arXiv subjects

N. Phuan Ong

Publications and source records attributed to N. Phuan Ong.

8 recordsLinked to original sources

arXiv2022

Colossal negative magnetoresistance in the complex charge density wave regime of an antiferromagnetic Dirac semimetal

Colossal magnetoresistance (MR) is a well-known phenomenon, notably observed in hole-doped ferromagnetic manganites. It remains a major research topic due to its potential in technological applications. Though topological semimetals also show large MR, its origin and nature are completely different. Here, we show that in the highly electron doped region, the Dirac semimetal CeSbTe demonstrates similar properties as the manganites. CeSb$_{0.11}$Te$_{1.90}$ hosts multiple charge density wave (CDW) modulation-vectors and has a complex magnetic phase diagram. We confirm that this compound is an antiferromagnetic Dirac semimetal. Despite having a metallic Fermi surface, the electronic transport properties are semiconductor-like and deviate from known theoretical models. An external magnetic field induces a semiconductor-metal-like transition, which results in a colossal negative MR. Moreover, signatures of the coupling between the CDW and a spin modulation are observed in resistivity. This spin modulation also produces a giant anomalous Hall response.

cond-mat.mtrl-sci↗

arXiv2022

Catalogue of Flat-Band Stoichiometric Materials

Topological electronic flatten bands near or at the Fermi level are a promising avenue towards unconventional superconductivity and correlated insulating states. However, the related experiments are mostly limited to the engineered materials, such as moire systems. Here we present a catalogue of all the three-dimensional stoichiometric materials with flat bands around the Fermi level that exist in nature. We consider 55,206 materials from the Inorganic Crystal Structure Database catalogued using the Topological Quantum Chemistry website which provides their structural parameters, space group (SG), band structure, density of states and topological characterization. We combine several direct signatures and properties of band flatness to a high-throughput analysis of all crystal structures. In particular, we identify materials hosting line-graph or bipartite sublattices - either in two or three dimensions - likely leading to flat bands. From this trove of information, we create the Materials Flatband Database website, a powerful search engine for future theoretical and experimental studies. We use it to extract a curated list of 2,379 materials, with among them 345 promising candidates, potentially hosting flat bands whose charge centers are not strongly localized on the atomic sites. We showcase five representative materials: KAg[CN]2 in SG 163 $(P\bar{3}1c)$, Pb2Sb2O7 in SG 227 $(Fd\bar{3}m)$, Rb2CaH4 in SG 139 $(I4/mmm)$, Ca2NCl in SG 166 $(R\bar{3}m)$ and WO3 in SG 221 $(Pm\bar{3}m)$. We provide a theoretical explanation for the origin of their flat bands close to the Fermi energy using the $S$-matrix method introduced in a parallel work [Calugaru et al., Nature Physics 18, 185 (2022)].

cond-mat.str-el↗

arXiv2022

A Class of Magnetic Topological Material Candidates with Hypervalent Bi Chains

The link between crystal and electronic structure is crucial for understanding structure-property relations in solid-state chemistry. In particular, it has been instrumental in understanding topological materials, where electrons behave differently than they would in conventional solids. Herein, we identify 1D Bi chains as a structural motif of interest for topological materials. We focus on Sm$_3$ZrBi$_5$, a new quasi-one-dimensional (1D) compound in the Ln$_3$MPn$_5$ (Ln = lanthanide; M = metal; Pn = pnictide) family that crystallizes in the P$6_{3}$/mcm space group. Density functional theory calculations indicate a complex, topologically non-trivial electronic structure that changes significantly in the presence of spin-orbit coupling. Magnetic measurements show a quasi-1D antiferromagnetic structure with two magnetic transitions at 11.7 and 10.7 K that are invariant to applied field up to 9 T, indicating magnetically frustrated spins. Heat capacity, electrical, and thermal transport measurements support this claim and suggest complex scattering behavior in Sm$_3$ZrBi$_5$. This work highlights 1D chains as an unexplored structural motif for identifying topological materials, as well as the potential for rich physical phenomena in the Ln$_3$MPn$_5$ family.

cond-mat.mtrl-sci↗

arXiv2021

Evolving Devil's staircase magnetization from tunable charge density waves in nonsymmorphic Dirac semimetals

While several magnetic topological semimetals have been discovered in recent years, their band structures are far from ideal, often obscured by trivial bands at the Fermi energy. Square-net materials with clean, linearly dispersing bands show potential to circumvent this issue. CeSbTe, a square-net material, features multiple magnetic field-controllable topological phases. Here, it is shown that in this material, even higher degrees of tunability can be achieved by changing the electron count at the square-net motif. Increased electron filling results in structural distortion and formation of charge density waves (CDWs). The modulation wave-vector evolves continuously leading to a region of multiple discrete CDWs and a corresponding complex "Devil's staircase" magnetic ground state. A series of fractionally quantized magnetization plateaus are observed, which implies direct coupling between CDW and a collective spin-excitation. It is further shown that the CDW creates a robust idealized non-symmorphic Dirac semimetal, thus providing access to topological systems with rich magnetism.

