Scanning tunnelling microscopy of charge-density waves in transition metal chalcogenides

Coleman, R. V.; Giambattista, B.; Hansma, P. K.; Johnson, Amber; McNairy, W. W.; Slough, C. G.

doi:10.1080/00018738800101439

Cited by 215 publications

(120 citation statements)

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“…1a). In 2H-NbSe 2 , we observe the hexagonal surface atomic lattice of Se atoms and the CDW order as previously reported in many other STM experiments and largely discussed in literature [27,28]. In 2H-NbS 2 , the STM image ( fig.…”

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confidence: 86%

Superconducting Density of States and Vortex Cores of 2H-NbS2

et al. 2008

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Scanning tunneling microscopy and spectroscopy (STM/S) measurements in the superconducting dichalcogenide 2H-NbS2 show a peculiar superconducting density of states with two well defined features at 0.97 meV and 0.53 meV, located respectively above and below the value for the superconducting gap expected from single band s-wave BCS model (∆=1.76kBTc=0.9 meV). Both features have a continuous temperature evolution and disappear at Tc = 5.7 K. Moreover, we observe the hexagonal vortex lattice with radially symmetric vortices and a well developed localized state at the vortex cores. The sixfold star shape characteristic of the vortex lattice of the compound 2H-NbSe2 is, together with the charge density wave order (CDW), absent in 2H-NbS2.PACS numbers: 71.45. Lr, 74.25.Jb,74.50.+r,74.70.Ad The study of the coexistence of superconductivity with competing physical phenomena such as magnetic or charge order has historically produced great interest on the scientific community. Anisotropies or modulations of the superconducting properties (in real and/or reciprocal space) often appear as a consequence of competing orders within the same system [1,2]. In the compound 2H-NbSe 2 , superconductivity appears within a CDW state (T CDW =33K and T c =7.2 K) [3]. Low lying excitations measured deep in the superconducting state long time ago by specific heat [4,5] have been explained by recent experiments and theoretical calculations with a multiband superconductivity and a peculiar anisotropy of the superconducting gap [6,7]. Recent angular resolved photoemission spectroscopy measurements demonstrate that the superconducting gap has, close to T c (at 5.7 K), largest values at k-space positions connected with CDW wavevectors [8,9]. Hess et al. [10,11,12] found that the local superconducting density of states (LDOS) at the center of the vortex core shows a high peak close to the Fermi level highlighting the lowest quasiparticle state bound within the vortex core well [13]. Around the vortex core, the LDOS is far from respecting in-plane symmetry and intriguing vortex lattice images with patterns showing strong in-plane LDOS modulations are obtained [10,11,12]. 2H-NbSe 2 belongs to the transition-metal dichalcogenides (2H-MX 2 with M = Ta, Nb and X = Se, S), a family of systems which is unique to study the interplay between CDW order and superconductivity. The 2H-MX 2 compounds share a double layered structure made of two hexagonal X sheets with an intercalated M sheet (X-M-X), connected through very weak van der Waals bonds [14]. This produces highly anisotropic, quasi two dimensional electronic properties. The features of the Fermi Surface (FS) expected to be common in all systems of the series are two concentric cylindrical FS sheets centered on both Γ and K points, derived from the transition-metal d bands [15,16,17,18]. When going over the series from 2H-TaSe 2 and 2H-TaS 2 to 2H-NbSe 2 and 2H-NbS 2 , the ratio of the intralayer lattice constant with the interlayer distance a/c increases, as well as T c , whereas T CDW decreas...

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confidence: 86%

Superconducting Density of States and Vortex Cores of 2H-NbS2

et al. 2008

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“…The gap edge can be obtained from the voltage position of the onset of the decrease of the tunneling conductance curves down to the V shape observed at low temperatures (dashed lines in Fig.2). We find ∆ = 2 meV, which gives about 2∆/k B T A ≈7, a value that is significantly larger than the mean field BCS result (2∆/k B T A = 3.7) previously used in the discussion of charge or spin density wave phenomena [17,23,24].…”

