Session P37: CDW Materials - Chalcogenides and other

Temperature- and pressure-dependent Raman scattering study of phase transitions in ZrTe$_{3}$

Pressure-induced superconductivity has been discovered in many classes of materials, such as the iron pnictides and transition metal chalcogenides. ZrTe$_{3}$ is a representative member of the latter whose ground state can be tuned between charge density wave and superconducting phases via pressure or intercalation. Microscopic information about the structural evolution of ZrTe$_{3}$ in response to applied pressure is lacking at present. In this talk, we describe a temperature- and pressure-dependent Raman scattering study of the structural evolution of ZrTe$_{3}$ through its temperature- and pressure-dependent phase transitions.   

Ultrafast Dynamics of a Charge Density Wave via Time-Resolved Resonant Diffraction

Understanding the emergence of collective behavior in correlated electron systems remains at the forefront of modern condensed matter physics. The key to such an understanding is unraveling the contributions from the coupling degrees of freedom in exotic many body states. Density waves, both of charge and spin, have been studied for decades and a wealth of information and insight has been gained. However, there are still open questions that need to be solved for a complete description of the phenomena as there are several existing density wave systems that exhibit prototypical behavior while violating traditional theory. Ultrafast dynamics of such a system, TbTe3, has been investigated via time-resolved resonant diffraction at the SXR endstation at LCLS. Oscillations of the amplitude mode and coherent phonons have been observed previously in time resolved photoemission and reflectivity measurement but, here we reveal a direct observation of the lattice response via resonant diffraction. Watching dynamics of the two dimensional Te plane density wave diffraction peak at a resonant energy of a bystander Tb atom reveals new insights into the coupling responsible for the formation of the state. Results and comparison with previous time resolved measurements will be discussed.   

Experimental Observation of Chiral Order in TiSe$_2$

Recent STM measurements on TiSe$_2$ were interpreted as evidence of chirality in the charge-density-wave order parameter, \textit{i.e.}, a rotation in the phase of the three in-plane components of the CDW order from one layer to the next. Recently, J. van Wezel has shown theoretically how a chiral state can arise from the onset of orbital order of the Ti 3$d$ and Se 4$p$ states in conjunction with the charge order at a temperature at or below the CDW transition temperature [arXiv:1106.1930v1 (2011)]. This theory predicts a lowering of the symmetry of the ordered phase from $P\bar{3}c1$ to $P2$. We present the results of synchrotron x-ray diffraction measurements on a single crystal of TiSe2 that provide evidence of a second structural phase transition 15K below the CDW transition, consistent with the proposed space group.   

Pressure-induced CDW suppression in 1T-TiSe$_2$

1T-TiSe$_2$ is a prototypical transition-metal dichalcogenide showing a commensurate CDW phase transition. It has been shown that hydrostatic pressure suppresses the CDW order and induces superconductivity. Here we present a high-pressure x-ray scattering study of the CDW order parameter and its fluctuations in TiSe$_2$. The integrated intensity at the base temperature as a function of pressure shows a positive curvature, indicating deviation from mean-field behavior. The ratio between the lattice distortion and the critical temperature is pressure-dependent, indicating a cross-over from strong to weak coupling limits. Using a Monte Carlo simulation of the three-component Potts model, we argue that the amount of anisotropy and the effective dimension must be taken into account to explain the properties of the phase transition.   

Chiral charge and orbital order in 1T-TiSe2

Helical arrangements of spins are common among magnetic materials. The first material to harbor a corkscrew pattern of charge density on the other hand, was discovered only very recently [1,2]. The nature of the order parameter is of key relevance, since rotating a magnetic vector around any propagation vector trivially yields a helical pattern. In contrast, the purely scalar charge density cannot straightforwardly support a chiral state. Here we resolve this paradox by identifying the microscopic mechanism underlying the formation of the chiral charge density wave in {\it 1T}-TiSe$_2$. It is shown that the emergence of chirality is accompanied by the simultaneous formation of orbital order [3] We show that this type of combined orbital and charge order may in fact be expected to be a generic property of a broad class of charge ordered materials and discuss the prerequisites for finding chiral charge order in other materials. \\[4pt] [1] J. Ishioka, Y. H. Liu, K. Shimatake, T. Kurosawa, K. Ichimura, Y. Toda, M. Oda and S. Tanda, {\it Phys. Rev. Lett.} \textbf{105}, 176401 (2010). \newline \noindent [2] J. van Wezel and P. B. Littlewood, {\it Physics} \textbf{3}, 87 (2010). \newline \noindent [3] J. van Wezel, arXiv:1106.1930v1 (2011).   

