Hybridization Gap Research Articles

Imaging emergent heavy Dirac fermions of a topological Kondo insulator

Kondo insulators are primary candidates in the search for strongly correlated topological quantum phases, which may host topological order, fractionalization, and non-Abelian statistics. Within some Kondo insulators, the hybridization gap is predicted to protect a nontrivial topological invariant and to harbor emergent heavy Dirac fermion surface modes. We use high-energy-resolution spectroscopic imaging in real and momentum space on the Kondo insulator, SmB$_6$. On cooling through $T^*_{\Delta}\approx$ 35 K we observe the opening of an insulating gap that expands to $\Delta\approx$ 10 meV at 2 K. Within the gap, we image the formation of linearly dispersing surface states with effective masses reaching $m^* = (410\pm20)m_e$. We thus demonstrate existence of a strongly correlated topological Kondo insulator phase hosting the heaviest known Dirac fermions.

Magnetoresistance from time-reversal symmetry breaking in topological materials

Magnetotransport measurements are a popular way of characterizing the electronic structure of topological materials and often the resulting datasets cannot be described by the well-known Drude model due to large, non-parabolic contributions. In this work, we focus on the effects of magnetic fields on topological materials through a Zeeman term included in the model Hamiltonian. To this end, we re-evaluate the simplifications made in the derivations of the Drude model and pinpoint the scattering time and Fermi velocity as Zeeman-term dependent factors in the conductivity tensor. The driving mechanisms here are the aligment of spins along the magnetic field direction, which allows for backscattering, and a significant change to the Fermi velocity by the opening of a hybridization gap. After considering 2D and 3D Dirac states, as well as 2D Rashba surface states and the quasi-2D bulk states of 3D topological insulators, we find that the 2D Dirac states on the surfaces of 3D topological insulators produce magnetoresistance, that is significant enough to be noticable in experiments. As this magnetoresistance effect is strongly dependent on the spin-orbit energy, it can be used as a telltale sign of a Fermi energy located close to the Dirac point.

Unusual pressure-induced metallic state in the correlated narrow band-gap semiconductor FeSi

Compressing FeSi induces a progressive semiconductor to metal transition, onset at P >= 15 GPa at temperatures below T-max determined by the degree of disorder in the sample. At high pressure preceding charge-gap closure, a broad maximum manifests in the rho(T) data at T-max and is a feature which persists into the metallic state. The extremum in rho(T) occurs at T-max similar to 40 K at similar to 11 GPa and shifts monotonically to similar to 240 K as pressure is increased to similar to 32 GPa, in the most detailed example of three series of measurements involving pressurized FeSi with different degrees of disorder. The transition to a metallic phase is an electronic change only, in that the B 20-type crystal structure is retained up to 30 GPa, with no evidence of a discontinuity in the volume-pressure equation of state data. Samples from the same ingot subjected to different quasihydrostatic conditions reveal different values of the critical pressure of the electronic transition, its width, and pressure dependences of T-max. This attests to sensitivity of the electronic transition to the degree of disorder in the investigated sample. The metallic state has neither Fermi-liquid nor non-Fermi-liquid behavior. Such an unusual pressure-induced correlated metallic state in FeSi is attributed to extended states within the 3d-3p hybridization gap originating from disorder and compression tuning of the mobility edge relative to the Fermi level. The metallic state has also been investigated in external magnetic fields up to 8 T at low temperatures (2 K <= T <= 20 K) at 15 and 19 GPa. This reveals a positive magnetoresistance, as observed in doped Fe1-xCoxSi samples at ambient pressure, suggesting that in the majority and minority spin bands there is a field-induced modification of the respective magnitudes of charge-carrier populations which have different mobilities.

Electrical tuning of helical edge states in topological multilayers

Mainstream among topological insulators, GaSb/InAs quantum wells present a broken gap alignment for the energy bands which supports the quantum spin Hall insulator phase and forms an important building block in the search of exotic states of matter. Such structures allow the band-gap inversion with electrons and holes confined in adjacent layers, providing a fertile ground to tune the corresponding topological properties. Using a full 3D eight-band method we investigate the inverted band structure of GaSb/InAs/GaSb and InAs/GaSb/InAs multilayers and the behavior of the helical edge states, under the influence of an electric field applied along the growth direction. By tuning the electric field modulus, we induce the change of the energy levels of both conduction and valence bands, resulting in a quantum spin Hall insulator phase where the helical edge states are predominantly confined in the GaSb layer. In particular, we found that InAs/GaSb/InAs has a large hybridization gap of about and, therefore, are promising to observe massless Dirac fermions with a large Fermi velocity. Our comprehensive characterization of GaSb/InAs multilayers creates a basis platform upon which further optimization of III–V heterostructures can be contrasted.

