-
Scanning tunneling microscopy study of hidden phases in atomically thin 1T-TaS$_2$
Authors:
Wooin Yang,
Dowook Kim,
Hyoung Kug Kim,
Tae-Hwan Kim
Abstract:
Lower thermal stability due to thinning often leads to unprecedented hidden phases in low-dimensional materials. Such hidden phases can coexist or compete with preexisting electronic phases. We investigate hidden phases observed in atomically thin (6-8 layers) 1T-TaS$_2$ with scanning tunneling microscopy. First, we can electrically induce a hidden stripe phase at room temperature. Such a uniaxial…
▽ More
Lower thermal stability due to thinning often leads to unprecedented hidden phases in low-dimensional materials. Such hidden phases can coexist or compete with preexisting electronic phases. We investigate hidden phases observed in atomically thin (6-8 layers) 1T-TaS$_2$ with scanning tunneling microscopy. First, we can electrically induce a hidden stripe phase at room temperature. Such a uniaxial stripe phase has three equivalent orientations by breaking three-fold symmetry of 1T-TaS$_2$. We also reveal that the hidden stripe phase coexists with nearly commensurate charge-density-wave phase. Next, we observe that the emergent stripe phase spontaneously appears without any electric excitation on a tiny flake ($160\times80$ nm$^2$). Our findings may provide a plausible explanation for the previously observed phase transition and two-fold optical response in thin 1T-TaS$_2$ devices at room temperature. Furthermore, the hidden stripe phase would be crucial to understand exotic CDW-related phenomena in 1T-TaS$_2$ for potential applications.
△ Less
Submitted 27 December, 2022;
originally announced December 2022.
-
Thickness dependent charge density wave networks on thin 1T-TaS$_2$
Authors:
Wooin Yang,
Dowook Kim,
Hyoung Kug Kim,
Tae-Hwan Kim
Abstract:
We investigate mechanically exfoliated thin 1T-TaS$_2$ with scanning tunneling microscopy at room temperature. Sample preparation without air exposure enables access to intrinsic charge-density-wave (CDW) phases of thin 1T-TaS$_2$. At room temperature, we can observe the expected nearly commensurate CDW (NCCDW) phase on thin flakes similar to bulk 1T-TaS$_2$. Further analysis reveals that the CDW…
▽ More
We investigate mechanically exfoliated thin 1T-TaS$_2$ with scanning tunneling microscopy at room temperature. Sample preparation without air exposure enables access to intrinsic charge-density-wave (CDW) phases of thin 1T-TaS$_2$. At room temperature, we can observe the expected nearly commensurate CDW (NCCDW) phase on thin flakes similar to bulk 1T-TaS$_2$. Further analysis reveals that the CDW domains in the NCCDW phase become smaller and have more anisotropic shape with decreasing thickness in the range of 8-28 layers. Our findings demonstrate that the anisotropic CDW nature of thin 1T-TaS$_2$ would be crucial to understand its exotic CDW-related phenomena and demand a systematic study on its correlation between the thickness-driven CDW domain anisotropy and the intermediate CDW states in thin 1T-TaS$_2$.
△ Less
Submitted 22 December, 2022;
originally announced December 2022.
-
In-situ scanning tunneling microscopy observation of thickness-dependent air-sensitive layered materials and heterodevices
Authors:
Hyoung Kug Kim,
Dowook Kim,
Dong Guk Lee,
Eun-Su Ahn,
Hyeon-Woo Jeong,
Gil-Ho Lee,
Jun Sung Kim,
Tae-Hwan Kim
Abstract:
Quasi-two-dimensional (Quasi-2D) van der Waals (vdW) materials can be mechanically or chemically exfoliated down to monolayer because of their strong intralayer bonding and the weak interlayer vdW interaction. Thanks to this unique property, one can often find exotic thickness-dependent electronic properties from these quasi-2D vdW materials, which can lead to band gap opening, emerging supercondu…
▽ More
Quasi-two-dimensional (Quasi-2D) van der Waals (vdW) materials can be mechanically or chemically exfoliated down to monolayer because of their strong intralayer bonding and the weak interlayer vdW interaction. Thanks to this unique property, one can often find exotic thickness-dependent electronic properties from these quasi-2D vdW materials, which can lead to band gap opening, emerging superconductivity, or enhanced charge density waves with decreasing thickness. Surface-sensitive scanning tunneling microscopy (STM) can provide direct observation of structural and electronic characteristics of such layered materials with atomic precision in real space. However, it is very challenging to preserve the intrinsic surfaces of air-sensitive quasi-2D materials between preparation and measurement. In addition, vdW 2D crystals after exfoliation are extremely hard to explore with a typical STM setup due to their small size (< 10 μm). Here, we present a straightforward method compatible with any STM setup having optical access: (1) exfoliating and/or stacking layered materials in a glove box, (2) transferring them to an ultra-high vacuum STM chamber using a suitcase without exposure to air, and (3) navigating surface to locate exfoliated vdW 2D flakes with different thicknesses. We successfully demonstrated that the clean surfaces of the air-sensitive Fe$_3$GeTe$_2$ can be effectively protected from unwanted oxidation during transfer. Furthermore, our method provides a simple but useful way to access a specific tiny stack of layered materials without any ex-situ fabrication processes for STM navigation. Our experimental improvement will open up a new way to investigate air-sensitive layered vdW materials with various thicknesses via surface-sensitive techniques including STM.
