arXiv subjects

N. P. Ong

Publications and source records attributed to N. P. Ong.

At least 55 records · Page 3Linked to original sources

arXiv2015

Thermal Hall conductivity in the frustrated pyrochlore magnet Tb2Ti2O7

In a ferromagnet, the spin excitations are the well-studied magnons. In frustrated quantum magnets, long-range magnetic order fails to develop despite a large exchange coupling between the spins. In contrast to the magnons in conventional magnets, their spin excitations are poorly understood. Are they itinerant or localized? Here we show that the thermal Hall conductivity $κ_{xy}$ provides a powerful probe of spin excitations in the "quantum spin ice" pyrochlore Tb$_2$Ti$_2$O$_7$. The thermal Hall response is large even though the material is transparent. The Hall response arises from spin excitations with specific characteristics that distinguish them from magnons. At low temperature ($T<$ 1 K), the thermal conductivity imitates that of a dirty metal. Using the Hall angle, we construct a phase diagram showing how the excitations are suppressed by a magnetic field.

cond-mat.str-el↗

arXiv2014

Z$_2$ topology and superconductivity from symmetry lowering of a 3D Dirac Metal Au$_2$Pb

3D Dirac semi-metals (DSMs) are materials that have massless Dirac electrons and exhibit exotic physical properties It has been suggested that structurally distorting a DSM can create a Topological Insulator (TI), but this has not yet been experimentally verified. Furthermore, quasiparticle excitations known as Majorana Fermions have been theoretically proposed to exist in materials that exhibit superconductivity and topological surface states. Here we show that the cubic Laves phase Au$_2$Pb has a bulk Dirac cone above 100 K that gaps out upon cooling at a structural phase transition to create a topologically non trivial phase that superconducts below 1.2 K. The nontrivial Z$_2$ = -1 invariant in the low temperature phase indicates that Au$_2$Pb in its superconducting state must have topological surface states. These characteristics make Au$_2$Pb a unique platform for studying the transition between bulk Dirac electrons and topological surface states as well as studying the interaction of superconductivity with topological surface states.

cond-mat.supr-con↗

arXiv2014

Ultrahigh mobility and giant magnetoresistance in the Dirac semimetal Cd$_3$As$_2$

Dirac semimetals and Weyl semimetals are 3D analogs of graphene in which crystalline symmetry protects the nodes against gap formation [1-3]. Na$_3$Bi and Cd$_3$As$_2$ were predicted to be Dirac semimetals [4,5], and recently confirmed to be so by photoemission [6-8]. Several novel transport properties in a magnetic field $\bf H$ have been proposed for Dirac semimetals [2,9-11]. Here we report an interesting property in Cd$_3$As$_2$ that was unpredicted, namely a remarkable protection mechanism that strongly suppresses back-scattering in zero $\bf H$. In single crystals, the protection results in a very high mobility that exceeds $>10^7$ cm$^2$/Vs below 4 K. Suppression of backscattering results in a transport lifetime 10$^4\times$ longer than the quantum lifetime. The lifting of this protection by $\bf H$ leads to an unusual giant $\bf H$-linear magnetoresistance that violates Kohler's rule. We discuss how this may relate to changes to the Fermi surface induced by $\bf H$.

cond-mat.str-el↗

arXiv2014

Titanic Magnetoresistance in WTe2

Magnetoresistance is the change of a material's electrical resistance in response to an applied magnetic field. In addition to its intrinsic scientific interest, it is a technologically important property, placing it in "Pasteur's quadrant" of research value: materials with large magnetorsistance have found use as magnetic sensors 1, in magnetic memory 2, hard drives 3, transistors 4, and are the subject of frequent study in the field of spintronics 5, 6. Here we report the observation of an extremely large one-dimensional positive magnetoresistance (XMR) in the layered transition metal dichalcogenide (TMD) WTe2; 452,700 percent at 4.5 Kelvin in a magnetic field of 14.7 Tesla, and 2.5 million percent at 0.4 Kelvin in 45 Tesla, with no saturation. The XMR is highly anisotropic, maximized in the crystallographic direction where small pockets of holes and electrons are found in the electronic structure. The determination of the origin of this effect and the fabrication of nanostructures and devices based on the XMR of WTe2 will represent a significant new direction in the study and uses of magnetoresistivity. *The published version of the paper includes co-authors Tian Liang and Max Hirschberger. **This paper has been published with new MR data to 60T where the MR of WTe2 reaches 13 million percent (at 0.5K) and still shows no signs of saturation. We also have new electron diffraction patterns to lower temperature (10K). We discuss the possible origin of the MR as coming from an electron-hole 'resonance' condition established by a perfect n/p ratio of 1 (more details in a new "extended data" section). This makes WTe2, possibly, the first realization of a perfectly balanced semimetal. ***The paper is published as "Large non-saturating magnetoresistance in WTe2" in Nature (2014), DOI:10.1038/nature13763

cond-mat.mtrl-sci↗

arXiv2014

Heavy Fermion Quantum Criticality and Destruction of the Kondo Effect in a Nickel Oxypnictide

