arXiv subjects

J. G. Checkelsky

Publications and source records attributed to J. G. Checkelsky.

17 recordsLinked to original sources

arXiv2020

Signatures of ultrafast reversal of excitonic order in Ta$_2$NiSe$_5$

In the presence of electron-phonon coupling, an excitonic insulator harbors two degenerate ground states described by an Ising-type order parameter. Starting from a microscopic Hamiltonian, we derive the equations of motion for the Ising order parameter in the phonon coupled excitonic insulator Ta$_2$NiSe$_5$ and show that it can be controllably reversed on ultrashort timescales using appropriate laser pulse sequences. Using a combination of theory and time-resolved optical reflectivity measurements, we report evidence of such order parameter reversal in Ta$_2$NiSe$_5$ based on the anomalous behavior of its coherently excited order-parameter-coupled phonons. Our work expands the field of ultrafast order parameter control beyond spin and charge ordered materials.

cond-mat.str-el↗

arXiv2014

Quantum Hall Effect on Top and Bottom Surface States of Topological Insulator (Bi1-xSbx)2Te3 Films

The three-dimensional (3D) topological insulator (TI) is a novel state of matter as characterized by two-dimensional (2D) metallic Dirac states on its surface. Bi-based chalcogenides such as Bi2Se3, Bi2Te3, Sb2Te3 and their combined/mixed compounds like Bi2Se2Te and (Bi1-xSbx)2Te3 are typical members of 3D-TIs which have been intensively studied in forms of bulk single crystals and thin films to verify the topological nature of the surface states. Here, we report the realization of the Quantum Hall effect (QHE) on the surface Dirac states in (Bi1-xSbx)2Te3 films (x = 0.84 and 0.88). With electrostatic gate-tuning of the Fermi level in the bulk band gap under magnetic fields, the quantum Hall states with filling factor ν= \pm 1 are resolved with quantized Hall resistance of Ryx = h/e2 and zero longitudinal resistance, owing to chiral edge modes at top/bottom surface Dirac states. Furthermore, the appearance of a ν= 0 state (σxy = 0) reflects a pseudo-spin Hall insulator state when the Fermi level is tuned in between the energy levels of the non-degenerate top and bottom surface Dirac points. The observation of the QHE in 3D TI films may pave a way toward TI-based electronics.

cond-mat.mtrl-sci↗

arXiv2014

Trajectory of Anomalous Hall Effect toward the Quantized State in a Ferromagnetic Topological Insulator

Topological insulators are bulk electronic insulators which possess symmetry protected gapless modes on their surfaces. Breaking the symmetries that underlie the gapless nature of the surface modes is predicted to give rise to exotic new states of matter. In particular, it has recently been predicted and shown that breaking of time reversal symmetry in the form of ferromagnetism can give rise to a gapped state characterized by a zero magnetic field quantized Hall response and dissipationless longitudinal transport known as the Quantum Anomalous Hall (QAH) state. A key question that has thus far remained experimentally unexplored is the relationship of this new type of quantum Hall state with the previously known orbitally driven quantum Hall states. Here, we show experimentally that a ferromagnetic topological insulator exhibiting the QAH state is well described by the global phase diagram of the quantum Hall effect. By mapping the behavior of the conductivity tensor in the parameter space of temperature, magnetic field, and chemical potential in the vicinity of the QAH phase, we find evidence for quantum criticality and delocalization behavior that can quantitatively be described by the renormalization group properties of the quantum Hall ground state. This result demonstrates that the QAH state observed in ferromagnetic topological insulators can be understood within the context of the law of corresponding states which governs the quantum Hall state. This suggests a roadmap for studying the QAH effect including transitions to possible adjacent topologically non-trivial states and a possible universality class for the QAH transition.

