Transition Metal Pentatellurides Research Articles

Probing the Spatial Homogeneity of Exfoliated HfTe5 Films.

In van der Waals materials, external strain is an effective tool to manipulate and control electronic responses by changing the electronic bands upon lattice deformation. In particular, the band gap of the layered transition metal pentatelluride HfTe5 is sufficiently small to be inverted by subtle changes of the lattice parameters resulting in a strain-tunable topological phase transition. In that case, knowledge about the spatial homogeneity of electronic properties becomes crucial, especially for the microfabricated thin film circuits used in typical transport measurements. Here, we reveal the homogeneity of exfoliated HfTe5 thin films by spatially resolved Raman microscopy. Comparing the Raman spectra under applied external strain to unstrained bulk references, we pinpoint local variations of Raman signatures to inhomogeneous strain profiles in the sample. Importantly, our results demonstrate that microfabricated contacts can act as sources of significant inhomogeneities. To mitigate the impact of unintentional strain and its corresponding modifications of the electronic structure, careful Raman microscopy constitutes a valuable tool for quantifying the homogeneity of HfTe5 films and circuits fabricated thereof.

Broadband Nonlinear Optical Modulator Enabled by Transition Metal Pentatelluride Nanosheets Towards Mid-Infrared Regime

Broadband Nonlinear Optical Modulator Enabled by Transition Metal Pentatelluride Nanosheets Towards Mid-Infrared Regime

Fractional electromagnetic response in a three-dimensional chiral anomalous semimetal

The magnetoelectric coupling of electrons in a three-dimensional solid can be effectively described by axion electrodynamics. Here we report the discovery of the fractional magnetoelectric effect in chiral anomalous semimetals of the three-dimensional massless Wilson fermions, which have linear dispersions at the energy crossing point, and break the chiral symmetry at generic momenta. In the presence of electric and magnetic fields, the time-reversal and parity symmetry breaking give rise to the quarter-quantized topological magnetoelectric effect, which is directly related to the winding number 1/2 of the band structure, and is only one half of that for topological insulators. The fractional electromagnetic response can be revealed by the surface Hall conductance and extracted from the measurement of the topological Kerr and Faraday rotation. The transition-metal pentatelluride \mathrm{ZrTe_{5}} with a strain-tunable band gap provides a potential platform to test the effect experimentally.

Magnetic field-induced non-linear transport in HfTe5.

The interplay of electron correlations and topological phases gives rise to various exotic phenomena including fractionalization, excitonic instability and axionic excitation. Recently discovered transition-metal pentatellurides can reach the ultra-quantum limit in low magnetic fields and serve as good candidates for achieving such a combination. Here, we report evidence of density wave and metal-insulator transition in HfTe5 induced by intense magnetic fields. Using the non-linear transport technique, we detect a distinct non-linear conduction behavior in the longitudinal resistivity within the a-c plane, corresponding to the formation of a density wave induced by magnetic fields. In high fields, the onset of non-linear conduction in the Hall resistivity indicates an impurity-pinned magnetic freeze-out as the possible origin of the insulating behavior. These frozen electrons can be gradually reactivated into mobile states above a threshold of electric field. This experimental evidence calls for further investigation into the underlying mechanism of the bulk quantum Hall effect and field-induced phase transitions in pentatellurides.

Highly efficient terahertz emission from layered ZrTe5 crystal with strong anisotropy and high in-plane carrier mobility

Intense, polarization controllable, and single-cycle terahertz pulses as light source are important tools to investigate physical and biological properties of materials. In this paper, we have detected efficient terahertz emission from layered transition-metal pentatelluride ZrTe5 excited by femtosecond laser pulse due to its strong anisotropy and high mobility of electrons. The physical mechanism of terahertz generation from this material is studied and the absolute predominance of diffusion current induced by photo-Dember effect is proved. In addition, we have achieved the modulation of terahertz polarization by adjusting the polarization states of the pump pulse. The generated elliptically polarized THz pulse is crucial for the investigation of vibration modes of chiral biological molecules, such as proteins and DNAs. Our findings indicate that strong and polarization-controllable THz emission can be generated by ZrTe5.

