arXiv subjects

Helmuth Berger

Publications and source records attributed to Helmuth Berger.

At least 55 records · Page 3Linked to original sources

arXiv2018

Site-Specific Spin Reorientation in Antiferromagnetic State of Quantum System SeCuO$_3$

We report on the magnetocrystalline anisotropy energy (MAE) and spin reorientation in antiferromagnetic state of spin $S=1/2$ tetramer system SeCuO$_3$ observed in torque magnetometry measurements in magnetic fields $H<5$~T and simulated using density functional calculations. We employ simple phenomenological model of spin reorientation in finite magnetic field to describe our experimental torque data. Our results strongly support collinear model for magnetic structure in zero field with possibility of only very weak canting. Torque measurements also indicate that, contrary to what is expected for uniaxial antiferromagnet, in SeCuO$_3$ only part of the spins exhibit spin flop instead all of them, allowing us to conclude that AFM state of SeCuO$_3$ is unconventional and comprised of two decoupled subsystems. Taking into account previously proposed site-selective correlations and dimer singlet state formation in this system, our results offer further proof that AFM state in SeCuO$_3$ is composed of a subsystem of AFM dimers forming singlets immersed in antiferromagnetically long-range ordered spins, where both states coexist on atomic scale. Furthermore, we show, using an ab-initio approach, that both subsystems contribute differently to the MAE, corroborating the existence of decoupled subnetworks in SeCuO$_3$. Combination of torque magnetometry, phenomenological approach and DFT simulations to magnetic anisotropy presented here represents a unique and original way to study site-specific reorientation phenomena in quantum magnets.

cond-mat.str-el↗

arXiv2018

Tunneling spectroscopy of gate-induced superconductivity in MoS$_2$

The ability to gate-induce superconductivity by electrostatic charge accumulation is a recent breakthrough in physics and nano-electronics. With the exception of LaAlO$_3$/SrTiO$_3$ interfaces, experiments on gate-induced superconductors have been largely confined to resistance measurements, which provide very limited information about the superconducting state. Here, we explore gate-induced superconductivity in MoS$_2$ by performing tunneling spectroscopy to determine the energy-dependent density of states (DOS) for different levels of electron density $\textit{n}$. In the superconducting state, the DOS is strongly suppressed at energy smaller than the gap, Δ, which is maximum (Δ~ 2 meV) for $\textit{n}$ of ~ 10$^{14}$ cm$^{-2}$ and decreases monotonously for larger $\textit{n}$. A perpendicular magnetic field $\textit{B}$ generates states at $E<Δ$ that fill the gap, but a 20% DOS suppression of superconducting origin unexpectedly persists much above the transport critical field. Conversely, an in-plane field up to 10 T leaves the DOS entirely unchanged. Our measurements exclude that the superconducting state in MoS$_2$ is fully gapped and reveal the presence of a DOS that vanishes linearly with energy, the explanation of which requires going beyond a conventional, purely phonon-driven Bardeen-Cooper-Schrieffer mechanism.

cond-mat.supr-con↗

arXiv2018

Disrupted orbital order and the pseudo-gap in layered 1T-TaS$_2$

We present a state-of-the-art density functional theory (DFT) study which models crucial features of the partially disordered orbital order stacking in the prototypical layered transition metal dichalcogenide 1T-TaS2 . Our results not only show that DFT models with realistic assumptions about the orbital order perpendicular to the layers yield band structures which agree remarkably well with experiments. They also demonstrate that DFT correctly predicts the formation of an excitation pseudo-gap which is commonly attributed to Mott-Hubbard type electron-electron correlations. These results highlight the importance of interlayer interactions in layered transition metal dichalcogenides and serve as an intriguing example of how disorder within an electronic crystal can give rise to pseudo-gap features.

