influences magnetic and spin transport properties, but also is essential to stabilizing magnetic order in the two-dimensional limit. Here, we report that hole doping effectively modulates the magnetic anisotropy of a van der Waals ferromagnet and explore the physical origin of this effect. Fe3-xGeTe2 nanoflakes show a significant suppression of the magnetic anisotropy with hole doping. Electronic structure measurements and calculations reveal that the chemical potential shift associated with hole doping is responsible for the reduced magnetic anisotropy by decreasing the energy gain from the spin-orbit induced band splitting. Our findings provide an understanding of the intricate connection between electronic structures and magnetic properties in two-dimensional magnets and propose a method to engineer magnetic properties through doping.

Superconducting states and Majorana modes in transition-metal dichalcogenides under inhomogeneous strain

We study the effect of inhomogeneous strain on transition-metal dichalcogenides with a large intrinsic gap in their spectrum. It is found that, by tuning the chemical potential, superconductivity can preserve within the strain-induced discrete pseudo Landau levels (LLs), which introduce interesting topological properties to these systems. As we show, the superconductivity for integer fillings is quantum critical, and the quantum critical coupling strength is determined by the spacing between the two LLs closest to the Fermi level. For partial fillings, the superconducting gap is scaled linearly with the coupling strength, and decreases rapidly when the chemical potential shifts away from the middle of each LL. In the presence of a Zeeman field, a pair of Majorana modes emerge simultaneously in the two valleys of strained dichalcogenides. When valley symmetry is further destroyed, a single Majorana mode can be expected to emerge at the edges of the strained monolayer dichalcogenides.

Thermoelectricity in B80-based single-molecule junctions: First-principles investigation

Thermoelectricity is a thermorelated property that is of great importance in single-molecule junctions. The electrical conductance (σ), electron-derived thermal conductance (κel) and Seebeck coefficient (S) of B80-based single-molecule junctions are investigated by using density functional theory in combination with non-equilibrium Green’s function. When the distance between the left/right electrodes is 11.4 A, the relationship between σ and κel obeys the Wiedemann–Franz law very well because of the strong hybridization between B80 molecular orbitals and the surface states of Au electrodes. Furthermore, the calculated Lorenz number is close to the famous value in metal or degenerate semiconductors. In addition, S is only −19.09 μV/K at 300 K, thus leading to the smaller electron’s thermoelectric figure of merit (ZelT = S2σT/κel). Interestingly, the strain and chemical potential can modulate B80-based single-molecule junctions from n-type to p-type when the compressive strain reaches −0.6 A or the chemical potential shifts to −0.16 eV. This might be attributed that S reflects the asymmetry in the electrical conductance with respect to the chemical potential and is proportional to the slopes of the transmission spectrum.

Investigation of the electronic structure and lattice dynamics of the thermoelectric material Na-doped SnSe

SnSe has drawn considerable attention on a global scale due to its intrinsic low thermal conductivity and large figure of merit along the $b$ axis. In Na-doped SnSe, further enhancement of the thermoelectric performance has been reported. Using angle-resolved photoemission spectroscopy and inelastic neutron scattering, we have studied how electronic structures and lattice dynamics evolve with temperature in Na-doped SnSe. Our data show that the effective mass of the Se ${p}_{z}$ orbital along the $\mathrm{\ensuremath{\Gamma}}\text{\ensuremath{-}}Z$ direction has a very weak temperature dependence, while the chemical potential shifts significantly along with the increase in the gap size evidenced by infrared absorption measurements. Inelastic neutron scattering reveals one acoustic TA and two low-lying optical (TO1 and TO2) phonon modes. Their temperature-dependent behaviors indicate that the TO1 and TA modes contribute more to the reduction of the lattice thermal conductivity with temperature increases. The estimated value of the lattice thermal conductivity based on the lattice dynamics is significantly larger than that determined by transport measurements, suggesting that extrinsic factors, such as the imperfection of the lattice, could drastically suppress the lattice thermal conductivity. Our data suggest that temperature-dependent properties of both electronic structures and phonon dynamics need to be taken into account for the investigation of the underlying physics of hole-doped SnSe.