Thermodynamically Induced Transport Anomaly in Dilute Metals ZrTe_{5} and HfTe_{5}.

A 40-year-old puzzle in transition metal pentatellurides ZrTe_{5} and HfTe_{5} is the anomalous peak in the temperature dependence of the longitudinal resistivity, which is accompanied by sign reverses of the Hall and Seebeck coefficients. We give a plausible explanation for these phenomena without assuming any phase transition or strong interaction effect. We show that, due to intrinsic thermodynamics and diluteness of the conducting electrons in these materials, the chemical potential displays a strong dependence on the temperature and magnetic field. With that, we compute resistivity, Hall and Seebeck coefficients in zero field, and magnetoresistivity and Hall resistivity in finite magnetic fields, in all of which we reproduce the main features that are observed in experiments.

Robust hybrid mode-locking operation with bulk-like transition metal pentatellurides

Robust hybrid mode-locking operation has been demonstrated based on bulk-like transition metal pentatellurides.

Dirac Polarons and Resistivity Anomaly in ZrTe_{5} and HfTe_{5}.

Resistivity anomaly, a sharp peak of resistivity at finite temperatures, in the transition-metal pentatellurides ZrTe_{5} and HfTe_{5} was observed four decades ago, and more exotic and anomalous behaviors of electric and thermoelectric transport were revealed in recent years. Here, we present a theory of Dirac polarons, composed by massive Dirac electrons and holes in an encircling cloud of lattice displacements or phonons at finite temperatures. The chemical potential of Dirac polarons sweeps the band gap of the topological band structure by increasing the temperature, leading to the resistivity anomaly. Formation of a nearly neutral state of Dirac polarons accounts for the anomalous behaviors of the electric and thermoelectric resistivity around the peak of resistivity.

Tunable discrete scale invariance in transition-metal pentatelluride flakes

Log-periodic quantum oscillations discovered in transition-metal pentatelluride give a clear demonstration of discrete scale invariance (DSI) in solid-state materials. The peculiar phenomenon is convincingly interpreted as the presence of two-body quasi-bound states in a Coulomb potential. However, the modifications of the Coulomb interactions in many-body systems having a Dirac-like spectrum are not fully understood. Here, we report the observation of tunable log-periodic oscillations and DSI in ZrTe5 and HfTe5 flakes. By reducing the flakes thickness, the characteristic scale factor is tuned to a much smaller value due to the reduction of the vacuum polarization effect. The decreasing of the scale factor demonstrates the many-body effect on the DSI, which has rarely been discussed hitherto. Furthermore, the cut-offs of oscillations are quantitatively explained by considering the Thomas-Fermi screening effect. Our work clarifies the many-body effect on DSI and paves a way to tune the DSI in quantum materials.

Unraveling the Topological Phase of ZrTe_{5} via Magnetoinfrared Spectroscopy.

For materials near the phase boundary between weak and strong topological insulators (TIs), their band topology depends on the band alignment, with the inverted (normal) band corresponding to the strong (weak) TI phase. Here, taking the anisotropic transition-metal pentatelluride ZrTe_{5} as an example, we show that the band inversion manifests itself as a second extremum (band gap) in the layer stacking direction, which can be probed experimentally via magnetoinfrared spectroscopy. Specifically, we find that the band anisotropy of ZrTe_{5} features a slow dispersion in the layer stacking direction, along with an additional set of optical transitions from a band gap next to the Brillouin zone center. Our work identifies ZrTe_{5} as a strong TI at liquid helium temperature and provides a new perspective in determining band inversion in layered topological materials.

