arXiv subjects

D. Graf

Publications and source records attributed to D. Graf.

At least 73 records · Page 4Linked to original sources

arXiv2017

Quantum oscillations in metallic Sb2Te2Se topological insulator

We have studied the magnetotransport properties of the metallic, p-type Sb2Te2Se which is a topological insulator. Magnetoresistance shows Shubnikov de Haas oscillations in fields above B=15 T. The maxima/minima positions of oscillations measured at different tilt angles with respect to the B direction align with the normal component of field Bcosine, implying the existence of a 2D Fermi surface in Sb2Te2Se. The value of the Berry phase determined from a Landau level fan diagram is very close to 0.5, further suggesting that the oscillations result from topological surface states. From Lifshitz-Kosevich analyses, the position of the Fermi level is found to be EF =250 meV, above the Dirac point. This value of EF is almost 3 times as large as that in our previous study on the Bi2Se2:1Te0:9 topological insulator; however, it still touches the tip of the bulk valence band. This explains the metallic behavior and hole-like bulk charge carriers in the Sb2Te2Se compound.

cond-mat.mes-hall↗

arXiv2016

Interlayer electronic transport in CaMnBi$_{2}$ antiferromagnet

We report interlayer electronic transport in CaMnBi$_{2}$ single crystals. Quantum oscillations and angular magnetoresistance suggest coherent electronic conduction and valley polarized conduction of Dirac states. Small cyclotron mass, large mobility of carriers and nontrivial Berry's phase are consistent with the presence of Dirac fermions on the side wall of the warped cylindrical Fermi surface. Similar to SrMnBi$_{2}$ that features an anisotropic Dirac cone, our results suggest that magnetic field-induced changes in the interlayer conduction are also present in layered bismuth-based materials with zero-energy line in momentum space created by the staggered alkaline earth atoms.

cond-mat.mes-hall↗

arXiv2016

Unusual interlayer quantum transport behavior caused by the zeroth Landau level in YbMnBi2

Relativistic fermions in topological quantum materials are characterized by linear energy-momentum dispersion near band crossing points. Under magnetic field, relativistic fermions acquire Berry phase of π in cyclotron motion, leading to a zeroth Landau level (LL) at the crossing point. Such field-independent zeroth LL, which distinguishes relativistic fermions from conventional electron systems, is hardly probed in transport measurements since the Fermi energy (EF) is usually not right at the band crossing points in most topological materials. Here we report the observation of exotic quantum transport behavior resulting from the zeroth LL in a multiband topological semimetal YbMnBi2 which possesses linear band crossings both at and away from the Fermi level (FL). We show that the Dirac bands with the crossing points being above or below the FL leads to Shubnikov de-Haas oscillations in the in-plane magnetoresistance, whereas the Dirac bands with the crossing points being at the FL results in unusual angular dependences of the out-of-plane magnetoresistance and in-plane Hall resistivity due to the dependence of the zeroth LL's degeneracy on field orientation. Our results shed light on the transport mechanism of the zeroth LL's relativistic fermions in layered materials.

cond-mat.mtrl-sci↗

arXiv2016

Nearly massless Dirac fermions hosted by Sb square net in BaMnSb2

Layered compounds AMnBi2 (A=Ca, Sr, Ba, or rare earth element) have been established as Dirac materials. Dirac electrons generated by the two-dimensional (2D) Bi square net in these materials are normally massive due to the presence of a spin-orbital coupling (SOC) induced gap at Dirac nodes. Here we report that the Sb square net in an isostructural compound BaMnSb2 can host nearly massless Dirac fermions. We observed strong Shubnikov-de Haas (SdH) oscillations in this material. From the analyses of the SdH oscillations, we find key signatures of Dirac fermions, including light effective mass (~0.052m0; m0, mass of free electron), high quantum mobility (1280 cm2V-1S-1) and a Pi Berry phase accumulated along cyclotron orbit. Compared with AMnBi2, BaMnSb2 also exhibits much more significant quasi two-dimensional (2D) electronic structure, with the out-of-plane transport showing nonmetallic conduction below 120K and the ratio of the out-of-plane and in-plane resistivity reaching ~670. Additionally, BaMnSb2 also exhibits an antiferromagnetic order with a weak ferromagnetic component. The combination of nearly massless Dirac fermions on quasi-2D planes with a magnetic order makes BaMnSb2 an intriguing platform for seeking novel exotic phenomena of massless Dirac electrons.