cond-mat.str-el↗

arXiv2018

Magnetic excitations and amplitude fluctuations in insulating cuprates

We present results from light scattering experiments on three insulating antiferromagnetic cuprates, YBa$_2$Cu$_3$O$_{6.05}$, Bi$_2$Sr$_2$YCu$_2$O$_{8+δ}$, and La$_2$CuO$_4$ as a function of polarization and excitation energy {using samples of the latest generation. From the raw data we derive symmetry-resolved spectra.} The spectral shape in $B_{1g}$ symmetry is found to be nearly universal and independent of the excitation energy. The spectra agree quantitatively with predictions by field theory [\onlinecite{Weidinger:2015}] facilitating the precise extraction of the Heisenberg coupling $J$. {In addition, the asymmetric line shape on the high-energy side is found to be related to amplitude fluctuations of the magnetization. In La$_2$CuO$_4$ alone minor contributions from resonance effects may be identified.} The spectra in the other symmetries are not universal. The variations may be traced back to weak resonance effects and extrinsic contributions. For all three compounds we find support for the existence of chiral excitations appearing as a continuum in $A_{2g}$ symmetry having an onset slightly below $3J$. In La$_2$CuO$_4$ an additional isolated excitation appears on top of the $A_{2g}$ continuum.

cond-mat.str-el↗

arXiv2018

Unusual suppression of the superconducting energy gap and critical temperature in atomically thin NbSe2

It is well known that superconductivity in thin films is generally suppressed with decreasing thickness. This suppression is normally governed by either disorder-induced localization of Cooper pairs, weakening of Coulomb screening, or generation and unbinding of vortex-antivortex pairs as described by the Berezinskii-Kosterlitz-Thouless (BKT) theory. Defying general expectations, few-layer NbSe2 - an archetypal example of ultrathin superconductors - has been found to remain superconducting down to monolayer thickness. Here we report measurements of both the superconducting energy gap and critical temperature in high-quality monocrystals of few-layer NbSe2, using planar-junction tunneling spectroscopy and lateral transport. We observe a fully developed gap that rapidly reduces for devices with the number of layers N < 5, as does their ctitical temperature. We show that the observed reduction cannot be explained by disorder, and the BKT mechanism is also excluded by measuring its transition temperature that for all N remains very close to Tc. We attribute the observed behavior to changes in the electronic band structure predicted for mono- and bi- layer NbSe2 combined with inevitable suppression of the Cooper pair density at the superconductor-vacuum interface. Our experimental results for N > 2 are in good agreement with the dependences of the gap and Tc expected in the latter case while the effect of band-structure reconstruction is evidenced by a stronger suppression of the gap and the disappearance of its anisotropy for N = 2. The spatial scale involved in the surface suppression of the density of states is only a few angstroms but cannot be ignored for atomically thin superconductors.

cond-mat.supr-con↗

arXiv2018

Singlet state formation and its impact on magnetic structure in tetramer system SeCuO$_3$

We present an experimental investigation of the magnetic structure in a tetramer system SeCuO$_3$ using neutron diffraction and nuclear resonance techniques. We establish a non-collinear, commensurate antiferromagnetic ordering with a propagation vector $\textbf{k} = \left(0,0,1 \right)$. The order parameter follows a critical behavior near $T_N = 8$ K, with a critical exponent $β= 0.32$ in agreement with a 3D universality class. Evidence is presented that a singlet state starts to form on tetramers at temperatures as high as 200 K, and its signature is preserved within the ordered state through a strong renormalization of the ordered magnetic moment on two non-equivalent copper sites, $m_{Cu1} \approx 0.4 μ_B$ and $m_{Cu2} \approx 0.7 μ_B$ at 1.5 K.

cond-mat.str-el↗

arXiv2017

Microscopic Origin of the Valley Hall Effect in Transition Metal Dichalcogenides Revealed by Wavelength Dependent Mapping

The band structure of many semiconducting monolayer transition metal dichalcogenides (TMDs) possesses two degenerate valleys, with equal and opposite Berry curvature. It has been predicted that, when illuminated with circularly polarized light, interband transitions generate an unbalanced non-equilibrium population of electrons and holes in these valleys, resulting in a finite Hall voltage at zero magnetic field when a current flows through the system. This is the so-called valley Hall effect that has recently been observed experimentally. Here, we show that this effect is mediated by photo-generated neutral excitons and charged trions, and not by inter-band transitions generating independent electrons and holes. We further demonstrate an experimental strategy, based on wavelength dependent spatial mapping of the Hall voltage, which allows the exciton and trion contributions to the valley Hall effect to be discriminated in the measurement. These results represent a significant step forward in our understanding of the microscopic origin of photo-induced valley Hall effect in semiconducting transition metal dichalcogenides, and demonstrate experimentally that composite quasi-particles, such as trions, can also possess a finite Berry curvature.