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confidence: 63%

Gap opening with ordering inPrFe4P12studied by local tunneling spectroscopy

et al. 2008

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We present measurements of the local tunneling density of states in the low temperature ordered state of PrFe4P12. The temperature dependencies of the Fermi level density of states, and of the integrated density of states at low bias voltages, show anomalies at T ≃ 6.5K, the onset of multipolar ordering as detected by specific heat and other macroscopic measurements. In the ordered phase, we find a local density of states with a V-shape form, indicating partial gap opening over the Fermi surface. The size of the gap according to the tunneling spectra is about 2 meV.PACS numbers: 71.27. a+,71.30 +h, 73.20.-r Recently, a new family of heavy fermion f-electron intermetallic compounds with the filled skutterudite structure RT 4 X 12 (R is a rare earth, T a transition metal and X a pnictogen) has been found to show many attractive phenomena, most of them appearing at low temperatures. Among others, metal-insulator transition, heavy fermion behavior, and superconductivity have been found in (R=Pr) systems [1,2,3,4,5,6].In particular, in PrFe 4 P 12 , a peculiar phase transition occurs at T A = 6.5 K, with distinct anomalies in both resistivity [7] and specific heat [8]. The intriguing order below T A is widely believed to be associated with orbital degrees of freedom [1]. A recent NMR study [9] has put severe restrictions on the symmetry of the order parameter. According to it, the ordering involves non-magnetic multipoles, which do not break the point symmetry of the crystal at the Pr sites. The scalar order with Γ 1g [10] is currently the most promising candidate to represent the order parameter. On the other hand, according to transport studies [6,7,11], the itinerant electrons are profoundly affected by this phase transition. The multifold increase in the Hall, Seebeck and Nernst coefficients all suggest a drastic decrease in the number of charge carriers [11]. This is compatible with the results of a de Haas van Alphen study [6], which found a small Fermi surface (0.0015 of the Brillouin zone) with a moderate effective mass (∼10m e ) in the ordered state. In higher magnetic fields (strong enough to destroy the ordering), the same study resolved several larger Fermi surfaces with heavier masses up to 81m e [6]. The consequences of the ordering on transport properties recall the case of URu 2 Si 2 [12]. In both cases, the ordering leads to an enhanced lattice thermal conductivity, a large Hall coefficient and anomalously large Nernst and Seebeck coefficients due to an incomplete metal-insulator transition, producing a heavy-Fermion semi-metal. In the case of URu 2 Si 2 , a partial gap opening has been observed using different techniques [13,14].On the other hand, scanning tunneling microscopy and spectroscopy (STM/S) techniques have been widely applied to systems where some kind of energy gap appears in the quasiparticle spectrum. The superconducting gap is certainly one of the most studied kinds of energy gaps viewed using this technique, although different kinds of charge ordered, and semimetallic or semicondu...

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“…Why? First, the CMP community believes that they have already seen this type of behavior in the metal chalcogenides [22], and second no new understanding of these CDW systems has come from experiments or calculations. It is cute, and the pictures made the cover of Nature, but that is all there is.…”

Section: Surface Physics and Cmpmentioning

confidence: 99%

The next 25 years of surface physics

Plummer¹

2001

Progress in Surface Science

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The 20th International Seminar on Surface Physics commemorates the last quarter of a century in surface physics and serves as the commencement of the next millennium. This article attempts to identify the guiding directions for the next 25 years of surface physics relevant to the field of condensed matter physics. The message is that Surface Science and specifically surface physics must undergo a Cultural Revolution from a technique-driven to science-driven discipline. There are tremendously exciting opportunities for surface physics but to take advantage of them will require a change in our culture and most importantly in the education of the next generation of surface scientist. Some of the opportunities that exist in complex highly correlated systems in reduced dimensionality will be presented to illustrate the pivotal role that surface physicists can play in contemporary Condensed Matter Physics. Ó

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Scanning tunnelling microscopy of charge-density waves in transition metal chalcogenides

Cited by 215 publications

References 83 publications

Superconducting Density of States and Vortex Cores of 2H-NbS2

Superconducting Density of States and Vortex Cores of 2H-NbS2

Gap opening with ordering inPrFe4P12studied by local tunneling spectroscopy

The next 25 years of surface physics

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