Bulk-boundary correspondence in the quantum Hall effect

We present a detailed microscopic study of edge excitations for n filled Landau levels. We show that the higher-level wavefunctions possess a non-trivial radial dependence that should be integrated over for properly defining the edge conformal field theory. This analysis let’s us clarify the role of the electron orbital spin s in the edge theory and to discuss its universality, thus providing a further instance of the bulk-boundary correspondence. We find that the values si for each level, parameterize a Casimir effect or chemical potential shift that could be experimentally observed. These results are generalized to fractional and hierarchical fillings by exploiting the W-infinity symmetry of incompressible Hall fluids.

Electronic Structure of Ce-Doped and -Undoped Nd_{2}CuO_{4} Superconducting Thin Films Studied by Hard X-Ray Photoemission and Soft X-Ray Absorption Spectroscopy.

In order to realize superconductivity in cuprates with the T^{'}-type structure, not only chemical substitution (Ce doping) but also postgrowth reduction annealing is necessary. In the case of thin films, however, well-designed reduction annealing alone without Ce doping can induce superconductivity in the T^{'}-type cuprates. In order to unveil the origin of superconductivity in the Ce-undoped T^{'}-type cuprates, we have performed bulk-sensitive hard x-ray photoemission and soft x-ray absorption spectroscopy on superconducting and nonsuperconducting Nd_{2-x}Ce_{x}CuO_{4} (x=0, 0.15, and 0.19) thin films. By postgrowth annealing, core-level spectra exhibited dramatic changes, which we attributed to the enhancement of core-hole screening in the CuO_{2} plane and the shift of chemical potential along with changes in the band filling. The result suggests that the superconducting Nd_{2}CuO_{4} film is doped with electrons despite the absence of the Ce substitution.

(Invited) Chemo-Mechanical Coupling Phenomena in Solid Oxide Fuel Cells

The materials and the structure of solid oxide fuel cells are designed to avoid thermo-mechanical damages under various operation conditions. However, inherent risk of chemo-mechanical failures are still not fully understood. This paper aims to review the recent works related to this topic, and to address some issues which have not been widely recognized. The coupling of chemistry and mechanics are classified into four types, i.e. (1) chemically driven strain, (2) chemically modified mechanical properties, (3) mechanically driven chemical reactions, and (4) mechanically modified chemical (physical) properties. Since chemical energies are much larger than mechanical energy accommodated in SOFC, the former two types (type(1) and (2)) of chemo-mechanical coupling have been recognized as more important than the others, and have been studied intensively. An example of type-1 phenomena is chemical expansion of mixed conducting oxides with e.g. (La,Sr)(Co,Fe)O3 cathode, LaCrO3 based interconnect, and CeO2 based or (La,Sr)(Ga,Mg,Co)O3 electrolytes. Since the transient behavior as well as steady state distribution of oxygen potential inside the constituent solids is essential to know the effect of the chemical strain, Terada et al. developed a computer code “SIMUDEL” of an FEM-based calculation of oxygen potential. This code considers “chemical capacitance” due to nonstoichiometry of the materials to treat the transient responses, and the results of the calculation can be transported into some of major commercial programs for structure analysis. Volume change of a nickel cermet anode is also an important feature of type-1 coupling which must be considered in determining fabrication and operation processes. The electrode shrinks on reduction and expands on re-oxidation as expected from the lattice size of the metal and the oxide. However, under certain conditions, a porous cermet was found to “shrink” upon oxidation. It took place only during light re-oxidation around 400C. Under this condition the formation of NiO was not obvious from XRD, whereas weight gain was observed by thermo-gravimetry. Careful observation of the microstructure of a porous Ni revealed that, upon shrinkage, the particle-to-particle separation changed partly due to the neck growth between the particles and to the change of the connection angle of the particles. Further study is underway to elucidate the detailed mechanism of the oxidation-induced shrinkage. The change of mechanical properties such as elastic moduli and fracture strength are also dependent on defect concentration and its motion in the lattice (type-2 coupling). Young’s modulus of nonstoichiometric oxides show dependences not only on temperature but also on pO2 through the change of defect concentration. Also, domain boundary shift of ferroelastic phase of LSCF was found to be correlated with the defect concentration. As is discussed for the anomaly of Young’s modulus of YSZ around 400˚C, the motion of oxide ion vacancies may also have correlation with the ferroelastic strain observed with Sc and Ce doped ZrO2 electrolyte above 300˚C. Another interesting type-2 coupling is with the lightly oxidized Ni cermet electrode. It was found that the creep rate of Ni-YSZ cermet at 400˚C was dramatically increased when oxygen-containing gas was introduced. This may be by a correlated mechanism with the above mentioned oxidation induced shrinkage. Several reports, including those from our group, have been published on the effect of mechanical stress on defect formation (type-3 coupling) of nonstoichiometric oxides determined by experiments or by calculation. As is expected from thermodynamic consideration, the experimentally determined effect was not large, e.g. 1G Pa stress was equivalent to 1/5 order of magnitude shift of chemical potential of oxygen for nonstoichiometry of LSCF. Similarly, only minor effect on a practical system was reported for type-4 coupling. However, those phenomena can have significant effect on long-term stability if cation mobility and their driving force are modified at a strained interfaces or grain boundaries.

