In-phase atomic vibrations as a pathway to modulate the electrical properties in a compensated semimetal

Compensated semimetals are materials where the electron and hole concentrations are equal, often accompained by non-saturating magnetoresistance. A promiment example is tungsten ditelluride. We show how in-phase atomic vibrations modulate the effective mass and could alter the transport properties.
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In-phase atomic vibrations as a pathway to modulate the electrical properties in  a compensated semimetal
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Semimetals are materials where both electron and holes charge carriers are present. Depending on the temperature and pressure conditions, it is in principle possible to modify their relative concentration and properties, e.g., their effective mass, which impacts the electrical transport of the system.

In particular, when the electron and hole concentrations are about equal (compensation), some semimetals present a very high magnetoresistance, only saturating at extremely large magnetic fields. This is typically explained in terms of a semiclassical two-band model in which the non-saturation occurs at the carrier densities compensation point. In this condition, the magnetoresistance depends on the mobilities, and, therefore, on the effective mass of electrons and holes [1]. Whether this mechanism is a sufficient or just a necessary condition for the high non-saturating magnetoresistance is still debated. Nonetheless, there seems to be a good correspondence with real-life materials.

Examples of this behavior are found in bismuth, graphite and layered transition metal dichalcogenides such as tungsten ditelluride (WTe2, Fig. 1b); the latter showing non-saturating magnetoresistance at low temperatures and external magnetic fields up to 60 Tesla [2].

Personally, I find systems like WTe2 very appealing due to their combination of a quasi-two-dimensional material character and several promising properties, e.g., aside from the magnetoresistance [2], a possible type-II Weyl semimetal character [3], quantum spin hall effect [4] and room-temperature ferroelectricity [5]. These characteristics are strongly linked to the electronic and crystal structure, which suggests that selective modifications may offer a way to modulate these emergent features. Their morphological characteristics make them promising candidates for applications based on these phenomena in new ultrathin devices.

After reading the previous reports from the literature, we asked ourselves how and by how much it is possible to alter the transport properties in WTe2 through structural change. An approach is using the out-of-equilibrium pump-probe technique where, typically, an ultrashort laser pulse perturbs the material. The excitation and recovery dynamics are then studied probing a certain property as a function of the delay with respect to the perturbation. This transient modification involves not only the electronic occupation, but also a rearrangement of the atoms. In this respect, it is possible under certain conditions to drive in-phase large-amplitude normal modes, named coherent phonon modes.

In general, these oscillations can be exploited to learn more about the equilibrium state or even to transiently modify its properties. This method has been successfully applied in various past research works of the groups involved in this work, i.e., Ultrafast Dynamics Group (ETH Zurich, Switzerland) [6], T-ReX group (Elettra Sincrotrone Trieste, Italy) [7] and the SwissFEL (X-ray free electron laser within the Paul Scherrer Institute, Switzerland) team at the Bernina beamline [8].

In order to quantify the coherent phonon influence on WTe2, we performed a time-resolved X-ray diffraction study at the Bernina beamline, which was among the first experiments performed at SwissFEL by external users (scheme in Fig 1a). The coherent modulation of the atomic positions leads to clear oscillations in the intensity of diffraction peaks (Fig. 1c-e), which can be compared to predictions of a simple model to deduce the magnitude of the atomic motion.

Fig. 1. Experiment scheme and TRXD results. a Scheme of the time-resolved x-ray diffraction experiment. b WTe2 crystal structure, where the red rectangles delimit the orthorhombic unit cell. c Time-resolved modification of the diffraction intensity ∆I/I for various diffraction peaks.  d Reflection dynamics tenfold shorter time step. e Fourier transform of panel d after subtracting the non-oscillatory response. In the graphs, the markers correspond to experimental data, while the continuous lines are the fit curves.

We compare our findings with those from complementary time-resolved near-infrared reflectivity measurements, and we estimate through density functional theory (DFT) calculations the impact of these structural perturbations on the electronic structure (Fig. 2). We show that the coherent atomic motion perturbs the semimetallic pockets, leading to a periodic change of their energy levels and associated effective masses.  We estimate a relative change of the effective mass up to 20% in specific regions of the hole and electron pockets.

Fig. 2. DFT simulation results. a Fermi surface of WTe2. The most prominent coherent optical phonons at b ≈0.24 THz (shear mode) and c ≈2.4 THz. d Band dispersion along X* (0.3 0 0) - Γ (0 0 0) around the Fermi level. e Example of band structures modifications at P1 due to the phononic displacements (from -700 to 700 fm with the steps shown in panel g,i). f -i Effective mass changes around the regions highlighted in panel d. W#1 refers to the labelling in Fig. 1b.

The impact of the coherent phonon oscillations is expected to influence various transport properties such as electrical transport through a high-frequency modulation of its extremely large non-saturating magnetoresistance and thermoelectric performance [9], which is of particular interest for low effective mass systems. Based on previous experimental work and simulations by Sie et al. [10], the magnitude of the shear phonon mode at 0.24 THz we obtained should be enough to transiently modulate its topological properties, together with a slower drift of the structure towards a topologically trivial phase after excitation.

Such oscillations could be implemented as modulators, simple function generators and test devices for high-frequency applications which are expected to become increasingly relevant for fast big-data transfer and telecommunications.

 

References

[1] Lv, H. Y. et al., Europhys. Lett. 110, 37004 (2015)

[2] Ali, M. N. et al., Nature 514, 205-208 (2014)

[3] Soluyanov, A. A. et al., Nature 527, 495–498 (2015)

[4] Wu, S. et al., Science 359, 76–79 (2018)

[5] Sharma, P. et al., Sci. Adv. 5, eaax5080 (2019)

[6] Johnson, S. L., Faraday Discuss., 237, 9-26 (2022)

[7] Soranzio, D. et al., Phys. Rev. Res. 1, 032033(R) (2019)

[8] Ingold, G. et al., J. Synchrotron. Radiat. 26, 874–886 (2019)

[9] Pei, Y. et al., Energy Environ. Sci. 5, 7963–7969 (2012)

[10] Sie, E. J. et al., Nature 565, 61–66 (2019)

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