Evolution of the Charge Density Wave state in Cu$_x$TiSe$_2$

1T-TiSe$_2$ is a quasi two-dimensional CDW material showing a 2x2x2 superlattice modulation below 200 K. Upon Cu doping the CDW is suppressed and superconductivity is induced. We present scanning tunneling microscopy and spectroscopy measurements of the charge-density wave state in 1T-TiSe$_2$, Cu$_{0.05}$TiSe$_2$ and Cu$_{0.06}$TiSe$_2$ single crystals. Topography images at 4.2 K reveal that the charge density waves are present in all samples studied, although the amplitude of the charge modulation decreases with the Cu-doping. Moreover, the chiral phase of the charge density wave is preserved also in Cu-doped samples. Tunneling spectroscopy shows that there is only a partial gap in the pure compound, with bands crossing the Fermi surface. In the Cu-doped samples the system becomes more metallic due to the increase of the chemical potential.   

Effect of dimensionality on charge density wave instabilities in TaS$_2$ and TaSe$_2$

Recent successes in making exfoliated single-layer transition-metal dichalcogenides has brought new interest to these materials, particularly with respect to the effects of dimensionality. As layered bulk materials, the 1T and 2H polymorphs of TaS$_2$ and TaSe$_2$ undergo a number of charge-density-wave (CDW) transitions. However, recent experiments have found that the CDW instability does not survive in nanopatches of 2H-TaS$_2$. Here we present a density-functional theory investigation of the CDW instability in single- and few-layer TaS$_2$ and TaSe$_2$, focusing on the role of the interlayer interactions. The effects of dimensionality on structure, electronic structure, and electronic-phonon coupling will be discussed.   

Disorder Induced Melting of Charge Density Wave Order in doped 2H-NbSe2 systems

Using a combination of Angle Resolved Photoemission Spectroscopy (ARPES), X-ray diffraction, transport and Scanning Tunneling Microscopy (STM) measurements on pristine as well as disordered 2H-NbSe2 samples, we have found that the onset Temperature Tcdw for Long Ranged Charge Density Wave (CDW) order gets quickly suppressed with concentration of disorder ions (X) and at certain critical concentration (Xc) it undergoes a quantum melting. Our STM measurements provide the evidence for local CDW ordering in doped samples for temperatures way above Tcdw. On the other hand, our ARPES measurements have found evidences for the presence of energy gap for both T$>$Tcdw {\&} X$>$Xc. We argue, all these experimental observations from completely different probes hint towards phase fluctuations of the order parameter as the mechanism behind the destruction of CDW order in quasi 2-d systems, such as 2H-NbSe2.   

Spectroscopic imaging of the charge density wave in $2H$-NbSe$_2$

Transition metal dichalcogenides are an ideal playground to study the interplay between charge density waves (CDWs) and superconductivity. We perform atomically resolved scanning tunneling microscopy and spectroscopy at cryogenic temperatures on the chalcogenide polytype $2H$-NbSe$_2$ to study the energy, temperature and spatial dependence of the CDW. By comparing our results with a tight-binding model, we disentangle the spectral behavior of the CDW phase in the material.   

Strong Periodic Lattice Distortion in Transition Metal Dichalcogenides

The charge density wave (CDW) instability was initially proposed to be the result of the Peierls mechanism in which a divergence in electronic response function results in a periodic charge redistribution; i.e. the electron gas itself is unstable with respect to the formation of a periodically varying electron charge density. However, the mechanism of CDW in many 2D Transition Metal Dichalcogenide (TMD) is still under debate. Fermi surface nesting was originally believed to act as the driving mechanism of CDW transitions in these materials; however, recent reports from both theoretical and experimental studies are not quite within this simple model. We use Spectroscopic Imaging Scanning Tunneling Microscope (SI-STM) to study the surfaces of 2H-TaSe$_{2}$, 2H-TaS$_{2}$, and 2H-NbSe$_{2}$ at various temperatures from 6K to above 100K. Topographic images and differential conductance data were recorded and analyzed in order to help understanding the underlying physics of CDW phases. Our results shows that Periodic Lattice Distortion (PLD) likely plays a more important role than the charge modulation in 2D TMD.   

Low frequency resistance fluctuations in nanoribbons of charge density wave (CDW) conductor NbSe$_{3}$