cond-mat.mtrl-sci↗

arXiv2021

Band Engineering of Dirac Semimetals using Charge Density Waves

New developments in the field of topological matter are often driven by materials discovery, including novel topological insulators, Dirac semimetals and Weyl semimetals. In the last few years, large efforts have been performed to classify all known inorganic materials with respect to their topology. Unfortunately, a large number of topological materials suffer from non-ideal band structures. For example, topological bands are frequently convoluted with trivial ones, and band structure features of interest can appear far below the Fermi level. This leaves just a handful of materials that are intensively studied. Finding strategies to design new topological materials is a solution. Here we introduce a new mechanism that is based on charge density waves and non-symmorphic symmetry to design an idealized Dirac semimetal. We then show experimentally that the antiferromagnetic compound GdSb$_{0.46}$Te$_{1.48}$ is a nearly ideal Dirac semimetal based on the proposed mechanism, meaning that most interfering bands at the Fermi level are suppressed. Its highly unusual transport behavior points to a thus far unknown regime, in which Dirac carriers with Fermi energy very close to the node seem to gradually localize in the presence of lattice and magnetic disorder.

cond-mat.mtrl-sci↗

arXiv2019

High mobility in a van der Waals layered antiferromagnetic metal

Magnetic van der Waals (vdW) materials have been heavily pursued for fundamental physics as well as for device design. Despite the rapid advances, so far magnetic vdW materials are mainly insulating or semiconducting, and none of them possesses a high electronic mobility - a property that is rare in layered vdW materials in general. The realization of a magnetic high-mobility vdW material would open the possibility for novel magnetic twistronic or spintronic devices. Here we report very high carrier mobility in the layered vdW antiferromagnet GdTe3. The electron mobility is beyond 60,000 cm2 V-1 s-1, which is the highest among all known layered magnetic materials, to the best of our knowledge. Among all known vdW materials, the mobility of bulk GdTe3 is comparable to that of black phosphorus, and is only surpassed by graphite. By mechanical exfoliation, we further demonstrate that GdTe3 can be exfoliated to ultrathin flakes of three monolayers, and that the magnetic order and relatively high mobility is retained in approximately 20-nm-thin flakes.

cond-mat.mtrl-sci↗

arXiv2019

Enhanced thermal Hall conductivity below 1 Kelvin in the pyrochlore magnet Yb$_2$Ti$_2$O$_7$

In this letter, we report the gigantic thermal Hall effect $κ_{xy}$ in the low-field correlated-paramagnetic state of the frustrated pyrochlore Yb$_2$Ti$_2$O$_7$. We observed a record magnitude for the thermal Hall angle in an insulator, $\left|κ_{xy}\right|/κ_{xx}\sim 2\,\%$. The signal onsets at $T\sim 3\,$K and is severely weakened around the transition to canted ferromagnetic order at $T_\text{CFM}=0.275\,$K. Besides the large $κ_{xy}>0$ of the fluctuating regime, a sign change towards negative $κ_{xy}$ occurs at the lowest temperatures and in moderate field, where sharp magnon excitations appear in the inelastic neutron scattering spectra. We analyze the magnon-Hall signal and its suppression with field semi-quantitatively. A contribution of phonon skew scattering to $κ_{xy}$ is ruled out by a comparison of $κ_{xx}$ for Tb-, Yb-, and Y-based rare earth pyrochlore titanates. These results represent the first report of non-vanishing $κ_{xy}$ measured in a dilution refrigerator ($T < 0.29\,$K).

cond-mat.str-el↗

arXiv2017

High-Pressure Synthesis and Characterization of $β$-GeSe - A Semiconductor with Six-Rings in an Uncommon Boat Conformation

Two-dimensional materials have significant potential for the development of new devices. Here we report the electronic and structural properties of $β$-GeSe, a previously unreported polymorph of GeSe, with a unique crystal structure that displays strong two-dimensional structural features. $β$-GeSe is made at high pressure and temperature and is stable under ambient conditions. We compare it to its structural and electronic relatives $α$-GeSe and black phosphorus. The $β$ form of GeSe displays a boat conformation for its Ge-Se six-ring, while the previously known $α$ form, and black phosphorus, display the more common chair conformation for their six-rings. Electronic structure calculations indicate that $β$-GeSe is a semiconductor, with an approximate bulk band gap of $Δ~\approx$ 0.5 eV, and, in its monolayer form, $Δ~\approx$ 0.9 eV. These values fall between those of $α$-GeSe and black phosphorus, making $β$-GeSe a promising candidate for future applications. The resistivity of our $β$-GeSe crystals measured in-plane is on the order of $ρ\approx$ 1 $Ω$cm, while being essentially temperature independent.

cond-mat.mtrl-sci↗