Experimental observation of conductive edge states in weak topological insulator candidate HfTe5

The quantum spin Hall (QSH) effect is widely studied as a novel quantum state in condensed matter physics over the past decade. Recently, it is predicted that the transition metal pentatelluride XTe5 (X = Zr, Hf) has a large bandgap in its bulk form and a single layer of XTe5 is a QSH insulator candidate. However, the topological nature of the bulk material is still under debate because it is located close to the phase boundary of a strong topological insulator and a weak topological insulator (WTI). Here, using angle-resolved photoemission spectroscopy and scanning tunneling microscopy (STM)/scanning tunneling spectroscopy, we systematically studied the electronic structures of bulk HfTe5. Both the large bulk bandgaps and conductive edge states in the vicinity of the step edges in HfTe5 were observed, strongly suggesting a WTI phase in bulk HfTe5. Moreover, our STM experiment for the first time reveals the bulk band bending due to the broken symmetry near the step edge, making it an ideal platform for studying the development of edge states in the WTI and QSH insulator.

Electrical, magneto-transport and significant thermoelectric properties of Te-rich ZrTe5+δ polycrystals

The transition metal pentatelluride ZrTe5 has been studied extensively due to high thermoelectric power factors for potential applications in low-temperature refrigeration devices. Recent studies reveal that the physical properties of ZrTe5 are strongly dependent on Te concentration. Here, we have synthesized the Te-rich ZrTe5+δ polycrystalline by direct solid-state reaction and systematically investigated the electrical, magneto-transport and thermoelectric properties. Temperature-dependent resistivity and magneto-transport characterizations substantiate that ZrTe5+δ is a p-type (hole-doped) semiconductor. Besides, Seebeck coefficient is positive and linearly increases with T when T is within 10–150 K, and saturated to 120 μV K−1. Remarkably, a significant figure of merit ZT is obtained (∼0.037 at T = 300 K), which is higher than those in polycrystalline ZrTe5 (∼0.026) and sister compound HfTe5 (∼0.016) measured at 300 K. The significant thermoelectric performance of ZrTe5+δ is attributed to its low electrical resistivity (∼5 mΩ·cm), relative low thermal conductivity (∼2.0 W·K−1·m−1) and moderate Seebeck coefficient (∼120 μV K−1) at 300 K. Our work demonstrates the defect engineering, through delicate synthesis method, is an effective way to improve the thermoelectric property of ZrTe5.

First-principles study of lattice thermal conductivity in ZrTe5 and HfTe5

Recently, the layered transition-metal pentatellurides ZrTe5 and HfTe5 have attracted increasing attention because of their interesting topological electronic properties. Nevertheless, some of their other good physical properties seem to be ignored now. Actually, both ZrTe5 and HfTe5 have high electric conductivities (>105 Ω−1 m−1) and Seebeck coefficients (> 100 μV/K) at room temperature, thus making them promising thermoelectric materials. However, the disadvantage is that the thermal conductivities of the two materials are relatively high according to the few available experiments; meanwhile, the detailed mechanism of the intrinsic thermal conductivity has not been studied yet. Based on the density functional theory and the Boltzmann transport theory, we present here the theoretical study of the intrinsic lattice thermal conductivities of ZrTe5 and HfTe5, which are found to be in the range of 5–8 W/m⋅K at room temperature and well consistent with the experimental results. We also find that the thermal conductivities of the two materials are anisotropic, which are mainly caused by their anisotropic crystal structures. Based on the detailed analysis, we proposed that the thermal conductivities of the two materials could possibly be reduced by different kinds of structural engineering at the atomic and mesoscopic scales, such as alloying, doping, nano-structuring, and polycrystalline structuring, which could make ZrTe5 and HfTe5 good thermoelectric materials for room temperature thermoelectric applications.