cond-mat.mes-hall↗

arXiv2017

Strongly bound excitons in anatase TiO2 single crystals and nanoparticles

Anatase TiO$_2$ is among the most studied materials for light-energy conversion applications, but the nature of its fundamental charge excitations is still unknown. Yet it is crucial to establish whether light absorption creates uncorrelated electron-hole pairs or bound excitons and, in the latter case, to determine their character. Here, by combining steady-state angle-resolved photoemission spectroscopy and spectroscopic ellipsometry with state-of-the-art ab initio calculations, we demonstrate that the direct optical gap of single crystals is dominated by a strongly bound exciton rising over the continuum of indirect interband transitions. This exciton possesses an intermediate character between the Wannier-Mott and Frenkel regimes and displays a peculiar two-dimensional wavefunction in the three-dimensional lattice. The nature of the higher-energy excitations is also identified. The universal validity of our results is confirmed up to room temperature by observing the same elementary excitations in defect-rich samples (doped single crystals and nanoparticles) via ultrafast two-dimensional deep-ultraviolet spectroscopy.

cond-mat.mtrl-sci↗

arXiv2017

Dimensional Crossover in a Charge Density Wave Material Probed by Angle-Resolved Photoemission Spectroscopy

High-resolution angle-resolved photoemission spectroscopy (ARPES) data reveal evidence of a crossover from one-dimensional (1D) to three-dimensional (3D) behavior in the prototypical charge density wave (CDW) material NbSe3. In the low-temperature 3D regime, gaps in the electronic structure are observed due to two incommensurate CDWs, in agreement with x-ray diffraction and electronic-structure calculations. At higher temperatures we observe a spectral weight depletion that approaches the power-law behavior expected in 1D. From the warping of the quasi-1D Fermi surface at low temperatures, we extract the energy scale of the dimensional crossover. This is corroborated by a detailed analysis of the density of states, which reveals a change in dimensional behavior dependent on binding energy. Our results offer an important insight into the dimensionality of excitations in quasi-1D materials.

cond-mat.str-el↗

arXiv2017

Scattering dominated high-temperature phase of 1T-TiSe2: an optical conductivity study

The controversy regarding the precise nature of the high-temperature phase of 1T-TiSe2 lasts for decades. It has intensified in recent times when new evidence for the excitonic origin of the low-temperature charge-density wave state started to unveil. Here we address the problem of the high-temperature phase through precise measurements and detailed analysis of the optical response of 1T-TiSe2 single crystals. The separate responses of electron and hole subsystems are identified and followed in temperature. We show that neither semiconductor nor semimetal pictures can be applied in their generic forms as the scattering for both types of carriers is in the vicinity of the Ioffe-Regel limit with decay rates being comparable to or larger than the offsets of band extrema. The nonmetallic temperature dependence of transport properties comes from the anomalous temperature dependence of scattering rates. Near the transition temperature the heavy electrons and the light holes contribute equally to the conductivity. This surprising coincidence is regarded as the consequence of dominant intervalley scattering that precedes the transition. The low-frequency peak in the optical spectra is identified and attributed to the critical softening of the L-point collective mode.

cond-mat.str-el↗

arXiv2017

Electron-energy-loss and time-dependent density functional theory study on the plasmon dispersion in 2H-NbS2

We examine the experimental and theoretical electron-energy loss spectra in 2$H$-Cu$_{0.2}$NbS$_2$ and find that the 1 eV plasmon in this material does not exhibit the regular positive quadratic plasmon dispersion that would be expected for a normal broad-parabolic-band system. Instead we find a nearly non-dispersing plasmon in the momentum-transfer range $q<0.35$ Å$^{-1}$. We argue that for a stoichiometric pure 2$H$-NbS$_2$ the dispersion relation is expected to have a negative slope as is the case for other transition-metal dichalcogenides. The presence of Cu impurities, required to stabilize the crystal growth, tends to shift the negative plasmon dispersion into a positive one, but the doping level in the current system is small enough to result in a nearly-non-dispersing plasmon. We conclude that a negative-slope plasmon dispersion is not connected with the existence of a charge-density-wave order in transition metal dichalcogenides.

