arXiv subjects

H. Berger

Publications and source records attributed to H. Berger.

At least 37 records · Page 2Linked to original sources

arXiv2017

Low spin wave damping in the insulating chiral magnet Cu$_{2}$OSeO$_{3}$

Chiral magnets with topologically nontrivial spin order such as Skyrmions have generated enormous interest in both fundamental and applied sciences. We report broadband microwave spectroscopy performed on the insulating chiral ferrimagnet Cu$_{2}$OSeO$_{3}$. For the damping of magnetization dynamics we find a remarkably small Gilbert damping parameter of about $1\times10^{-4}$ at 5 K. This value is only a factor of 4 larger than the one reported for the best insulating ferrimagnet yttrium iron garnet. We detect a series of sharp resonances and attribute them to confined spin waves in the mm-sized samples. Considering the small damping, insulating chiral magnets turn out to be promising candidates when exploring non-collinear spin structures for high frequency applications.

cond-mat.str-el↗

arXiv2017

Linearly polarized GHz magnetization dynamics of spin helix modes in the ferrimagnetic insulator Cu$_{2}$OSeO$_{3}$

Linear dichroism -- the polarization dependent absorption of electromagnetic waves -- is routinely exploited in applications as diverse as structure determination of DNA or polarization filters in optical technologies. Here filamentary absorbers with a large length-to-width ratio are a prerequisite. For magnetization dynamics in the few GHz frequency regime strictly linear dichroism was not observed for more than eight decades. Here, we show that the bulk chiral magnet Cu$_{2}$OSeO$_{3}$ exhibits linearly polarized magnetization dynamics at an unexpectedly small frequency of about 2 GHz. Unlike optical filters that are assembled from filamentary absorbers, the magnet provides linear polarization as a bulk material for an extremely wide range of length-to-width ratios. In addition, the polarization plane of a given mode can be switched by 90$^\circ$ via a tiny variation in width. Our findings shed a new light on magnetization dynamics in that ferrimagnetic ordering combined with anisotropic exchange interaction offers strictly linear polarization and cross-polarized modes for a broad spectrum of sample shapes. The discovery allows for novel design rules and optimization of microwave-to-magnon transduction in emerging microwave technologies.

cond-mat.str-el↗

arXiv2017

Dynamical defects in rotating magnetic skyrmion lattices

The chiral magnet Cu$_{2}$OSeO$_{3}$ hosts a skyrmion lattice, that may be equivalently described as a superposition of plane waves or lattice of particle-like topological objects. A thermal gradient may break up the skyrmion lattice and induce rotating domains raising the question which of these scenarios better describes the violent dynamics at the domain boundaries. Here we show that in an inhomogeneous temperature gradient caused by illumination in a Lorentz Transmission Electron Microscope different parts of the skyrmion lattice can be set into motion with different angular velocities. Tracking the time dependence we show that the constant rearrangement of domain walls is governed by dynamic 5-7 defects arranging into lines. An analysis of the associated defect density is described by Frank's equation and agrees well with classical 2D-Monte Carlo simulations. Fluctuations of boundaries show surge-like rearrangement of skyrmion clusters driven by defect rearrangement consistent with simulations treating skyrmions as point particles. Our findings underline the particle character of the skyrmion.

cond-mat.mes-hall↗

arXiv2017

Magnetic and dielectric order in the kagome-like francisite Cu$_3$Bi(SeO$_3$)$_2$O$_2$Cl

We report a single-crystal neutron diffraction and inelastic neutron scattering study on the spin 1/2 cuprate Cu$_3$Bi(SeO$_3$)$_2$O$_2$Cl, complemented by dielectric and electric polarization measurements. The study clarifies a number of open issues concerning this complex material, whose frustrated interactions on a kagome-like lattice, combined with Dzyaloshinskii-Moriya interactions, are expected to stabilize an exotic canted antiferromagnetic order. In particular, we determine the nature of the structural transition occurring at 115 K, the magnetic structure below 25 K resolved in the updated space group, and the microscopic ingredients at the origin of this magnetic arrangement. This was achieved by an analysis of the measured gapped spin waves, which signifies the need of an unexpected and significant anisotropic exchange beyond the proposed Dzyaloshinskii-Moriya interactions. Finally, we discuss the mutliferroic properties of this material with respect to the space group symmetries.