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

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↗

arXiv2010

Superconductivity and non-metallicity induced by doping the topological insulators Bi2Se3 and Bi2Te3

We show that by Ca-doping the Bi2Se3 topological insulator, the Fermi level can be fine tuned to fall inside the band gap and therefore suppress the bulk conductivity. Non-metallic Bi2Se3 crystals are obtained. On the other hand, the Bi2Se3 topological insulator can also be induced to become a bulk superconductor, with Tc ~ 3.8 K, by copper intercalation in the van der Waals gaps between the Bi2Se3 layers. Likewise, an as-grown crystal of metallic Bi2Te3 can be turned into a non-metallic crystal by slight variation of the Te content. The Bi2Te3 topological insulator shows small amounts of superconductivity with Tc ~ 5.5 K when reacted with Pd to form materials of the type PdxBi2Te3.

cond-mat.str-el↗

arXiv2010

The development of ferromagnetism in the doped topological insulator Bi2-xMnxTe3

The development of ferromagnetism in Mn-doped Bi2Te3 is characterized through measurements on a series of single crystals with different Mn content. Scanning tunneling microscopy analysis shows that the Mn substitutes on the Bi sites, forming compounds of the type Bi2-xMnxTe3, and that the Mn substitutions are randomly distributed, not clustered. Mn doping first gives rise to local magnetic moments with Curie-like behavior, but by the compositions Bi1.96Mn0.04Te3 and Bi1.91Mn0.09Te3 a second order ferromagnetic transition is observed, with Tc ~ 9-12 K. The easy axis of magnetization in the ferromagnetic phase is perpendicular to the Bi2Te3 basal plane. Thermoelectric power and Hall effect measurements show that the Mn-doped Bi2Te3 crystals are p-type. Angle resolved photoemission spectroscopy measurements show that the topological surface states that are present in pristine Bi2Te3 are also present in ferromagnetic Mn-doped Bi2-xMnxTe3, and that the dispersion relations of the surface states are changed in a subtle fashion.

cond-mat.mtrl-sci↗

arXiv2010

First observation of Spin-Momentum Helical Locking in Bi2Se3 and Bi2Te3, demonstration of Topological-Order at 300K and a realization of topological-transport-regime

Both the theoretical and experimental discovery of single-Dirac-cone topological-insulator-class was reported at arXiv:0812.2078 (2008) [Y. Xia et.al., Nature Physics 5, 398-402 (2009) http://www.nature.com/nphys/journal/v5/n6/full/nphys1294.html]. Here we report the first observation of Spin-Momentum Helical Locking and Spin-Vortex structures in Bi2Se3 and Bi2Te3, demonstrate the existence of Topological-Order at 300K and report a material realization of topological-transport-regime for helical Dirac fermions. Our results reveal a one-to-one spin-momentum locked Dirac structure in Bi2Se3 and Bi2Te3 that is nearly 100% spin-polarized, which exhibits a tunable topological fermion density in the vicinity of the Kramers' point and can be driven to the long-sought topological-transport-regime. The observed topological nodal Dirac ground state is found to be protected even up to room temperature (300 K). Our results pave the way for future transport based studies of topological insulators, and possible room temperature applications of protected spin-polarized edge channels we observe with spin-ARPES in spintronic technology. All of these new results are made possible due to the Spin-resolved-ARPES (Mott polarimetric) technique [http://www.nature.com/nature/journal/v460/n7259/full/nature08234.html ] .

cond-mat.mes-hall↗

arXiv2009

Quantum interference in macroscopic crystals of non-metallic Bi$_2$Se$_3$

Photoemission experiments have shown that Bi$_2$Se$_3$ is a topological insulator. By controlled doping, we have obtained crystals of Bi$_2$Se$_3$ with non-metallic conduction. At low temperatures, we uncover a novel type of magnetofingerprint signal which involves the spin degrees of freedom. Given the mm-sized crystals, the observed amplitude is 200-500$\times$ larger than expected from universal conductance fluctuations. The results point to very long phase breaking lengths in an unusual conductance channel in these non-metallic samples. We discuss the nature of the in-gap conducting states and their relation to the topological surface states.

cond-mat.mes-hall↗

arXiv2009

Superconductivity in CuxBi2Se3 and its implications for pairing in the undoped topological insulator