cond-mat.mtrl-sci↗

arXiv2016

Large magnetothermopower and Fermi surface reconstruction in Sb$_2$Te$_2$Se

We report the magnetoresistance, magnetothermopower and quantum oscillation study of Sb$_2$Te$_2$Se single crystal. The in-plane transverse magnetoresistance exhibits a crossover at a critical field $B^*$ from semiclassical weak-field $B^2$ dependence to the high-field unsaturated linear magnetoresistance which persists up to the room temperature. The low-temperature Seebeck coefficient is negative in zero field contrary to the positive Hall resistivity, indicating the multiband effect. The magnetic field induced the sign reversion of the Seebeck coefficient between 2 K and 150 K, . The quantum oscillation of crystals reveals the quasi-two-dimensional (quasi-2D) Fermi surface. These effects are possibly attributed to the large Fermi surface which touches Brillouin zone boundary to becomes quasi-2D and the variation in the chemical potential induced by the magnetic field.

cond-mat.mes-hall↗

arXiv2016

Quasi-two-dimensional Dirac fermions and quantum magnetoresistance in LaAgBi$_2$

We report quasi-two-dimensional Dirac fermions and quantum magnetoresistance in LaAgBi$_2$. The band structure shows several narrow bands with nearly linear energy dispersion and Dirac-cone-like points at the Fermi level. The quantum oscillation experiments revealed one quasi-two-dimensional Fermi pocket and another complex pocket with small cyclotron resonant mass. The in-plane transverse magnetoresistance exhibits a crossover at a critical field $B^*$ from semiclassical weak-field $B^2$ dependence to the high-field unsaturated linear magnetoresistance which is attributed to the quantum limit of the Dirac fermions. Our results suggest the existence of quasi 2D Dirac fermions in rare-earth based layered compounds with two-dimensional double-sized Bi square nets, similar to (Ca,Sr)MnBi$_{2}$, irrespective of magnetic order.

cond-mat.mes-hall↗

arXiv2016

Critical Doping for the Onset of Fermi-Surface Reconstruction by Charge-Density-Wave Order in the Cuprate Superconductor La$ _{2-x} $Sr$_{x} $CuO$ _{4}$

The Seebeck coefficient $S$ of the cuprate superconductor La$ _{2-x} $Sr$_{x} $CuO$ _{4}$ (LSCO) was measured in magnetic fields large enough to access the normal state at low temperatures, for a range of Sr concentrations from $x = 0.07$ to $x = 0.15$. For $x = 0.11$, 0.12, 0.125 and 0.13, $S/T$ decreases upon cooling to become negative at low temperatures. The same behavior is observed in the Hall coefficient $R_{H}(T)$. In analogy with other hole-doped cuprates at similar hole concentrations $p$, the negative $S$ and $R_{H}$ show that the Fermi surface of LSCO undergoes a reconstruction caused by the onset of charge-density-wave modulations. Such modulations have indeed been detected in LSCO by X-ray diffraction in precisely the same doping range. Our data show that in LSCO this Fermi-surface reconstruction is confined to $0.085 < p < 0.15$. We argue that in the field-induced normal state of LSCO, charge-density-wave order ends at a critical doping $p_{\rm CDW} = 0.15 \pm 0.005$, well below the pseudogap critical doping $p^\star \simeq 0.19$.

cond-mat.supr-con↗

arXiv2016

Two-dimensional Dirac fermions in YbMnBi2 antiferromagnet

We report two-dimensional quantum transport and Dirac fermions in YbMnBi2 single crystals. YbMnBi2 is a layered material with anisotropic conductivity and magnetic order below 290 K. Magnetotransport properties, nonzero Berry phase and small cyclotron mass indicate the presence of quasi two dimensional Dirac fermions. Quantum oscillations in Hall resistivity suggest the presence of both electron and hole parts of the Fermi surface whereas the Berry phase suggests spin-orbit coupling.

cond-mat.mes-hall↗

arXiv2016

Electron-hole asymmetry, Dirac fermions, and quantum magnetoresistance in BaMnBi2

We report two-dimensional quantum transport and Dirac fermions in BaMnBi2 single crystals. BaMnBi2 is a layered bad metal with highly anisotropic conductivity and magnetic order below 290 K. Magnetotransport properties, nonzero Berry phase, small cyclotronmass, and the first-principles band structure calculations indicate the presence of Dirac fermions in Bi square nets. Quantum oscillations in the Hall channel suggest the presence of both electron and hole pockets, whereas Dirac and parabolic states coexist at the Fermi level.