Unusually large chemical potential shift in a degenerate semiconductor: Angle-resolved photoemission study of SnSe and Na-doped SnSe

We have studied the electronic structure of SnSe and Na-doped SnSe by means of angle-resolved photoemission spectroscopy. The valence-band top reaches the Fermi level by the Na doping, indicating that Na-doped SnSe can be viewed as a degenerate semiconductor. However, in the Na-doped system, the chemical potential shift with temperature is unexpectedly large and is apparently inconsistent with the degenerate semiconductor picture. The large chemical potential shift and anomalous spectral shape are key ingredients for an understanding of the novel metallic state with the large thermoelectric performance in Na-doped SnSe.

Finite-size effects in a nanowire strongly coupled to a thin superconducting shell

We study the proximity effect in a one-dimensional nanowire strongly coupled to a finite superconductor with a characteristic size which is much shorter than its coherence length. Such geometries have become increasingly relevant in recent years in the experimental search for Majorana fermions with the development of thin epitaxial Al shells which form a very strong contact with either InAs or InSb nanowires. So far, however, no theoretical treatment of the proximity effect in these systems has accounted for the finite size of the superconducting film. We show that the finite-size effects become very detrimental when the level spacing of the superconductor greatly exceeds its energy gap. Without any fine-tuning of the size of the superconductor (on the scale of the Fermi wavelength), the tunneling energy scale must be larger than the level spacing in order to reach the hard gap regime which is seen ubiquitously in the experiments. However, in this regime, the large tunneling energy scale induces a large shift in the effective chemical potential of the nanowire and pushes the topological phase transition to magnetic field strengths which exceed the critical field of Al.

Van der Waals Schottky barriers as interface probes of the correlation between chemical potential shifts and charge density wave formation in 1T−TiSe2 and 2H−NbSe2

Layered transition metal dichalcogenide (TMD) materials, i.e. 1T-TiSe$_2$ and 2H-NbSe$_2$, harbor a second order charge density wave (CDW) transition where phonons play a key role for the periodic modulations of conduction electron densities and associated lattice distortions. We systematically study the transport and capacitance characteristics over a wide temperature range of Schottky barriers formed by intimately contacting freshly exfoliated flakes of 1T-TiSe$_2$ and 2H-NbSe$_2$ to \textit{n}-type GaAs semiconductor substrates. The extracted temperature-dependent parameters (zero-bias barrier height, ideality and built-in potential) reflect changes at the TMD/GaAs interface induced by CDW formation for both TMD materials. The measured built-in potential reveals chemical-potential shifts relating to CDW formation. With decreasing temperature a peak in the chemical-potential shifts during CDW evolution indicates a competition between electron energy re-distributions and a combination of lattice strain energies and Coulomb interactions. These modulations of chemical potential in CDW systems, such as 1T-TiSe$_2$ and 2H-NbSe$_2$ harboring second-order phase transitions, reflect a corresponding conversion from short to long-range order.