Bi2Se3 is one of a handful of known topological insulators. Here we show that copper intercalation in the van der Waals gaps between the Bi2Se3 layers, yielding an electron concentration of ~ 2 x 10^20cm-3, results in superconductivity at 3.8 K in CuxBi2Se3 for x between 0.12 and 0.15. This demonstrates that Cooper pairing is possible in Bi2Se3 at accessible temperatures, with implications for study of the physics of topological insulators and potential devices.

cond-mat.supr-con↗

arXiv2009

p-type Bi2Se3 for topological insulator and low temperature thermoelectric applications

The growth and elementary properties of p-type Bi2Se3 single crystals are reported. Based on a hypothesis about the defect chemistry of Bi2Se3, the p-type behavior has been induced through low level substitutions (1 percent or less) of Ca for Bi. Scanning tunneling microscopy is employed to image the defects and establish their charge. Tunneling and angle resolved photoemission spectra show that the Fermi level has been lowered into the valence band by about 400 meV in Bi1.98Ca0.02Se3 relative to the n-type material. p-type single crystals with ab plane Seebeck coefficients of +180 microVK-1 at room temperature are reported. These crystals show a giant anomalous peak in the Seebeck coefficient at low temperatures, reaching +120 microVK-1 at 7 K, giving them a high thermoelectric power factor at low temperatures. In addition to its interesting thermoelectric properties, p-type Bi2Se3 is of substantial interest for studies of technologies and phenomena proposed for topological insulators.

cond-mat.mtrl-sci↗

arXiv2008

Tightly-bound Cooper pair, quasiparticle kinks and clues on the pairing potential in a high Tc FeAs Superconductor

We present a systematic photoemission study of the newly discovered high Tc superconductor class (Sr/Ba)1-xKxFe2As2. By utilizing a unique photon energy range and scattering geometry we resolve the details of the single particle dynamics of interacting electrons on the central Fermi surfaces of this series which shows overall strong coupling behavior (2D/kBTc = 6). Quasiparticle dispersion kinks are observed in a binding energy range of 15 to 50 meV which matches the magnetic excitation energy scales (parameterized by J1,J2). The size of the Cooper pair wavefunction is found to be less than 20A indicating a short in-plane scale uncharacteristic of a BCS-phonon scenario but suggestive of a phase factor in the global order parameter. The kink likely reflects contributions from the strongly frustrated fluctuating spin excitations and the soft phonons around 20-40 meV. Our results provide important clue to the nature of the pairing potential realized in these superconductors.

cond-mat.supr-con↗

arXiv2007

The anomalous Hall Effect and magnetoresistance in the layered ferromagnet Fe_{1/4}TaS_2: the inelastic regime

The large magnetic anisotropy in the layered ferromagnet Fe_{1/4}TaS_2 leads to very sharp reversals of the magnetization $\bf M$ at the coercive field. We have exploited this feature to measure the anomalous Hall effect (AHE), focussing on the AHE conductivity $σ^A_{xy}$ in the inelastic regime. At low temperature T (5-50 K), $σ^A_{xy}$ is T-independent, consistent with the Berry-phase/Karplus-Luttinger theory. Above 50 K, we extract an inelastic AHE conductivity $σ^{in}_{xy}$ that scales as the square of $Δρ$ (the T dependent part of the resistivity $ρ$). The term $σ^{in}_{xy}$ clarifies the T dependence and sign-reversal of the AHE coefficient R_s(T). We discuss the possible ubiquity of $σ^{in}_{xy}$ in ferromagnets, and ideas for interpreting its scaling with $(Δρ)^2$. Measurements of the magnetoresistance (MR) reveal a rich pattern of behavior vs. T and field tilt-angle. We show that the 2 mechanisms, the anisotropic MR effect and field-suppression of magnons, account for the intricate MR behavior, including the bow-tie features caused by the sharp reversals in $\bf M$.

cond-mat.str-el↗

arXiv2007

Online Supplementary Material: Low temperature vortex liquid in $\rm La_{2-x}Sr_xCuO_4$