cond-mat.mtrl-sci↗

arXiv2016

Emerging one-dimensionality from self-organization of electrons in NbSe$_3$

Materials where the electron filling is close to commensurate filling provide one of the great challenges in materials science. Several proposals of unconventional orderings, where the electronic liquid self-organizes into components with distinct properties, were recently put forward, in particular in cuprates and pnictides where electronic nematic orders have been observed. The electrons self-organization is expected to yield complex intra and inter unit cell patterns, and a reduction of dimensionality. Nevertheless, an unambiguous experimental proof of such complex orders, namely the direct observation of distinct dispersions, is still missing. Here we report a Nano Angle Resolved Photo-emission Spectroscopy (Nano-ARPES) study of NbSe$_3$, a material that has been considered a paradigm of charge order. The new data (Fig.1) invalidate the canonical picture of imperfect nesting and reveals the emergence of a novel order. The electrons self-organization uncovers the one-dimensional (1D) physics hidden in a material which naively should be the most 3D of all columnar chalcogenides.

cond-mat.str-el↗

arXiv2016

Imaging and manipulation of skyrmion lattice domains in Cu2OSeO3

Nanoscale chiral skyrmions in noncentrosymmetric helimagnets are promising binary state variables in high-density, low-energy nonvolatile memory. Skyrmions are ubiquitous as an ordered, single-domain lattice phase, which makes it difficult to write information unless they are spatially broken up into smaller units, each representing a bit. Thus, the formation and manipulation of skyrmion lattice domains is a prerequisite for memory applications. Here, using an imaging technique based on resonant magnetic x-ray diffraction, we demonstrate the mapping and manipulation of skyrmion lattice domains in Cu2OSeO3. The material is particularly interesting for applications owing to its insulating nature, allowing for electric field-driven domain manipulation.

cond-mat.mes-hall↗

arXiv2016

Pressure-induced superconductivity in the giant Rashba system BiTeI

At ambient pressure, BiTeI is the first material found to exhibit a giant Rashba splitting of the bulk electronic bands. At low pressures, BiTeI undergoes a transition from trivial insulator to topological insulator. At still higher pressures, two structural transitions are known to occur. We have carried out a series of electrical resistivity and AC magnetic susceptibility measurements on BiTeI at pressure up to ~40 GPa in an effort to characterize the properties of the high-pressure phases. A previous calculation found that the high-pressure orthorhombic P4/nmm structure BiTeI is a metal. We find that this structure is superconducting with Tc values as high as 6 K. AC magnetic susceptibility measurements support the bulk nature of the superconductivity. Using electronic structure and phonon calculations, we compute Tc and find that our data is consistent with phonon-mediated superconductivity.

cond-mat.supr-con↗

arXiv2016

Revealing the role of electrons and phonons in the ultrafast recovery of charge density wave correlations in 1$T$-TiSe$_2$

Using time- and angle-resolved photoemission spectroscopy with selective near- and mid-infrared photon excitations, we investigate the femtosecond dynamics of the charge density wave (CDW) phase in 1$T$-TiSe$_2$, as well as the dynamics of CDW fluctuations at 240 K. In the CDW phase, we observe the coherent oscillation of the CDW amplitude mode. At 240 K, we single out an ultrafast component in the recovery of the CDW correlations, which we explain as the manifestation of electron-hole correlations. Our momentum-resolved study of femtosecond electron dynamics supports a mechanism for the CDW phase resulting from the cooperation between the interband Coulomb interaction, the mechanism of excitonic insulator phase formation, and electron-phonon coupling.

cond-mat.str-el↗

arXiv2016

BiTeCl and BiTeBr: a comparative high-pressure optical study

We here report a detailed high-pressure infrared transmission study of BiTeCl and BiTeBr. We follow the evolution of two band transitions: the optical excitation $β$ between two Rashba-split conduction bands, and the absorption $γ$ across the band gap. In the low pressure range, $p< 4$~GPa, for both compounds $β$ is approximately constant with pressure and $γ$ decreases, in agreement with band structure calculations. In BiTeCl, a clear pressure-induced phase transition at 6~GPa leads to a different ground state. For BiTeBr, the pressure evolution is more subtle, and we discuss the possibility of closing and reopening of the band gap. Our data is consistent with a Weyl phase in BiTeBr at 5$-$6~GPa, followed by the onset of a structural phase transition at 7~GPa.