A quantum critical point arises at a continuous transformation between distinct phases of matter at zero temperature. Studies in antiferromagnetic heavy fermion materials have revealed that quantum criticality has several classes, with an unconventional type that involves a critical destruction of the Kondo entanglement. In order to understand such varieties, it is important to extend the materials basis beyond the usual setting of intermetallic compounds. Here we show that a nickel oxypnictide, CeNiAsO, displays a heavy-fermion antiferromagnetic quantum critical point as a function of either pressure or P/As substitution. At the quantum critical point, non-Fermi liquid behavior appears, which is accompanied by a divergent effective carrier mass. Across the quantum critical point, the low-temperature Hall coefficient undergoes a rapid sign change, suggesting a sudden jump of the Fermi surface and a destruction of the Kondo effect. Our results imply that the enormous materials basis for the oxypnictides, which has been so crucial to the search for high temperature superconductivity, will also play a vital role in the effort to establish the universality classes of quantum criticality in strongly correlated electron systems.

cond-mat.str-el↗

arXiv2014

Paramagnetic to ferromagnetic phase transition in lightly Fe-doped Cr$_2$B

Cr$_2$B displays temperature independent paramagnetism. We induce ferromagnetism by replacing less than $3\,\%$ of the Cr atoms by Fe. By the lowest Fe doping level made, Curie-Weiss behavior is observed; $Θ_{CW}$ changes from $-20\,$K for $0.5\,\%$ Fe-doped Cr$_2$B to positive values of about 50 K by $5\,\%$ Fe doping. The ferromagnetic T$_C$ is 8 K for $2.5\,\%$ Fe doping and increases linearly to 46 K by $5\,\%$ doping; we infer that a quantum phase transition occurs near the $2.0\,\%$ Fe level. Magnetic fluctuations at the intermediate doping levels are reflected in the linear resistance and an anomalous heat capacity at low temperatures. Imaging and chemical analysis down to the atomic scale show that the Fe dopant is randomly distributed.

cond-mat.str-el↗

arXiv2014

Comparison of Sn-doped and nonstoichiometric vertical-Bridgman-grown crystals of the topological insulator Bi2Te2Se

A comparative study of the properties of topological insulator Bi2Te2Se (BTS) crystals grown by the vertical Bridgeman method is described. Two defect mechanisms that create acceptor impurities to compensate for the native n-type carriers are compared: Bi excess, and light Sn doping. Both methods yield low carrier concentrations and an n-p crossover over the length of the grown crystal boules, but lower carrier concentrations and higher resistivities are obtained for the Sn-doped crystals, which reach carrier concentrations as low as 8 x 1014 cm-3. Further, the temperature dependent resistivities for the Sn-doped crystals display strongly activated behavior at high temperatures, with a characteristic energy of half the bulk band gap. The (001) cleaved Sn-doped BTS crystals display high quality Shubnikov de Haas (SdH) quantum oscillations due to the topological surface state electrons. Angle resolved photoelectron spectroscopy (ARPES) characterization shows that the Fermi energy (EF) for the Sn-doped crystals falls cleanly in the surface states with no interference from the bulk bands, that the Dirac point for the surface states lies approximately 60 meV below the top of the bulk valence band maximum, and allows for a determination of the bulk and surface state carrier concentrations as a function of Energy near EF. Electronic structure calculations that compare Bi excess and Sn dopants in BTS demonstrate that Sn acts as a special impurity, with a localized impurity band that acts as a charge buffer occurring inside the bulk band gap. We propose that the special resonant level character of Sn in BTS gives rise to the exceptionally low carrier concentrations and activated resistivities observed.

cond-mat.mtrl-sci↗

arXiv2013

Evidence for massive bulk Dirac Fermions in Pb$_{1-x}$Sn$_x$Se from Nernst and thermopower experiments

The lead chalcogenides (Pb,Sn)Te and (Pb,Sn)Se are the first examples of topological crystalline insulators (TCI) predicted \cite{Fu,Hsieh} (and confirmed \cite{Hasan,Story,Takahashi}) to display topological surface Dirac states (SDS) that are protected by mirror symmetry. A starting premise \cite{Hsieh} is that the SDS arise from bulk states describable as massive Dirac states \cite{Wallis,Svane}, but this assumption is untested. Here we show that the thermoelectric response of the bulk states display features specific to the Dirac spectrum. We show that, in the quantum limit, the lowest Landau Level (LL) is singly spin-degenerate, whereas higher levels are doubly degenerate. The abrupt change in spin degeneracy leads to a large step-decrease in the thermopower $S_{xx}$. In the lowest LL, $S_{xx}$ displays a striking linear increase vs. magnetic field. In addition, the Nernst signal undergoes an anomalous sign change when the bulk gap inverts at 180 K.