We report here the supplementary material for Ref. [1]. Using torque magnetometry, we measured the magnetization anisotropy of 7 crystals of $\rm La_{2-x}Sr_xCuO_4$ (LSCO). In optimally-doped cuprates, the bulk susceptibility is dominated by the paramagnetic van-Vleck orbital term $χ_{orb}$ which has a significant anisotropy ($χ^{orb}_c>χ^{orb}_{ab}$) that changes weakly with $T$ (subscripts $c$ and ${ab}$ identify quantities measured with $\bf H||c$ and $\bf H\perp c$, respectively). Moreover, in the lightly-doped regime, the paramagnetic spin susceptibilities $χ^s_c$ and $χ^s_{ab}$ become significantly large below the interval 40-60 K. However, the spin susceptibility is very nearly isotropic (except below 10 K where its anisotropy becomes measurable). Against the large orbital and spin terms, the weak diamagnetic signal is very difficult to resolve using standard bulk magnetometry in lightly-doped cuprates. By contrast, torque magnetometry selectively detects the orbital diamagnetism generated by supercurrents confined to the CuO$_2$ layers while ignoring the large spin contribution when it is isotropic. The orbital van-Vleck contribution is also detected, but as a ``background'' that is $H$-linear to intense fields and only mildly $T$ dependent.

cond-mat.supr-con↗

arXiv2007

Low temperature vortex liquid in $\rm La_{2-x}Sr_xCuO_4$

In the cuprates, the lightly-doped region is of major interest because superconductivity, antiferromagnetism, and the pseudogap state \cite{Timusk,Lee,Anderson} come together near a critical doping value $x_c$. These states are deeply influenced by phase fluctuations \cite{Emery} which lead to a vortex-liquid state that surrounds the superconducting region \cite{WangPRB01,WangPRB06}. However, many questions \cite{Doniach,Fisher,FisherLee,Tesanovic,Sachdev} related to the nature of the transition and vortex-liquid state at very low tempera- tures $T$ remain open because the diamagnetic signal is difficult to resolve in this region. Here, we report torque magnetometry results on $\rm La_{2-x}Sr_xCuO_4$ (LSCO) which show that superconductivity is lost at $x_c$ by quantum phase fluctuations. We find that, in a magnetic field $H$, the vortex solid-to-liquid transition occurs at field $H_m$ much lower than the depairing field $H_{c2}$. The vortex liquid exists in the large field interval $H_m \ll H_{c2}$, even in the limit $T\to$0. The resulting phase diagram reveals the large fraction of the $x$-$H$ plane occupied by the quantum vortex liquid.

cond-mat.supr-con↗

arXiv2007

Sharp switching of the magnetization in Fe1/4TaS2

Anisotropic magneto-transport measurements are reported on Fe1/4TaS2 single crystals grown by vapor transport. Both the magnetization and resistivity are extremely anisotropic, with the magnetic moments aligned parallel to the c crystallographic direction. Fe1/4TaS2 orders ferromagnetically below TC = 160 K and displays very sharp hysteresis loops in the ordered state for H||c. The corresponding magnetoresistance is negative, and it qualitatively reproduces the features observed in the M(H) data, by showing a sharp drop around the critical field Hs for the moment reversal. The magnetization switching time shows an unusual increase with increasing temperature. For field applied within the ab plane, the magnetization remains small and linear in field up to 5 T, and the magnetoresistance is positive and quadratic in field, with no visible hysteresis. The squareness of the H||c M(H) loops and the high critical field for the magnetization switch (Hs = 3.7 T at T = 2 K) allow us to classify Fe1/4TaS2 as a strong ferromagnet.

cond-mat.str-el↗

arXiv2006

Depairing field, onset temperature and the nature of the transition in cuprates

The depairing (upper critical) field $H_{c2}$ in hole-doped cuprates has been inferred from magnetization curves $M$-$H$ measured by torque magnetometry in fields $H$ up to 45 T. We discuss the implications of the results for the pair binding energy, the Nernst onset temperature, fluctuations and the nature of the Meissner transition at $T_c$.

cond-mat.supr-con↗