cond-mat.mtrl-sci↗

arXiv2016

Local resilience of the $1T$-TiSe$_2$ charge density wave to Ti self-doping

In Ti-intercalated self-doped $1T$-TiSe$_2$ crystals, the charge density wave (CDW) superstructure induces two nonequivalent sites for Ti dopants. Recently, it has been shown that increasing Ti doping dramatically influences the CDW by breaking it into phase-shifted domains. Here, we report scanning tunneling microscopy and spectroscopy experiments that reveal a dopant-site dependence of the CDW gap. Supported by density functional theory, we demonstrate that the loss of the longrange phase coherence introduces an imbalance in the intercalated-Ti site distribution and restrains the CDW gap closure. This local resilient behavior of the $1T$-TiSe$_2$ CDW reveals a novel mechanism between CDW and defects in mutual influence.

cond-mat.mtrl-sci↗

arXiv2016

Exchange anisotropy as mechanism for spin-stripe formation in frustrated spin chains

We investigate the spin-stripe mechanism responsible for the peculiar nanometer modulation of the incommensurate magnetic order that emerges between the vector-chiral and the spin-density-wave phase in the frustrated zigzag spin-1/2 chain compound $β$-TeVO$_4$. A combination of magnetic-torque, neutron-diffraction and spherical-neutron-polarimetry measurements is employed to determine the complex magnetic structures of all three ordered phases. Based on these results, we develop a simple phenomenological model, which exposes the exchange anisotropy as the key ingredient for the spin-stripe formation in frustrated spin systems.

cond-mat.str-el↗

arXiv2016

Evidence for a Strong Topological Insulator Phase in $\mathrm{ZrTe_5}$

The complex electronic properties of $\mathrm{ZrTe_5}$ have recently stimulated in-depth investigations that assigned this material to either a topological insulator or a 3D Dirac semimetal phase. Here we report a comprehensive experimental and theoretical study of both electronic and structural properties of $\mathrm{ZrTe_5}$, revealing that the bulk material is a strong topological insulator (STI). By means of angle-resolved photoelectron spectroscopy, we identify at the top of the valence band both a surface and a bulk state. The dispersion of these bands is well captured by ab initio calculations for the STI case, for the specific interlayer distance measured in our x-ray diffraction study. Furthermore, these findings are supported by scanning tunneling spectroscopy revealing the metallic character of the sample surface, thus confirming the strong topological nature of $\mathrm{ZrTe_5}$.

cond-mat.mtrl-sci↗

arXiv2016

Orbital textures and charge density waves in transition metal dichalcogenides

Low-dimensional electron systems, as realized naturally in graphene or created artificially at the interfaces of heterostructures, exhibit a variety of fascinating quantum phenomena with great prospects for future applications. Once electrons are confined to low dimensions, they also tend to spontaneously break the symmetry of the underlying nuclear lattice by forming so-called density waves; a state of matter that currently attracts enormous attention because of its relation to various unconventional electronic properties. In this study we reveal a remarkable and surprising feature of charge density waves (CDWs), namely their intimate relation to orbital order. For the prototypical material 1T-TaS2 we not only show that the CDW within the two-dimensional TaS2-layers involves previously unidentified orbital textures of great complexity. We also demonstrate that two metastable stackings of the orbitally ordered layers allow to manipulate salient features of the electronic structure. Indeed, these orbital effects enable to switch the properties of 1T-TaS2 nanostructures from metallic to semiconducting with technologically pertinent gaps of the order of 200 meV. This new type of orbitronics is especially relevant for the ongoing development of novel, miniaturized and ultra-fast devices based on layered transition metal dichalcogenides.

cond-mat.str-el↗

arXiv2016

Magnetic anisotropy in the frustrated spin chain compound $β$-TeVO$_4$

Isotropic and anisotropic magnetic behavior of the frustrated spin chain compound $β$-TeVO$_4$ is reported. Three magnetic transitions observed in zero magnetic field are tracked in fields applied along different crystallographic directions using magnetization, heat capacity, and magnetostriction measurements. Qualitatively different temperature-field diagrams are obtained below 10 T for the field applied along $a$ or $b$ and along $c$, respectively. In contrast, a nearly isotropic high-field phase emerges above 18 T and persists up to the saturation that occurs around 22.5 T. Upon cooling in low fields, the transitions at $T_{\rm N1}$ and $T_{\rm N2}$ toward the spin-density-wave and stripe phases are of the second order, whereas the transition at $T_{\rm N3}$ toward the helical state is of the first order and entails a lattice component. Our microscopic analysis identifies frustrated $J_1-J_2$ spin chains with a sizable antiferromagnetic interchain coupling in the $bc$ plane and ferromagnetic couplings along the $a$ direction. The competition between these ferromagnetic interchain couplings and the helical order within the chain underlies the incommensurate order along the $a$-direction, as observed experimentally. Although a helical state is triggered by the competition between $J_1$ and $J_2$ within the chain, the plane of the helix is not uniquely defined because of competing magnetic anisotropies. Using high-resolution synchrotron diffraction and $^{125}$Te nuclear magnetic resonance, we also demonstrate that the crystal structure of $β$-TeVO$_4$ does not change down to 10 K, and the orbital state of V$^{4+}$ is preserved.