Suppression of the antiferromagnetic metallic state in the pressurized MnBi2Te4 single crystal

We study the effect of hydrostatic pressure on the electrical transport, magnetic, and structural properties of $\mathrm{MnB}{\mathrm{i}}_{2}\mathrm{T}{\mathrm{e}}_{4}$ by measuring its resistivity, Hall effect, and x-ray diffraction under pressures up to 12.8 GPa supplemented by the first-principles calculations. At ambient pressure, $\mathrm{MnB}{\mathrm{i}}_{2}\mathrm{T}{\mathrm{e}}_{4}$ shows a metallic conducting behavior with a cusplike anomaly at around ${T}_{\mathrm{N}}\ensuremath{\approx}24\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, where it undergoes a long-range antiferromagnetic (AF) transition. With increasing pressure, ${T}_{\mathrm{N}}$ determined from the resistivity anomaly first increases slightly with a maximum at around 2 GPa and then decreases until vanishing completely at about 7 GPa. Intriguingly, its resistivity is enhanced gradually by pressure and even evolves from metallic to semimetal or semiconductinglike behavior as ${T}_{\mathrm{N}}$ is suppressed. However, the density of the $n$-type charge carrier that remains dominant under pressure increases with pressure. In addition, the interlayer AF coupling seems to be strengthened under compression, since the critical field ${H}_{\mathrm{c}1}$ for the spin-flop transition to the canted AF state is found to increase with pressure. No structural transition was evidenced up to 12.8 GPa, but some lattice softening was observed at about 2 GPa, signaling the occurrence of an electronic transition or crossover from a localized to itinerant state. We have rationalized these experimental findings by considering the pressure-induced enhancement of antiferromagnetic/ferromagnetic competition and partial delocalization of $\mathrm{Mn}\text{\ensuremath{-}}3d$ electrons, which not only destroys long-range AF order but also promotes charge-carrier localization through enhanced spin fluctuations and/or the formation of a hybridization gap at high pressure.

Breakdown of the strong multiplet description of the Sm2+ ion in the topological Kondo insulator SmB6: specific heat studies

We have theoretically confirmed the existence of in-gap real quantum-mechanical states in SmB6, which have been suggested by experiments. These in-gap states, below the hybridization gap of 20 meV, are related to the Sm2+ ion states and can be revealed by calculations within the spin-orbital |LSLzSz〉 space, with L = 3 and S = 3. Our approach overcomes difficulties related to the singlet J = 0 multiplet ground state. The in-gap states originate from the 49-fold degenerated term 7F (4f 6), which is split by cubic crystal-field (CEF) and spin-orbit (s − o) interactions. There is competition between these interactions: the six-order CEF interactions produce a 7-fold degenerated ground state, whereas the s − o interactions, even the weakest one, produce a singlet (J = 0) ground state. We have found preliminary CEF and s − o parameters that produce the lowest states at 0 K (singlet) and 91 K (triplet) and the next triplet at 221 K, i.e., within the hybridization gap. The derived states well explain the large extra specific heat of SmB6, confirming the consistency and adequateness of our theoretical approach with the breakdown of the strong multiplet description of the Sm2+ ion in SmB6.

Interplay between hybridisation gaps and unusual magnetic orders in Kondo semiconductors CeT2Al10 (T = Ru and Os)

ABSTRACTKondo semiconductors based on rare-earth elements possess a narrow gap in the renormalised density of states near the Fermi level. Their ground states remain nonmagnetic due to strong hybridisation of the 4f state with conduction bands. However, in the orthorhombic compounds CeT2Al10 (T = Ru, Os), opening of a hybridisation gap is followed by an antiferromagnetic (AFM) order at a rather high temperature TN ≅ 28 K. Our systematic studies of the effects of electron/hole doping on the magnetic and transport properties, electron tunnelling, and spin-gap formation revealed that the hybridisation gap is indispensable for the unusual AFM order. Under uniaxial pressure, TN increases linearly as the orthorhombic b-axis parameter is decreased. Our findings support the model that a kind of charge-density wave developing along the b axis in the presence of the hybridisation gap induces the AFM order in these systems.