cond-mat.mtrl-sci↗

arXiv2016

Wiedemann-Franz law in the underdoped cuprate superconductor YBa2Cu3Oy

The recent detection of charge-density modulations in YBa2Cu3Oy and other cuprate superconductors raises new questions about the normal state of underdoped cuprates. In one class of theories, the modulations are intertwined with pairing in a dual state, expected to persist up to high magnetic fields as a vortex liquid. In support of such a state, specific heat and magnetisation data on YBa2Cu3Oy have been interpreted in terms of a vortex liquid persisting above the vortex-melting field Hvs at T = 0. Here we report high-field measurements of the electrical and thermal Hall conductivities in YBa2Cu3O6.54 that allow us to probe the Wiedemann-Franz law, a sensitive test of the presence of superconductivity in a metal. In the T = 0 limit, we find that the law is satisfied for fields immediately above Hvs. This rules out the existence of a vortex liquid and it places strict constraints on the nature of the normal state in underdoped cuprates.

cond-mat.supr-con↗

arXiv2016

Inter-planar coupling dependent magnetoresistivity in high purity layered metals

The magnetic field-induced changes in the conductivity of metals are the subject of intense interest, both for revealing new phenomena and as a valuable tool for determining their Fermi surface. Here, we report a hitherto unobserved magnetoresistive effect in ultra-clean layered metals, namely a negative longitudinal magnetoresistance that is capable of overcoming their very pronounced orbital one. This effect is correlated with the inter-layer coupling disappearing for fields applied along the so-called Yamaji angles where the inter-layer coupling vanishes. Therefore, it is intrinsically associated with the Fermi points in the field-induced quasi-one-dimensional electronic dispersion, implying that it results from the axial anomaly among these Fermi points. In its original formulation, the anomaly is predicted to violate separate number conservation laws for left- and right-handed chiral- (e.g. Weyl) fermions. Its observation in PdCoO$_2$, PtCoO$_2$ and Sr$_2$RuO$_4$ suggests that the anomaly affects the transport of clean conductors, particularly near the quantum limit.

cond-mat.mes-hall↗

arXiv2015

Temperature - Pressure phase diagram of the cubic Laves phase Au$_2$Pb

The temperature ($T$) as a function of pressure ($P$) phase diagram is reported for the cubic Laves phase compound Au$_2$Pb, which was recently proposed to support linearly dispersing "topological" bands, together with conventional quadratic bands. At ambient pressure, Au$_2$Pb exhibits several structural phase transitions at $T_1$ $=$ 97 K, $T_2$ $=$ 51 K, and $T_3$ $=$ 40 K with superconductivity below $T_{\rm{c}}$ $=$ 1.2 K. Applied pressure results in a rich phase diagram where $T_1$, $T_2$, and $T_3$ evolve strongly with $P$ and a new phase is stabilized for $P$ $>$ 0.64 GPa that also supports superconductivity below 1.1 K. These observations suggest that Au$_2$Pb is an ideal system in which to investigate the relationship between structural degrees of freedom, band topology, and resulting anomalous behaviors.

cond-mat.mtrl-sci↗

arXiv2015

Large Fermi Surface of Heavy Electrons at the Border of Mott Insulating State in NiS2

One of the early triumphs of quantum physics is the explanation why some materials are metallic whereas others are insulating. While a treatment based on single electron states correctly predicts the character of most materials this approach can fail spectacularly, when the electrostatic repulsion between electrons causes strong correlations. Not only can these favor new and subtle forms of order in metals, such as magnetism or superconductivity, they can even cause the electrons in a half-filled energy band to lock into position altogether, producing a correlated, or Mott insulator. Arguably the most extreme manifestation of electronic correlations in dense electronic matter, the transition into the Mott insulating state raises a number of fundamental questions. Foremost among these is the fate of the electronic Fermi surface and the associated charge carrier mass, as the Mott transition is approached at low temperature, which have been particularly controversial in the high temperature superconducting cuprates. We report the first direct observation of the Fermi surface on the metallic side of a Mott insulating transition by high pressure quantum oscillatory measurements in NiS2. Our results point at a large Fermi surface consistent with Luttinger's theorem and a strongly enhanced carrier effective mass, suggesting that electron localization occurs via a diverging effective mass and concomitant slowing down of charge carriers as predicted theoretically.