Tuneable strong optical absorption in a graphene-insulator-metal hybrid plasmonic device

An optical device configuration allowing efficient electrical tuning of near total optical absorption in monolayer graphene is reported. This is achieved by combining a two-dimensional gold coated diffraction grating with a transparent spacer and a suspended graphene layer to form a doubly resonant plasmonic structure. Electrical tuneability is achieved with the inclusion of an ionic gel layer which plays the role of the gate dielectric. The underlying grating comprises a 2-dimensional array of inverted pyramids with a triple layer coating consisting of a reflective gold layer and two transparent dielectric spacers, also forming a vertical micro-cavity known as a Salisbury screen. Resonant coupling of plasmons between the gold grating and graphene result in strong enhancement of plasmon excitations in the atomic monolayer. Plasmon excitations can be dynamically switched off by lowering the chemical potential of graphene. Very high absorption values for an atomic monolayer and large tuning range, extremely large electrostatically induced changes in absorption over very small shifts in chemical potential are possible thus allowing for very sharp transitions in the optical behavior of the device. Overall this leads to the possibility of making electrically tunable plasmonic switches and optical memory elements by exploiting slow modes.

Experimental search for one-dimensional edge states at surface steps of the topological insulator Bi2Se3 : Distinguishing between effects and artifacts

The results of a detailed study of the topological insulator Bi$_2$Se$_3$ surface state energy structure in the vicinity of surface steps using scanning tunneling microscopy and spectroscopy methods are presented. An increase in the chemical potential level $\mu$ near the step edge is observed. The value of the increase $\delta \mu\sim 0.1$~eV is found to correlate with the step height. The effect is caused by redistribution of electron wave functions between the outer and inner edges of surface steps, as known for normal metals. The smaller value of the chemical potential shift and its larger characteristic length of $\sim 10$~nm reflect specifics of the helical surface states. This increase is accompanied by enlargement of the normalized differential tunneling conductance in the helical surface states energy region and thereby produces the illusion of the appearance of edge states. We show that the enlargement is reproduced in the framework of the tunneling model taking into account the tunneling gap transparency change when the chemical potential moves away from the Dirac point.

Energy gap in tunneling spectroscopy: Effect of the chemical potential shift

We study the effect of a shift of the chemical potential level on the tunneling conductance spectra. In the systems with gapped energy spectra, significant chemical-potential dependent distortions of the differential tunneling conductance curves, $dI/dV$, arise in the gap region. An expression is derived for the correction of the $dI/dV$, which in a number of cases was found to be large. The sign of the correction depends on the chemical potential level position with respect to the gap. The correction of the $dI/dV$ associated with the chemical potential shift has a nearly linear dependence on the tip-sample separation $z$ and vanishes at $z\to 0$.

Work function of bulk-insulating topological insulator Bi2–xSbxTe3–ySey

Recent discovery of bulk insulating topological insulator (TI) Bi2–xSbxTe3–ySey paved a pathway toward practical device application of TIs. For realizing TI-based devices, it is necessary to contact TIs with a metal. Since the band-bending at the interface dominates the character of devices, knowledge of TIs' work function is of essential importance. We have determined the compositional dependence of the work function in Bi2–xSbxTe3–ySey by high-resolution photoemission spectroscopy. The obtained work-function values (4.95–5.20 eV) track the energy shift of the surface chemical potential seen by angle-resolved photoemission spectroscopy. The present result serves as a useful guide for developing TI-based electronic devices.