cond-mat.str-el↗

arXiv2013

Oscillatory surface dichroism of an insulating topological insulator Bi2Te2Se

Using circular dichroism-angle resolved photoemission spectroscopy (CD-ARPES), we report a study of the effect of angular momentum transfer between polarized photons and topological surface states on the surface of highly bulk insulating topological insulator Bi2Te2Se. The photoelectron dichroism is found to be strongly modulated by the frequency of the helical photons including a dramatic sign-flip. Our results suggest that the observed dichroism and its sign-flip are consequences of strong coupling between the photon field and the spin-orbit nature of the Dirac modes on the surface. Our studies reveal the intrinsic dichroic behavior of topological surface states and point toward the potential utility of bulk insulating topological insulators in device applications.

cond-mat.mes-hall↗

arXiv2013

Tuning the quantum oscillations of surface Dirac electrons in the topological insulator Bi$_2$Te$_2$Se by liquid gating

In Bi$_2$Te$_2$Se, the period of quantum oscillations arising from surface Dirac fermions can be increased 6-fold using ionic liquid gating. At large gate voltages, the Fermi energy reaches the $N$ = 1 Landau level in a 14-Tesla field. This enables the $\frac12$-shift predicted for the Dirac spectrum to be measured accurately. A surprising result is that liquid gating strongly enhances the surface mobility. By analyzing the Hall conductivity, we show that the enhancement occurs on only one surface. We present evidence that the gating process is fully reversible (hence consistent with band-bending by the $E$-field from the anion layer accumulated). In addition to the surface carriers, the experiment yields the mobility and density of the bulk carriers in the impurity band. By analyzing the charge accumulation vs. gate voltage, we also obtain estimates of the depletion width and the areal depletion capacitance $C_d/A$. The value of $C_d/A$ implies an enhanced electronic polarizability in the depletion region.

cond-mat.mes-hall↗

arXiv2013

Phase fluctuations versus Gaussian fluctuations in optimally-doped YBa$_2$Cu$_3$O$_7$

We analyze recent torque measurements of the magnetization $M_d$ vs. magnetic field $H$ in optimally doped YBa$_2$Cu$_3$O$_{7-y}$ (OPT YBCO) to argue against a recent proposal by Rey et al that the magnetization results above $T_c$ are consistent with Gaussian fluctuations. We find that, despite its strong interlayer coupling, OPT YBCO displays an anomalous non-monotonic dependence of $M_d$ on $H$ which represents direct evidence for the locking of the pair wavefunction phase $θ_n$ at $T_c$ and the subsequent unlocking by a relatively weak $H$. These unusual features characterize the unusual nature of the transition to the Meissner state in cuprate superconductors. They are absent in low-$T_c$ superconductors to our knowledge. We also stress the importance of the vortex liquid state, as well as the profiles of the melting field $H_m(T)$ and the upper critical field curve $H_{c2}(T)$ in the $T$-$H$ plane. Contrary to the claims of Rey et al, we show that the curves of the magnetization and the Nernst signal illustrate the inaccessibility of the $H_{c2}$ line near $T_c$. The prediction of the $H_{c2}$ line by Rey et al is shown to be invalid in OPT YBCO.

cond-mat.supr-con↗

arXiv2012

High-Field Shubnikov-de Haas Oscillations in the Topological Insulator Bi$_2$Te$_2$Se

We report measurements of the surface Shubnikov de Haas oscillations (SdH) on crystals of the topological insulator Bi$_2$Te$_2$Se. In crystals with large bulk resistivity ($\sim$4 $Ω$cm at 4 K), we observe $\sim$15 surface SdH oscillations (to the $n$ = 1 Landau Level) in magnetic fields $B$ up to 45 Tesla. Extrapolating to the limit $1/B\to 0$, we confirm the $\frac12$-shift expected from a Dirac spectrum. The results are consistent with a very small surface Lande $g$-factor.

cond-mat.mes-hall↗

arXiv2011

Low carrier concentration crystals of the topological insulator Bi$_2$Te$_2$Se

We report the characterization of Bi$_2$Te$_2$Se crystals obtained by the modified Bridgman and Bridgman-Stockbarger crystal growth techniques. X-ray diffraction study confirms an ordered Se-Te distribution in the inner and outer chalcogen layers, respectively, with a small amount of mixing. The crystals displaying high resistivity ($> 1 \mathrm{Ωcm}$) and low carrier concentration ($\sim 5\times 10^{16}$/cm$^3$) at 4 K were found in the central region of the long Bridgman-Stockbarger crystal, which we attribute to very small differences in defect density along the length of the crystal rod. Analysis of the temperature dependent resistivities and Hall coefficients reveals the possible underlying origins of the donors and acceptors in this phase.