Geometric phase invariance in spatiotemporal modulated elastic system

We study the topological characteristics of elastic waves in a one-dimensional mass and spring elastic superlattice that exhibits non-reciprocal elastic wave propagation due to extrinsic application of a single sinusoidal spatiotemporal modulation of its spring stiffness. We employ a computational procedure to generate the band structure, traveling modes' amplitudes and phases, and subsequently the geometric phases to characterize the global vibrational behavior of the system and its topological character. The elastic superlattice demonstrates the notion of non-conventional band structure, where hybridization gaps arise as bands appear and cross for certain wavenumber values. We estimate these hybridization points using multiple time scale perturbation theory for low modulation velocity. At the hybridization points, both the theoretical and numerical analyses display a discontinuity in the traveling modes’ amplitudes. Consequently, we find a multiple of π, sometimes zero, geometric phase value in the spatiotemporal modulated elastic system, if the unit cell has inversion symmetry as imposed by the values of the spring constant at the initial time. The temporal modulation, which creates hybridization gaps, changes the nature of the geometric phase from a closed loop geometric phase when the modulation is only spatial, to an open loop geometric phase. This open loop geometric phase is invariant to the temporal modulation.

Putative hybridization gap in CaMn2Bi2 under applied pressure

We report electrical transport measurements on CaMn$_{2}$Bi$_{2}$ single crystals under applied pressure. At ambient pressure and high temperatures, CaMn$_{2}$Bi$_{2}$ behaves as a single-band semimetal hosting N\'{e}el order at $T_{N}=150$~K. At low temperatures, multi-band behavior emerges along with an activated behavior typical of degenerate semiconductors. The activation gap is estimated to be $\Delta \sim 20$~K. Applied pressure not only favors the antiferromagnetic order at a rate of 0.40(2)~K/kbar, but also enhances the activation gap at $20$~kbar by about $70$~\%. This gap enhancement is typical of correlated narrow-gap semiconductors such as FeSi and Ce$_{3}$Bi$_{4}$Pt$_{3}$, and places CaMn$_{2}$Bi$_{2}$ as a Mn-based hybridization-gap semiconductor candidate. \textit{Ab initio} calculations based on the density functional theory are shown to be insufficient to describe the ground state of CaMn$_{2}$Bi$_{2}$.

End-Bonded Metal Contacts on WSe2 Field-Effect Transistors.

Contact engineering has been the central issue in the context of high-performance field-effect transistors (FETs) made of atomic thin transition metal dichalcogenides (TMDs). Conventional metal contacts on TMDs have been made on top via a lithography process, forming a top-bonded contact scheme with an appreciable contact barrier. To provide a more efficient pathway for charge injection, an end-bonded contact scheme has been proposed, in which covalent bonds are formed between the contact metal and channel edges. Yet, little efforts have been made to realize this contact configuration. Here, we bridge this gap and demonstrate seeded growth of end-bonded contact with different TMDs by means of chemical vapor deposition (CVD). Monolayer WSe2 FETs with a CVD-grown channel and end contacts exhibit improved performance metrics, including an on-current density of 30 μA/μm, a hole mobility of 90 cm2/V·s, and a subthreshold swing of 94 mV/dec, an order of magnitude superior than those of top-contact FET counterparts that share the same channel material. A fundamental NOT logic gate constructed using top-gated and end-bonded WSe2 and MoS2 FETs is also demonstrated. Calculations using density functional theory indicate that the superior device performance stems mainly from the stronger metal-TMD hybridization and substantial gap states in the end-contact configuration.

Character of defect states in vacancy-doped MoTe2 monolayer: Spatial localization, flat bands and hybridization gap

Abstract The structural and electronic properties of vacancy-doped 1H and 1T′ MoTe2 monolayers are systematically investigated based on first-principles calculations. Te vacancy is found energetically favored over Mo vacancy for both phases, and Te double vacancy is more favored as it is formed by missing two Te atoms from opposite sides. Vacancy induced defect states are strongly spatially localized on the atoms within two or three shell surrounding the point defect. This strongly localized nature leads to weak dependence of the defect formation energy on defect concentrations, sharply peaked density of states, and flat bands. Moreover, the calculation suggests that Te vacancy opens a band gap in the 1T′ MoTe2 monolayer, which can be tuned by lattice strain or external force. These results might give a guidance for the research on the application of MoTe2 in electronics and optoelectronics.

