arXiv subjects

Christopher Sims

Publications and source records attributed to Christopher Sims.

26 records · Page 2Linked to original sources

arXiv2019

Observation of topological surface state in a superconducting material

The discovery of topological insulator phase has ignited massive research interests in novel quantum materials. Topological insulators with superconductivity further invigorate the importance of materials providing the platform to study the interplay between these two unique states. However, the candidates of such materials are rare. Here, we report a systematic angle-resolved photoemission spectroscopy (ARPES) study of a superconducting material CaBi2 [Tc = 2 K], corroborated by the first principles calculations. Our study reveals the presence of Dirac cones with a topological protection in this system. Systematic topological analysis based on symmetry indicator shows the presence of weak topological indices in this material. Furthermore, our transport measurements show the presence of large magnetoresistance in this compound. Our results indicate that CaBi2 could potentially provide a material platform to study the interplay between superconductivity and topology.

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arXiv2019

Temperature Dependent Electronic Structure in a Higher Order Topological Insulator Candidate EuIn$_2$As$_2$

The higher order topological insulator (HOTI) has enticed enormous research interests owing to its novelty in supporting gapless states along the hinges of the crystal. Despite several theoretical predictions, enough experimental confirmation of HOTI state in crystalline solids is still lacking. It has been well known that interplay between topology and magnetism can give rise to various magnetic topological states including HOTI and Axion insulator states. Here using the high-resolution angle-resolved photoemission spectroscopy (ARPES) combined with the first-principles calculations, we report a systematic study on the electronic band topology across the magnetic phase transition in EuIn2As2 which possesses an antiferromagnetic ground state below 16 K. Antiferromagnetic EuIn2As2 has been predicted to host both the Axion insulator and HOTI phase. Our experimental results show the clear signature of the evolution of the topological state across the magnetic transition. Our study thus especially suited to understand the interaction of higher order topology with magnetism in materials.

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arXiv2019

Dirac state switching in transition metal diarsenides

Topological Dirac and Weyl semimetals, which support low-energy quasiparticles in condensed matter physics, are currently attracting intense interest due to exotic physical properties such as large magnetoresistance and high carrier mobilities. Transition metal diarsenides such as MoAs2 and WAs2 have been reported to harbor very high magnetoresistance suggesting the possible existence of a topological quantum state, although this conclusion remains dubious. Here, based on systematic angle-resolved photoemission spectroscopy (ARPES) measurements and parallel first-principles calculations, we investigate the electronic properties of TAs2 (T = Mo, W). Importantly, clear evidence for switching the single-Dirac cone surface state in MoAs2 with the cleaving plane is observed, whereas a Dirac state is not observed in WAs2 despite its high magnetoresistance. Our study thus reveals the key role of the terminated plane in a low-symmetry system, and provides a new perspective on how termination can drive dramatic changes in electronic structures.

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arXiv2019

Termination Dependent Topological Surface States in Nodal Loop Semimetal HfP2

Symmetry plays a major role in all disciplines of physics. Within the field of topological materials there is a great interest in understanding how the mechanics of crystalline and internal symmetries protect crossings between the conduction and valence bands. Additionally, exploring this direction can lead to a deeper understanding on the topological properties of crystals hosting a variety of symmetries. For the first time, we report the experimental observation of topological surface states in the nodal loop semimetal HfP2 using angle resolved photoemission spectroscopy (ARPES) which is supported by our first principles calculations. Our study shows termination dependent surface states in this compound. Our experimental data reveal surface states linked to three unique nodal loops confirmed by theoretical calculation to be topologically non-trivial. This work demonstrates that transition metal dipnictides provide a good platform to study non-trivial topological states protected by nonsymmorphic symmetry.

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arXiv2018

Observation of topological nodal-loop state in RAs3 (R = Ca, Sr)

Topological nodal-line semimetals (NLSs) are unique materials, which harbor one-dimensional line nodes along with the so-called drumhead surface states arising from nearly dispersionless two dimensional surface bands. However, a direct observation of these drumhead surface states in the currently realized NLSs has remained elusive. Here, by using high-resolution angle-resolved photoemission spectroscopy (ARPES) along with parallel first principles calculations, we examine the topological characteristics of SrAs3 and CaAs3. SrAs3 is found to show the presence of a topological nodal-loop, while CaAs3 is found to lie near a topologically trivial phase. Our analysis reveals that the surface projections of the bulk nodal-points in SrAs3 are connected by drumhead surface states. Notably, the topological states in SrAs3 and CaAs3 are well separated from other irrelevant bands in the vicinity of the Fermi level. These compounds thus provide a hydrogen-like simple platform for developing an in-depth understanding of the quantum phase transitions of NLSs.

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arXiv2018

Observation of Dirac state in half-Heusler material YPtBi

The prediction of non-trivial topological electronic states hosted by half-Heusler compounds makes them prime candidates for discovering new physics and devices as they harbor a variety of electronic ground states including superconductivity, magnetism, and heavy fermion behavior. Here we report normal state electronic properties of a superconducting half-Heusler compound YPtBi using angle-resolved photoemission spectroscopy (ARPES). Our data reveal the presence of a Dirac state at the zone center of the Brillouin zone at 500 meV below the chemical potential. We observe the presence of multiple Fermi surface pockets including two concentric hexagonal and six half oval shaped pockets at the gamma and K points of the Brillouin zone, respectively. Furthermore, our measurements show Rashba-split bands and multiple surface states crossing the chemical potential which are supported by the first-principles calculations. Our finding of a Dirac state in YPtBi plays a significant role in establishing half-Heusler compounds as a new potential platform for novel topological phases and explore their connection with superconductivity.

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arXiv2017

Observation of Gapless Dirac Surface States in ZrGeTe

The experimental discovery of the topological Dirac semimetal establishes a platform to search for various exotic quantum phases in real materials. ZrSiS-type materials have recently emerged as topological nodal-line semimetals where gapped Dirac-like surface states are observed. Here, we present a systematic angle-resolved photoemission spectroscopy (ARPES) study of ZrGeTe, a nonsymmorphic symmetry protected Dirac semimetal. We observe two Dirac-like gapless surface states at the same $\bar X$ point of the Brillouin zone. Our theoretical analysis and first-principles calculations reveal that these are protected by crystalline symmetry. Hence, ZrGeTe appears as a rare example of a naturally fine tuned system where the interplay between symmorphic and non-symmorphic symmetry leads to rich phenomenology, and thus opens for opportunities to investigate the physics of Dirac semimetallic and topological insulating phases realized in a single material.

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arXiv2017

Distinct multiple fermionic states in a single topological metal

Among the quantum materials that gained interest recently are the topological Dirac/Weyl semimetals, where conduction and valence bands touch at points in reciprocal (k)-space, and the Dirac nodal-line semimetals, where these bands touch along a line or a loop in k-space. However, the coexistence of multiple fermion phases in one and the same material has not been verified yet. Using angle-resolved photoemission spectroscopy (ARPES) and first-principles electronic structure calculations, we systematically study the metallic topological quantum material, Hf2Te2P. Our investigations discover various properties that are rare and never observed in a single Dirac material. We observe the coexistence of both weak and strong topological surface states in the same material and interestingly, at the same momentum position. An one-dimensional Dirac crossing{the Dirac-node arc-along a high-symmetry direction is revealed by our first-principles calculations and confirmed by our ARPES measurements. This novel state is associated with the surface bands of a weak topological insulator protected by in-plane time-reversal invariance. Ternary compound Hf2Te2P thus emerges as an intriguing platform to study the coexistence and competition of multi-fermionic states in one material.

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