cond-mat.str-el↗

arXiv2011

Thermal Hall Conductivity as a Probe of Gap Structure in Multi-band Superconductors: The Case of $\rm Ba_{1-x}K_xFe_2As_2$

The sign and profile of the thermal Hall conductivity $κ_{xy}$ gives important insights into the gap structure of multi-band superconductors. With this perspective, we have investigated $κ_{xy}$ and the thermal conductivity $κ_{xx}$ in $\rm Ba_{1-x}K_xFe_2As_2$ which display large peak anomalies in the superconducting state. The anomalies imply that a large hole-like quasiparticle (qp) population exists below the critical temperature $T_c$. We show that the qp mean-free-path inferred from $κ_{xx}$ reproduces the observed anomaly in $κ_{xy}$, providing a consistent estimate of a large qp population. Further, we demonstrate that the hole-like signal is consistent with a theoretical scenario where despite potentially large gap variations on the electron pockets, the minimal homogeneous gap of the superconducting phase resides at a hole pocket. Implications for probing the gap structure in the broader class of pnictide superconductors are discussed.

cond-mat.supr-con↗

arXiv2011

Unusual Nernst effect suggestive of time-reversal violation in the striped cuprate La$_{2-x}$Ba$_x$CuO$_4$

The striped cuprate La$_{2-x}$Ba$_x$CuO$_4$ ($x=\frac18)$ undergoes several transitions below the charge-ordering temperature $T_{co}$ = 54 K. From Nernst experiments, we find that, below $T_{co}$, there exists a large, anomalous Nernst signal $e_{N,even}(H,T)$ that is symmetric in field $H$, and remains finite as $H\to 0$. The time-reversal violating signal suggests that, below $T_{co}$, vortices of one sign are spontaneously created to relieve interlayer phase frustration.

cond-mat.supr-con↗

arXiv2011

Effect of hydrostatic pressure on transport in the topological insulator Bi$_2$Te$_2$Se

The Hall coefficient $R_H$ and resistivity $ρ$ of the topological insulator Bi$_2$Te$_2$Se display a number of puzzling features vs. temperature $T$. We propose a model that describes well the non-monotonic variation of $R_H(T)$. A key feature of the model is a chemical potential that is weakly $T$-dependent. From the fit to the model, we infer a "transport" gap $Δ_T$ of 50 mV. We find that hydrostatic pressure $P$ (0-27 kbar) has a pronounced effect on both $R_H$ and $ρ$. We show that these changes arise from decreases in both $Δ_T$ and the hole effective mass under pressure.

cond-mat.mes-hall↗

arXiv2011

Bulk Band Gap and Surface State Conduction Observed in Voltage-Tuned Crystals of the Topological Insulator Bi$_2$Se$_3$

We report a transport study of exfoliated few monolayer crystals of topological insulator Bi$_2$Se$_3$ in an electric field effect (EFE) geometry. By doping the bulk crystals with Ca, we are able to fabricate devices with sufficiently low bulk carrier density to change the sign of the Hall density with the gate voltage $V_g$. We find that the temperature $T$ and magnetic field dependent transport properties in the vicinity of this $V_g$ can be explained by a bulk channel with activation gap of approximately 50 meV and a relatively high mobility metallic channel that dominates at low $T$. The conductance (approximately 2 $\times$ 7$e^2/h$), weak anti-localization, and metallic resistance-temperature profile of the latter lead us to identify it with the protected surface state. The relative smallness of the observed gap implies limitations for EFE topological insulator devices at room temperature.

cond-mat.mes-hall↗

arXiv2011

Quantum Oscillations in a Topological Insulator Bi$_2$Te$_2$Se with Large Bulk Resistivity (6 $Ω$cm)

We report the observation of prominent Shubnikov-de Haas oscillations in a Topological Insulator, Bi$_2$Te$_2$Se, with large bulk resistivity (6 $Ω$cm at 4 K). By fitting the SdH oscillations, we infer a large metallicity parameter $k_F\ell$ = 41, with a surface mobility ($μ_s\sim$ 2,800 cm$^2$/Vs) much larger than the bulk mobility ($μ_b\sim$ 50 cm$^2$/Vs). The plot of the index fields $B_ν$ vs. filling factor $ν$ shows a $\frac12$-shift, consistent with massless, Dirac states. Evidence for fractional-filling states is seen in an 11-T field.

cond-mat.mes-hall↗