cond-mat.mtrl-sci↗

arXiv2016

Multidomain Skyrmion Lattice State in Cu$_2$OSeO$_3$

Magnetic skyrmions in chiral magnets are nanoscale, topologically-protected magnetization swirls that are promising candidates for spintronics memory carriers. Therefore, observing and manipulating the skyrmion state on the surface level of the materials are of great importance for future applications. Here, we report a controlled way of creating a multidomain skyrmion state near the surface of a Cu$_{2}$OSeO$_{3}$ single crystal, observed by soft resonant elastic x-ray scattering. This technique is an ideal tool to probe the magnetic order at the $L_{3}$ edge of $3d$ metal compounds giving a depth sensitivity of ${\sim}50$ nm. The single-domain sixfold-symmetric skyrmion lattice can be broken up into domains overcoming the propagation directions imposed by the cubic anisotropy by applying the magnetic field in directions deviating from the major cubic axes. Our findings open the door to a new way to manipulate and engineer the skyrmion state locally on the surface, or on the level of individual skyrmions, which will enable applications in the future.

cond-mat.mtrl-sci↗

arXiv2016

Resonant Elastic X-ray Scattering from the Skyrmion Lattice in Cu$_{2}$OSeO$_{3}$

We report the study of the skyrmion state near the surface of Cu$_2$OSeO$_3$ using soft resonant elastic x-ray scattering (REXS) at the Cu $L_3$ edge. Within the lateral sampling area of $200 \times 200$ $μ$m$^2$, we found a long-range-ordered skyrmion lattice phase as well as the formation of skyrmion domains via the multiple splitting of the diffraction spots. In a recent REXS study of the skyrmion phase of Cu$_2$OSeO$_3$ [Phys. Rev. Lett. 112, 167202 (2014)], Langner et al. reported the observation of the unexpected existence of two distinct skyrmion sublattices that arise from inequivalent Cu sites, and that the rotation and superposition of the two periodic structures leads to a moiré pattern. However, we find no energy splitting of the Cu peak in x-ray absorption measurements and, instead, discuss alternative origins of the peak splitting. In particular, we find that for magnetic field directions deviating from the major cubic axes, a multidomain skyrmion lattice state is obtained, which consistently explains the splitting of the magnetic spots into two - and more - peaks.

cond-mat.mtrl-sci↗

arXiv2016

Discovery of Weyl semimetal state violating Lorentz invariance in MoTe2

A new type of Weyl semimetal state, in which the energy values of Weyl nodes are not the local extrema, has been theoretically proposed recently, namely type II Weyl semimetal. Distinguished from type I semimetal (e.g. TaAs), the Fermi surfaces in a type II Weyl semimetal consist of a pair of electron and hole pockets touching at the Weyl node. In addition, Weyl fermions in type II Weyl semimetals violate Lorentz invariance. Due to these qualitative differences distinct spectroscopy and magnetotransport properties are expected in type II Weyl semimetals. Here, we present the direct observation of the Fermi arc states in MoTe2 by using angle resolved photoemission spectroscopy. Two arc states are identified for each pair of Weyl nodes whoes surface projections of them possess single topological charge, which is a unique property for type II Weyl semimetals. The experimentally determined Fermi arcs are consistent with our first principle calculations. Our results unambiguously establish that MoTe2 is a type II Weyl semimetal, which serves as a great test bed to investigate the phenomena of new type of Weyl fermions with Lorentz invariance violated.

cond-mat.mtrl-sci↗

arXiv2016

Short-range phase coherence and origin of the 1T-TiSe2 charge density wave

The impact of variable Ti self-doping on the 1T-TiSe2 charge density wave (CDW) is studied by scanning tunneling microscopy. Supported by density functional theory we show that agglomeration of intercalated-Ti atoms acts as preferential nucleation centers for the CDW that breaks up in phaseshifted CDW domains whose size directly depends on the intercalated-Ti concentration and which are separated by atomically-sharp phase boundaries. The close relationship between the diminution of the CDW domain size and the disappearance of the anomalous peak in the temperature dependent resistivity allows to draw a coherent picture of the 1T-TiSe2 CDW phase transition and its relation to excitons.

cond-mat.mtrl-sci↗