cond-mat.str-el↗

arXiv2016

Probing the coupling between a doublon excitation and the charge-density wave in TaS2 by ultrafast optical spectroscopy

Recently, the switching between the different charge-ordered phases of 1T-TaS2 has been probed by ultrafast techniques, revealing unexpected phenomena such as "hidden" metastable states and peculiar photoexcited charge patterns. Here, we apply broadband pump-probe spectroscopy with varying excitation energy to study the ultrafast optical properties of 1T-TaS2 in the visible regime. By scanning the excitation energy in the near-IR region we unravel the coupling between different charge excitations and the low-lying charge-density wave state. We find that the amplitude mode of the charge-density wave exhibits strong coupling to a long-lived doublon state that is photoinduced in the center of the star-shaped charge-ordered Ta clusters by the near-IR optical excitation.

cond-mat.str-el↗

arXiv2016

Origin and magnitude of 'designer' spin-orbit interaction in graphene on semiconducting transition metal dichalcogenides

We use a combination of experimental techniques to demonstrate a general occurrence of spin-orbit interaction (SOI) in graphene on transition metal dichalcogenide (TMD) substrates. Our measurements indicate that SOI is ultra-strong and extremely robust, despite it being merely interfacially-induced, with neither graphene nor the TMD substrates changing their structure. This is found to be the case irrespective of the TMD material used, of the transport regime, of the carrier type in the graphene band, and of the thickness of the graphene multilayer. Specifically, we perform weak antilocalization measurements as the simplest and most general diagnostic of SOI, and show that the spin relaxation time is very short in all cases regardless of the elastic scattering time. Such a short spin-relaxation time strongly suggests that the SOI originates from a modification of graphene band structure. We confirmed this expectation by measuring a gate-dependent beating, and a corresponding frequency splitting, in the low-field Shubnikov-de Haas magneto-resistance oscillations in high quality bilayer graphene on WSe$_2$. These measurements provide an unambiguous diagnostic of a SOI-induced splitting in the electronic band structure, and their analysis allows us to determine the SOI coupling constants for the Rashba term and the so-called spin-valley coupling term, i.e., the terms that were recently predicted theoretically for interface-induced SOI in graphene. The magnitude of the SOI splitting is found to be on the order of 10 meV, more than 100 times greater than the SOI intrinsic to graphene. Both the band character of the interfacially induced SOI, as well as its robustness and large magnitude make graphene-on-TMD a promising system to realize and explore a variety of spin-dependent transport phenomena, such as, in particular, spin-Hall and valley-Hall topological insulating states.

cond-mat.mes-hall↗

arXiv2016

Gate tuning of electronic phase transitions in two-dimensional NbSe$_2$

Recent experimental advances in atomically thin transition metal dichalcogenide (TMD) metals have unveiled a range of interesting phenomena including the coexistence of charge-density-wave (CDW) order and superconductivity down to the monolayer limit. The atomic thickness of two-dimensional (2D) TMD metals also opens up the possibility for control of these electronic phase transitions by electrostatic gating. Here we demonstrate reversible tuning of superconductivity and CDW order in model 2D TMD metal NbSe$_2$ by an ionic liquid gate. A variation up to ~ 50% in the superconducting transition temperature has been observed, accompanied by a correlated evolution of the CDW order. We find that the doping dependence of the superconducting and CDW phase transition in 2D NbSe$_2$ can be understood by a varying electron-phonon coupling strength induced by the gate-modulated carrier density and the electronic density of states near the Fermi surface.