Possible orbital crossover in the ferromagnetic Kondo lattice compound CeAgSb2

Utilizing femtosecond time-resolved pump probe spectroscopy, we investigated the ultrafast quasiparticle dynamics of the ferromagnetic Kondo lattice compound ${\mathrm{CeAgSb}}_{2}$. The photoinduced reflectivity demonstrates a positive relaxation process connected with simple metal physics at room temperature, while an additional negative decay channel associated with Kondo screening effect can be recognized immediately with temperature decreasing. The temperature-dependent amplitude of the latter exhibits a kink at ${T}^{*}\ensuremath{\approx}$ 45 K, which is close to the energy scale of the first excited crystal field splitting Ce $4f$ level. We propose that an orbital crossover is responsible for this intriguing feature. The dominating $4f$ orbitals that participated in the Kodno $c\ensuremath{-}f$ hybridization varies from the ground state below ${T}^{*}$ to multiple crystal field splitting levels. In the meantime, the hybridization gap changes from 12.6 to 220.4 K correspondingly, which is also in agreement with the fluence-dependent measurements.

Quasiparticle relaxation dynamics in URu2−xFexSi2 single crystals

We investigate quasiparticle relaxation dynamics in URu$_{2-x}$Fe$_{x}$Si$_{2}$ single crystals using ultrafast optical-pump optical-probe (OPOP) spectroscopy as a function of temperature ($T$) and Fe substitution ($x$), crossing from the hidden order (HO) phase ($x$ = 0) to the large moment antiferromagnet (LMAFM) phase ($x$ = 0.12). At low $T$, the dynamics for $x$ = 0 and $x$ = 0.12 are consistent with the low energy electronic structure of the HO and LMAFM phases that emerge from the high $T$ paramagnetic (PM) phase. In contrast, for $x$ = 0.1, two transitions occur over a narrow $T$ range (from ~15.5 - 17.5 K). A PM to HO transition occurs at an intermediate $T$ followed by a transition to the LMAFM phase at lower $T$. While the data at low $T$ are consistent with the expected coexistence of LMAFM and HO, the data in the intermediate $T$ phase are not, and instead suggest the possibility of an unexpected coexistence of HO and PM. Additionally, the dynamics in the PM phase reflect the presence of a hybridization gap as well as strongly interacting spin and charge degrees of freedom. OPOP yields insights into meV-scale electrodynamics with sub-Kelvin $T$ resolution, providing a complementary approach to study low energy electronic structure in quantum materials.

Impact of the Rashba spin-orbit coupling on f -electron materials

The combination of strong spin orbit coupling and strong correlations holds tremendous potential for interesting physical phenomena as well as applications in spintronics and quantum computation. In this context, we here study the interplay between the Rashba spin-orbit coupling (RSOC) and the Kondo screening in noncentrosymmetric $f$-electron materials. We show that the Kondo coupling of the $f$-electrons becomes anisotropic at high temperatures due to the RSOC. However, an isotropic Kondo effect is restored at low temperature which leads to a complete Kondo screening. We furthermore demonstrate that the Kondo effect has influence on the Rashba splitting in the band structure, which becomes temperature dependent. Although the $f$-electrons are localized at high temperature, a helical spin polarization of the conduction band emerges due to the scattering with the $f$-electrons. With decreasing temperature, the Kondo screening occurs, which leads to drastic changes in the band structure. Remarkably, these changes in the band structure depend on the helical spin polarization. For strong RSOC, we observe that the hybridization gap of one of the helical bands is closed at low temperature and a helical half-metal is formed.

Emergent Coherent Lattice Behavior in Kondo Nanosystems.

How many magnetic moments periodically arranged on a metallic surface are needed to generate a coherent Kondo lattice behavior? We investigate this fundamental issue within the particle-hole symmetric Kondo lattice model using quantum MonteCarlo simulations. Extra magnetic atoms forming closed shells around the initial impurity induce a fast splitting of the Kondo resonance at the inner shells, which signals the formation of composite heavy-fermion bands. The onset of the hybridization gap matches well the enhancement of antiferromagnetic spin correlations in the plane perpendicular to the applied magnetic field, a genuine feature of the coherent Kondo lattice. In contrast, the outermost shell remains dominated by a local Kondo physics with spectral features resembling the single-impurity behavior.