△ Less
Submitted 22 December, 2022;
originally announced December 2022.
-
Dimensional crossover of charge order in IrTe$_2$ with strong interlayer coupling
Authors:
Hyoung Kug Kim,
So Young Kim,
C. J. Won,
Sang-Wook Cheong,
Jonghwan Kim,
Jun Sung Kim,
Tae-Hwan Kim
Abstract:
Tuning dimensionality in van der Waals materials with finite interlayer coupling has introduced various electronic phase transitions by conventional mechanical exfoliation. Particularly when the electronic order is tied to the modulation of the interlayer coupling, such dimensional tunability has a strong impact on its stability and properties, which has rarely been investigated experimentally. He…
▽ More
Tuning dimensionality in van der Waals materials with finite interlayer coupling has introduced various electronic phase transitions by conventional mechanical exfoliation. Particularly when the electronic order is tied to the modulation of the interlayer coupling, such dimensional tunability has a strong impact on its stability and properties, which has rarely been investigated experimentally. Here, we demonstrate a dimensional crossover of charge order in IrTe$_2$ from genuine two- to quasi-three-dimension using low-temperature scanning tunneling microscopy and spectroscopy. Employing atomically thin IrTe$_2$ flakes ranging from monolayer to multilayer, we observe a gradual phase transition of charge order and exponential decay of Coulomb gap with increasing thickness. Moreover, we find a suppression of the density of states emerging at an abrupt lateral interface between two- and three-dimension. These findings are attributed to the interplay between the strongly coupled layers and substrate-driven perturbation, which can provide a new insight into the dimensional crossover of strongly coupled layered materials with hidden electronic phases.
△ Less
Submitted 22 December, 2022;
originally announced December 2022.
-
Superconductivity emerging from a stripe charge order in IrTe2 nanoflakes
Authors:
Sungyu Park,
So Young Kim,
Hyoung Kug Kim,
Min Jeong Kim,
Hoon Kim,
Gyu Seung Choi,
C. J. Won,
Sooran Kim,
Kyoo Kim,
Evgeny F. Talantsev,
Kenji Watanabe,
Takashi Taniguchi,
Sang-Wook Cheong,
B. J. Kim,
H. W. Yeom,
Jonghwan Kim,
Tae-Hwan Kim,
Jun Sung Kim
Abstract:
Superconductivity in the vicinity of a competing electronic order often manifests itself with a superconducting dome, centred at a presumed quantum critical point in the phase diagram. This common feature, found in many unconventional superconductors, has supported a prevalent scenario that fluctuations or partial melting of a parent order are essential for inducing or enhancing superconductivity.…
▽ More
Superconductivity in the vicinity of a competing electronic order often manifests itself with a superconducting dome, centred at a presumed quantum critical point in the phase diagram. This common feature, found in many unconventional superconductors, has supported a prevalent scenario that fluctuations or partial melting of a parent order are essential for inducing or enhancing superconductivity. Here we present a contrary example, found in IrTe2 nanoflakes of which the superconducting dome is identified well inside the parent stripe charge ordering phase in the thickness-dependent phase diagram. The coexisting stripe charge order in IrTe2 nanoflakes significantly increases the out-of-plane coherence length and the coupling strength of superconductivity, in contrast to the doped bulk IrTe2. These findings clarify that the inherent instabilities of the parent stripe phaseare sufficient to induce superconductivity in IrTe2 without its complete or partial melting. Our study highlights the thickness control as an effective means to unveil intrinsic phase diagrams of correlated vdW materials.
△ Less
Submitted 26 September, 2020;
originally announced September 2020.