We investigate finite size effects in the low-frequency (1 mHz $<$ f $<$ 10 Hz) resistance fluctuations of individual nanoribbons of single-crystalline NbSe$_{3}$ (cross sections of 10$^{4}$ nm$^{2})$ across the two CDW transitions ($\sim $ 59 K and $\sim $ 141 K). This ultra sensitive frequency-dependent study of the electrical noise is crucial in improving our understanding of the mechanisms that generate noise around CDW transitions. The power spectral density, S$_{R}$, of the resistance noise has a generic form, S$_{R} \quad \sim $ 1/f$^{\alpha }$, typical of a diffusive metallic conductor. Below the CDW transition at 59 K, where the CDW is pinned by disorder, S$_{R}$ (at 1 Hz) shows a non-monotonic behavior with a maximum magnitude around 45 K. A similar peak in S$_{R}$ is also observed at 125 K, below the second CDW transition. Also, it is well known that the CDW state can be depinned by an application of a high bias voltage or current and S$_{R}$ is measured as a function of current across the pinning-depinning of CDW. S$_{R}$ shows a complex, non-monotonic dependence and is extremely sensitive to temperature below the CDW transition. In contrast, S$_{R}$ is bias independent above the CDW transitions as expected from a metal. The implications of these noise behaviors in understanding the pinning and depinning of CDW in NbSe$_{3}$ will be discussed.   

Extrinsic control of collective transport in quasi-1D materials with end contacts geometry

End contacts to mesowires of NbSe$_{3}$ and TaS$_{3}$ were nanofabricated and tested with transport, noise and X-ray microdiffraction measurements. We measured unusual and unexpected weak dependence of collective current on temperature in the [70K, 90K] range, close to 2/3T$_{P1}$ point, indicating a modification of CDW condensate transport due to the end contact geometry. This is accompanied with modifications to the temperature dependence to of the phase slip voltage. We also report a partial control of the threshold field (E$_{T})$ for CDW sliding, below T$_{P2}$, with the decrease in E$_{T}$ by as much as one order of magnitude in a limited temperature range below 2/3T$_{P2}$. These changes can be also seen in electric field modified X-ray topography images performed with sub-micron focused synchrotron X-rays (X13B beamline at NSLS). The most likely causes of these phenomena when end contacts are applied, are in modifications of: (a) carrier injection efficiency and, (b) the phase loop formation mechanism.   

Spectroscopic evidence of the Peierls transition in three-dimensional charge density wave solids

We studied the ultrafast photoinduced charge-density-wave to metal phase transition in a complex solid, namely Lu5Ir4Si10. After melting the charge ordering using infrared laser pulses, the consequent charge redistribution is probed with fs time resolution through the spectral weight analysis of the transient optical response over a broad energy range. The time-dependent spectral weight reveals a signature of the CDW melting and the time-scale of this photo-induced phase transition. This new kind of analysis allows us to show that the charge order remains preserved until the lattice distorts sufficiently to induce the phase transition. These results are completed by ab-initio modeling of the electronic band structure, identifying the orbitals involved in the CDW and the electronic transitions leading to the photo-induced melting of the charge order. This allows us to reveal the Peierls origin of multiple CDW in this three-dimensional solid.   

Gapped sliding phononic modes in the incommensurate structure of the ladder-chain system Sr$_{14}$Cu$_{24}$O$_{41}$

We report on low energy Raman and infra-red (IR) excitations in Sr$_{14}$Cu$_{24}$O$_{41}$. Two modes are observed starting from room temperature in the 1-2 meV range. One is a fully symmetric Raman mode and the other, observed in c-axis reflectance, is an excitation carrying a dipole moment along the chain/ladder direction. We associate these modes with the gapped c-axis sliding motions of the charged, incommensurate CuO$_{2}$ chains and Sr$_{2}$Cu$_{2}$O$_{3}$ ladder layers. This approach is able to quantitatively explain the range and relative energies of these excitations which are sensitive probes of the charge and spin density-wave ordering in the ``14-24-41'' systems.   

Dynamics of Sr$_{14}$Cu$_{24}$O$_{41}$ in a transient high intensity THz field

Over the past few years new techniques have become available to generate electromagnetic radiation with high electric field values in the THz range [1], allowing the dynamic study of materials whose excitations lie in the far infrared frequency range. This is the case of many charge density wave compounds, which exhibit a collective response due to pinning. Sr$_{14}$Cu$_{24}$O$_{41}$ is a charge density wave compound of particular interest, given its quasi one-dimensional structure consisting of alternating layers of Cu$_{2}$O$_{3}$ chains and CuO$_2$ ladders [2]. Understanding the dynamics of Sr$_{14}$Cu$_{24}$O$_{41}$ excitations in the far infrared has the potential not only to shed light onto the complex nature of charge ordering in this material, but also to help provide a better understanding of the nature of superconducting behavior in two-dimensional high temperature superconducting cuprates. In our work, THz pulses generated using a LiNbO$_3$ crystal interact with single crystal Sr$_{14}$Cu$_{24}$O$_{41}$ samples grown by the traveling solvent floating zone method. We will present preliminary results of a high field THz study of the dynamics of Sr$_{14}$Cu$_{24}$O$_{41}$. [1] Jepsen et al., Laser Phot Rev 5 124 (2011) [2] Gorshunov et al., Phys Rev B 66 060508 (2002)