Energy spectrum and optical absorption of superlattices under strong band inversion conditions

We show that in superlattices with a strong band inversion no hybridization gap exists. There are two points where the bands are crossing and the spectrum has a shape of Dirac cones. Due to the absence of the hybridization gap the optical absorption does not have a threshold and goes to zero gradually with the decrease of optical energy.

Tunable phononic structures using Lamb waves in a piezoceramic plate

Phononic crystals made of piezoceramic materials allow for frequency band structure tunability due to the ease of electric command use. In this paper, we develop a full elastodynamic model, taking into account piezoelectric coupling effects for the band-structure calculation of Lamb eigenmodes of a phononic crystal consisting of a piezoceramic plate with a one-dimensional periodic array of electrodes set on both sides. The dispersion-relation characteristics of these eigenmodes are nondestructively tuned via external circuit impedance loads coupled to the phononic plate, e.g., through inductance loads inducing tunable resonant flat bands that hybridize with the Lamb modes of the elastic plate, thus opening up hybridization gaps. It is shown that additional control of the shape of these resonant bands can be achieved by incorporating an impedance interconnecting two adjacent electrodes of the same side, the whole structure forming an electric quadripole. This behavior can be easily predicted with the help of our formalism combined with a simplified electric line model of the phononic crystal plate.

Physical properties of SmxB6 single crystals

We study the magnetic susceptibility, electrical resistivity, Hall effect, and heat capacity of single crystals of ${\mathrm{Sm}}_{x}{\mathrm{B}}_{6}$ for $x=1,0.94,0.8,$ and 0.75. Remarkably, the overall properties of the crystals do not qualitatively depend on the density of vacancies. The topological surface states are seen at low temperatures in the electrical transport properties as the bulk conductivity freezes out. Even a large number of Sm vacancies does not close the hybridization gap. The linear term in the specific heat $\ensuremath{\gamma}$, of unclear origin, remains large but decreases gradually with $x$. The shoulder in the density of states, brought by the hybridized $4f$ states and observed in the susceptibility and the specific heat, does not move but decreases its height with decreasing $x$. The specific heat also displays a weak Schottky anomaly at about 1.5 K for all $x$, except for the $x$ = 0.75 sample. This Schottky anomaly is only slightly field dependent and of unknown origin.

Signatures of long-range-correlated disorder in the magnetotransport of ultrathin topological insulators

In an ultrathin topological insulator (TI) film, a hybridization gap opens in the TI surface states, and the system is expected to become either a trivial insulator or a quantum spin Hall insulator when the chemical potential is within the hybridization gap. Here we show, however, that these insulating states are destroyed by the presence of a large and long-range-correlated disorder potential, which converts the expected insulator into a metal. We perform transport measurements in ultrathin, dual-gated topological insulator films as a function of temperature, gate voltage, and magnetic field, and we observe a metallic-like, non-quantized conductivity, which exhibits a weak antilocalization-like cusp at the low magnetic field and gives way to a nonsaturating linear magnetoresistance at large field. We explain these results by considering the disordered network of electron- and hole-type puddles induced by charged impurities. We argue theoretically that such disorder can produce an insulator-to-metal transition as a function of increasing disorder strength, and we derive a condition on the band gap and the impurity concentration necessary to observe the insulating state. We also explain the linear magnetoresistance in terms of strong spatial fluctuations of the local conductivity, using both numerical simulations and a theoretical scaling argument.

Heavy quasiparticle bands in the underscreened quasiquartet Kondo lattice

We study the quasiparticle spectrum in an underscreened Kondo-lattice (KL) model that involves a single spin degenerate conduction band and two crystalline-electric-field (CEF) split Kramers doublets coupled by both orbital-diagonal and non-diagonal exchange interactions. We find the three quasiparticle bands of the model using a constrained fermionic mean field approach. While two bands are similar to the one-orbital model a new genuinely heavy band inside the main hybridization gap appears in the quasiquartet model. Its dispersion is due to effective hybridization with conduction states but the bandwidth is controled by the size of the CEF splitting. Furthermore several new indirect and direct hybridztion gaps may be identified. By solving the selfconsistency equation we calculate the CEF- splitting and exchange dependence of effective Kondo low energy scale, hybridization gaps and band widths. We also derive the quasiparticle spectral densities and their partial orbital contributions. We suggest that the two-orbital KL model can exhibit mixed CEF/Kondo excitonic magnetism.