-
Giant component in a configuration-model power-law graph with a variable number of links
Authors:
Heung Kyung Kim,
Mi Jin Lee,
Matthieu Barbier,
Sung-Gook Choi,
Min Seok Kim,
Hyung-Ha Yoo,
Deok-Sun Lee
Abstract:
We generalize an algorithm used widely in the configuration model such that power-law degree sequences with the degree exponent $λ$ and the number of links per node $K$ controllable independently may be generated. It yields the degree distribution in a different form from that of the static model or under random removal of links while sharing the same $λ$ and $K$. With this generalized power-law d…
▽ More
We generalize an algorithm used widely in the configuration model such that power-law degree sequences with the degree exponent $λ$ and the number of links per node $K$ controllable independently may be generated. It yields the degree distribution in a different form from that of the static model or under random removal of links while sharing the same $λ$ and $K$. With this generalized power-law degree distribution, the critical point $K_c$ for the appearance of the giant component remains zero not only for $λ\leq 3$ but also for $3<λ<λ_l \simeq 3.81$. This is contrasted with $K_c=0$ only for $λ\leq 3$ in the static model and under random link removal. The critical exponents and the cluster-size distribution for $λ<λ_l$ are also different from known results. By analyzing the moments and the generating function of the degree distribution and comparison with those of other models, we show that the asymptotic behavior and the degree exponent may not be the only properties of the degree distribution relevant to the critical phenomena but that its whole functional form can be relevant. These results can be useful in designing and assessing the structure and robustness of networked systems.
△ Less
Submitted 24 November, 2019;
originally announced November 2019.
-
30 inch Roll-Based Production of High-Quality Graphene Films for Flexible Transparent Electrodes
Authors:
Sukang Bae,
Hyeong Keun Kim,
Youngbin Lee,
Xianfang Xu,
Jae-Sung Park,
Yi Zheng,
Jayakumar Balakrishnan,
Danho Im,
Tian Lei,
Young Il Song,
Young Jin Kim,
Kwang S. Kim,
Barbaros Özyilmaz,
Jong-Hyun Ahn,
Byung Hee Hong,
Sumio Iijima
Abstract:
We report that 30-inch scale multiple roll-to-roll transfer and wet chemical doping considerably enhance the electrical properties of the graphene films grown on roll-type Cu substrates by chemical vapor deposition. The resulting graphene films shows a sheet resistance as low as ~30 Ohm/sq at ~90 % transparency which is superior to commercial transparent electrodes such as indium tin oxides (ITO).…
▽ More
We report that 30-inch scale multiple roll-to-roll transfer and wet chemical doping considerably enhance the electrical properties of the graphene films grown on roll-type Cu substrates by chemical vapor deposition. The resulting graphene films shows a sheet resistance as low as ~30 Ohm/sq at ~90 % transparency which is superior to commercial transparent electrodes such as indium tin oxides (ITO). The monolayer of graphene shows sheet resistances as low as ~125 Ohm/sq with 97.4% optical transmittance and half-integer quantum Hall effect, indicating the high-quality of these graphene films. As a practical application, we also fabricated a touch screen panel device based on the graphene transparent electrodes, showing extraordinary mechanical and electrical performances.
△ Less
Submitted 16 March, 2010; v1 submitted 30 December, 2009;
originally announced December 2009.
-
Jamming transition in a highly dense granular system under vertical vibration
Authors:
Kipom Kim,
Jong Kyun Moon,
Jong Jin Park,
Hyung Kook Kim,
Hyuk Kyu Pak
Abstract:
The dynamics of the jamming transition in a three-dimensional granular system under vertical vibration is studied using diffusing-wave spectroscopy. When the maximum acceleration of the external vibration is large, the granular system behaves like a fluid, with the dynamic correlation function G(t) relaxing rapidly. As the acceleration of vibration approaches the gravitational acceleration g, th…
▽ More
The dynamics of the jamming transition in a three-dimensional granular system under vertical vibration is studied using diffusing-wave spectroscopy. When the maximum acceleration of the external vibration is large, the granular system behaves like a fluid, with the dynamic correlation function G(t) relaxing rapidly. As the acceleration of vibration approaches the gravitational acceleration g, the relaxation of G(t) slows down dramatically, and eventually stops. Thus the system undergoes a phase transition and behaves like a solid. Near the transition point, we find that the structural relaxation shows a stretched exponential behavior. This behavior is analogous to the behavior of supercooled liquids close to the glass transition.
△ Less
Submitted 25 June, 2005; v1 submitted 13 January, 2005;
originally announced January 2005.