cond-mat.str-el↗

arXiv2015

Onset field for Fermi-surface reconstruction in the cuprate superconductor YBCO

Quantum oscillations and negative Hall and Seebeck coefficients at low temperature and high magnetic field have shown the Fermi surface of underdoped cuprates to contain a small closed electron pocket. It is thought to result from a reconstruction by charge order, but whether it is the order seen by NMR and ultrasound above a threshold field or the short-range modulations seen by X-ray diffraction in zero field is unclear. Here we use measurements of the thermal Hall conductivity in YBCO to show that Fermi-surface reconstruction occurs only above a sharply defined onset field, equal to the transition field seen in ultrasound. This reveals that electrons do not experience long-range broken translational symmetry in the zero-field ground state, and hence in zero field there is no quantum critical point for the onset of charge order as a function of doping.

cond-mat.supr-con↗

arXiv2015

Pi Berry phase and Zeeman splitting of TaP probed by high field magnetotransport measurements

The chiral anomaly-induced negative magnetoresistance and non-trivial Berry phase are two fundamental transport properties associated with the topological properties of Weyl fermions. In this work, we report the quantum transport of TaP single crystals in magnetic field up to 31T. Through the analyses of our magnetotransport data, we show TaP has the signatures of a Weyl state, including light effective quasiparticle masses, ultrahigh carrier mobility, as well as negative longitudinal magnetoresistance. Furthermore, we have generalized the Lifshitz-Kosevich formula for Shubnikov-de Haas (SdH) oscillations with multi-frequencies, and determined the non-trivial Berry phase of Pi for multiple Fermi pockets in TaP through the direct fitting of the quantum oscillations. In high fields, we also probed signatures of Zeeman splitting, from which the Landé g-factor is extracted.

cond-mat.mtrl-sci↗

arXiv2014

Tuning the Fermi level through the Dirac point of giant Rashba semiconductor BiTeI with pressure

We report measurements of Shubnikov-de Haas oscillations in the giant Rashba semiconductor BiTeI under applied pressures up to $\sim 2\,\mathrm{GPa}$. We observe one high frequency oscillation at all pressures and one low frequency oscillation that emerges between $\sim 0.3-0.7\,\mathrm{GPa}$ indicating the appearance of a second small Fermi surface. BiTeI has a conduction band bottom that is split into two sub-bands due to the strong Rashba coupling, resulting in a `Dirac point'. Our results suggest that the chemical potential starts below the Dirac point in the conduction band at ambient pressure and moves upward, crossing it as pressure is increased. The presence of the chemical potential above this Dirac point results in two Fermi surfaces. We present a simple model that captures this effect and can be used to understand the pressure dependence of our sample parameters. These extracted parameters are in quantitative agreement with first-principles calculations and other experiments. The parameters extracted via our model support the notion that pressure brings the system closer to the predicted topological quantum phase transition.

cond-mat.mtrl-sci↗

arXiv2014

Anomalous magnetic ground state in LaAlO3/SrTiO3 interface probed by transport through nanowires

Resistance as a function of temperature down to 20mK and magnetic fields up to 18T for various carrier concentrations is measured for nanowires made from the SrTiO3/LaAlO3 interface using a hard mask shadow deposition technique. The narrow width of the wires (of the order of 50nm) allows us to separate out the magnetic effects from the dominant superconducting ones at low magnetic fields. At this regime hysteresis loops are observed along with the superconducting transition. From our data analysis we find that the magnetic order probed by the giant magnetoresistance (GMR) effect vanishes at TCurie = 954 mK. This order is not a simple ferromagnetic state but consists of domains with opposite magnetization having a preferred in-plane orientation.

cond-mat.mes-hall↗

arXiv2014

Anisotropic giant magnetoresistance in NbSb2

The extremely large transverse magnetoreistance (the magnetoresistant ratio $\sim 1.3\times10^5\%$ in 2 K and 9 T field, and $4.3\times 10^6\%$ in 0.4 K and 32 T field, without saturation), and the metal-semiconductor crossover induced by magnetic field, are reported in NbSb$_2$ single crystal with electric current parallel to the $b$-axis. The metal-semiconductor crossover is preserved when the current is along the $ac$-plane but the magnetoresistant ratio is significantly suppressed. The sign reversal of the Hall resistivity in the field close to the crossover point, and the electronic structure calculation reveals the coexistence of a small number of holes with very high mobility and a large number of electrons with low mobility. These effects are attributed to the change of the Fermi surface induced by the magnetic field.

cond-mat.mtrl-sci↗