Polycrystalline ZrTe5 Parametrized as a Narrow-Band-Gap Semiconductor for Thermoelectric Performance

The exotic transport physics of single-crystalline transition-metal pentatellurides has been debated for years. The authors discover that in fact polycrystalline samples show the same unusual properties, and that viewing these compounds as semiconductors with bipolar conduction explains their behavior. This insight allows for the prediction and optimization of their thermoelectric properties. Considering the dearth of work on materials for waste-heat recovery at lower temperatures, and on generators employing a single compound for both legs, this study could change the way we think about what is possible in thermoelectric applications.

Anomalous Magneto-Transport Behavior in Transition Metal Pentatelluride HfTe5**Supported by the National Basic Research Program of China under Grant Nos 2015CB921303 and 2013CB921700, the National Key Research Program of China under Grant No 2016YFA0300604, and the ‘Strategic Priority Research Program (B)’ of the Chinese Academy of Sciences under Grant No XDB07020100.

There is a long-standing confusion concerning the physical origin of the anomalous resistivity peak in transition metal pentatelluride HfTe5. Several mechanisms, such as the formation of charge density wave or polaron, have been proposed, but so far no conclusive evidence has been presented. In this work, we investigate the unusual temperature dependence of magneto-transport properties in HfTe5. It is found that a three-dimensional topological Dirac semimetal state emerges only at around Tp (at which the resistivity shows a pronounced peak), as manifested by a large negative magnetoresistance. This accidental Dirac semimetal state mediates the topological quantum phase transition between the two distinct weak and strong topological insulator phases in HfTe5. Our work not only provides the first evidence of a temperature-induced critical topological phase transition in HfTe5 but also gives a reasonable explanation on the long-lasting question.

Observation of Optical and Electrical In-Plane Anisotropy in High-Mobility Few-Layer ZrTe5.

Transition metal pentatelluride ZrTe5 is a versatile material in condensed-matter physics and has been intensively studied since the 1980s. The most fascinating feature of ZrTe5 is that it is a 3D Dirac semimetal which has linear energy dispersion in all three dimensions in momentum space. Structure-wise, ZrTe5 is a layered material held together by weak interlayer van der Waals force. The combination of its unique band structure and 2D atomic structure provides a fertile ground for more potential exotic physical phenomena in ZrTe5 related to 3D Dirac semimentals. However, the physical properties of its few-layer form have yet to be thoroughly explored. Here we report strong optical and electrical in-plane anisotropy of mechanically exfoliated few-layer ZrTe5. Raman spectroscopy shows a significant intensity change with sample orientations, and the behavior of angle-resolved phonon modes at the Γ point is explained by theoretical calculations. DC conductance measurement indicates a 50% of difference along different in-plane directions. The diminishing of resistivity anomaly in few-layer samples indicates the evolution of band structure with a reduced thickness. A low-temperature Hall experiment sheds light on more intrinsic anisotropic electrical transport, with a hole mobility of 3000 and 1500 cm2/V·s along the a-axis and c-axis, respectively. Pronounced quantum oscillations in magnetoresistance are observed at low temperatures with the highest electron mobility up to 44 000 cm2/V·s.

Spectroscopic evidence for the gapless electronic structure in bulk ZrTe5

Recently, transition metal pentatellurides MTe5 (M=Zr, Hf) have inspired intensive research effort. Being predicted to be quantum spin Hall insulators (QSHI) with the bulk gap up to hundreds of meVs, it could lead to promising applications at unprecedented high temperature compared with previously discovered QSHI (e.g. HgTe/CdTe or InAs/GaSb quantum wells). However, the experimental works soon followed illustrated considerable discrepancies regarding to whether MTe5 compounds possess a full bulk gap, making their topological nature (topological insulators or Dirac semimetals) illusive. In this work, combining investigations of angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM), we systematically studied the electronic properties of ZrTe5. In intrinsic samples, we observed little evidence for the existence of topological surface states or large bulk gap. With bulk and surface doping to adjust the position of the Fermi-level, ARPES spectra indicate gapless and highly linear dispersions at the valance band top, in consistence with the STM measurements that show a V-shaped total density of states near the Fermi-level (i.e. suggesting a gapless nature of the electronic structure of ZrTe5). Moreover, near the terrace edge on the surface, we observed non-zero DOS, indicating the existence of edge states.