cond-mat.supr-con↗

arXiv2015

Gate-induced Superconductivity in atomically thin MoS2 crystals

When thinned down to the atomic scale, many layered van der Waals materials exhibit an interesting evolution of their electronic properties, whose main aspects can be accounted for by changes in the single-particle band structure. Phenomena driven by interactions are also observed, but identifying experimentally systematic trends in their thickness dependence is challenging. Here, we explore the evolution of gate-induced superconductivity in exfoliated MoS2 multilayers ranging from bulk-like to individual monolayers. We observe a clear transition for all the thicknesses down to the ultimate atomic limit, providing the first demonstration of superconductivity in atomically thin exfoliated crystals. Additionally, we characterize the superconducting state by measuring the critical temperature (TC) and magnetic field (BC) in a large number of multilayer devices, upon decreasing their thickness. The superconducting properties change smoothly down to bilayers, and a pronounced reduction in TC and BC is found to occur when going from bilayers to monolayers, for which we discuss possible microscopic mechanisms. Finding that gate-induced superconductivity persists in individual monolayers, which form the basic building blocks of more sophisticated van der Waals heterostructures, opens new possibilities for the engineering of the electronic properties of materials at the atomic scale.

cond-mat.supr-con↗

arXiv2015

Strong interface-induced spin-orbit coupling in graphene on WS2

Interfacial interactions allow the electronic properties of graphene to be modified, as recently demonstrated by the appearance of satellite Dirac cones in the band structure of graphene on hexagonal boron nitride (hBN) substrates. Ongoing research strives to explore interfacial interactions in a broader class of materials in order to engineer targeted electronic properties. Here we show that at an interface with a tungsten disulfide (WS2) substrate, the strength of the spin-orbit interaction (SOI) in graphene is very strongly enhanced. The induced SOI leads to a pronounced low-temperature weak anti-localization (WAL) effect, from which we determine the spin-relaxation time. We find that spin-relaxation time in graphene is two-to-three orders of magnitude smaller on WS2 than on SiO2 or hBN, and that it is comparable to the intervalley scattering time. To interpret our findings we have performed first-principle electronic structure calculations, which both confirm that carriers in graphene-on-WS2 experience a strong SOI and allow us to extract a spin-dependent low-energy effective Hamiltonian. Our analysis further shows that the use of WS2 substrates opens a possible new route to access topological states of matter in graphene-based systems.

cond-mat.mes-hall↗

arXiv2015

Evidence of Ising pairing in superconducting NbSe$_2$ atomic layers

Two-dimensional transition metal dichalcogenides with strong spin-orbit interactions and valley-dependent Berry curvature effects have attracted tremendous recent interests. Although novel single-particle and excitonic phenomena related to spin-valley coupling have been extensively studied, effects of spin-momentum locking on collective quantum phenomena remain unexplored. Here we report an observation of superconducting monolayer NbSe$_2$ with an in-plane upper critical field over six times of the Pauli paramagnetic limit by magneto-transport measurements. The effect can be understood in terms of the competing Zeeman effect and large intrinsic spin-orbit interactions in non-centrosymmetric NbSe$_2$ monolayers, where the electronic spin is locked to the out-of-plane direction. Our results provide a strong evidence of unconventional Ising pairing protected by spin-momentum locking and open up a new avenue for studies of non-centrosymmetric superconductivity with unique spin and valley degrees of freedom in the exact two-dimensional limit.

cond-mat.supr-con↗

arXiv2015

Strongly enhanced charge-density-wave order in monolayer NbSe$_2$

Two-dimensional (2D) atomic materials possess very different properties from their bulk counterparts. While changes in the single-particle electronic properties have been extensively investigated, modifications in the many-body collective phenomena in the exact 2D limit, where interaction effects are strongly enhanced, remain mysterious. Here we report a combined optical and electrical transport study on the many-body collective-order phase diagram of 2D NbSe$_2$. Both the charge density wave (CDW) and the superconducting phase have been observed down to the monolayer limit. While the superconducting transition temperature ($T_C$) decreases with lowering the layer thickness, the newly observed CDW transition temperature ($T_{\mathrm{CDW}}$) increases drastically from 33 K in the bulk to 145 K in the monolayers. Such highly unusual enhancement of CDWs in atomically thin samples can be understood as a result of significantly enhanced electron-phonon interactions in 2D NbSe$_2$, which cause a crossover from the weak coupling to the strong coupling limit. This is supported by the large blueshift of the collective amplitude vibrations observed in our experiment.

cond-mat.mtrl-sci↗