Enhancement of the power factor of the transition metal pentatelluride HfTe5 by rare-earth doping

The transition metal pentatellurides HfTe5 and ZrTe5 have been observed to possess interesting electrical transport properties with high thermopower and low resistivity values leading to high thermoelectric power factors. We have investigated the effect of doping HfTe5 with rare-earth elements by measuring the power factor data from about 10K to room temperature on single crystals of Hf1−xRxTe5, where R=Ce, Pr, Nd, Sm, Gd, Tb, Dy, and Ho. Samples that have been doped with Nd (Hf1−xNdxTe5) possess power factors more than a factor of 2 larger than that of the commonly used thermoelectric material Bi2Te3.

Raman scattering in doped transition metal pentatellurides

We have measured the Raman spectra of Sb doped (ZrTe5−xSbx; 0<x<0.25) and Hf doped (ZryHf1−yTe5; 0<y<1) pentatellurides in the 90–300 cm−1 range, and compared them to the corresponding spectrum for the parent material ZrTe5. X-ray diffraction data revealed that the pentatelluride structure prevails at all doping concentrations. With increasing Sb concentration, the linewidths for three of the four Raman peaks observed at 115, 120, and 147 cm−1 in the parent material broadened with insignificant change in the linewidth for the fourth peak at 180 cm−1. Interestingly, the peak intensity for the mode at 180 cm−1 gradually diminishes in ZrTe5−xSbx up to x=0.15, and completely vanishes in the x=0.20 compound. On the other hand, the electrical and Raman properties of ZryHf1−yTe5 exhibit a systematic shift from the corresponding properties of ZrTe5 to that of HfTe5 for increasing Hf concentration in the sample. Implications of the disappearance of the resistivity anomaly with concomitant vanishing of 180 cm−1 Raman mode in the Sb doped samples for x=0.20 composition are presented.

Suppression of the resistivity anomaly and corresponding thermopower behavior in the pentatelluride system by the addition of Sb:Hf1−XZrXTe5−YSbY

The electrical resistivity \ensuremath{\rho} and the thermopower \ensuremath{\alpha} of the transition-metal pentatelluride system ${\mathrm{Hf}}_{1\ensuremath{-}X}{\mathrm{Zr}}_{X}{\mathrm{Te}}_{5\ensuremath{-}Y}{\mathrm{Sb}}_{Y}$ have been measured over a broad range of temperature, $10\mathrm{K}<T<300\mathrm{K}.$ The systematic Sb doping of these materials has been performed over a range from $0<Y<0.75,$ where Y is the nominal Sb concentration. Both parent materials $({\mathrm{HfTe}}_{5}$ and ${\mathrm{ZrTe}}_{5})$ exhibit an anomalous resistive peak, ${T}_{P}\ensuremath{\approx}80\mathrm{K}$ for ${\mathrm{HfTe}}_{5}$ and ${T}_{P}\ensuremath{\approx}145\mathrm{K}$ for ${\mathrm{ZrTe}}_{5}.$ Each parent material displays a large positive (p-type) thermopower $(\ensuremath{\alpha}>~+125\ensuremath{\mu}\mathrm{V}/\mathrm{K})$ around room temperature, which undergoes a change to a large negative (n-type) thermopower $(\ensuremath{\alpha}<~\ensuremath{-}125\ensuremath{\mu}\mathrm{V}/\mathrm{K})$ below the peak temperature. At a specific level of Sb doping the resistive anomaly is no longer evident and results in a semimetallic temperature dependence. In addition the thermopower monotonically decreases with temperature with no change in sign as in the parent materials. X-ray-diffraction data reveals that the pentatelluride structure is still preserved at all doping concentrations.