Showing 1–50 of 56 results for author: Lee, S B

New Superconductors in the PtPb$_3$Bi Structure Type

Authors: Lior Verbitsky, Amira Merino, Scott B. Lee, Jaime M. Moya, Sigalit Aharon, Fatmagül Katmer, Sudipta Chatterjee, Grigorii Skorupskii, Josh Leeman, Gabrielle Carrel, Leslie M. Schoop

Abstract: The quest for new superconductors is of both fundamental and technological importance. Recently, an artificial intelligence method correctly predicted PtPb$_3$Bi to be a superconductor. In this work, we find superconductivity in the newly synthesized $M$Pb$_{4-x}$Bi$_x$ ($M$ = Au, Pd, and Rh), of which PtPb$_3$Bi is a member. When $M$ = Ni, whose radius is considerably smaller, the structure inste… ▽ More The quest for new superconductors is of both fundamental and technological importance. Recently, an artificial intelligence method correctly predicted PtPb$_3$Bi to be a superconductor. In this work, we find superconductivity in the newly synthesized $M$Pb$_{4-x}$Bi$_x$ ($M$ = Au, Pd, and Rh), of which PtPb$_3$Bi is a member. When $M$ = Ni, whose radius is considerably smaller, the structure instead collapses into the different, Pb-substituted NiBi$_3$ type. Interestingly, the stoichiometric parameter $x$ shifts across the three compounds to keep the total valence electron count close to 20 per formula unit. The superconducting transitions occur at 4.9, 4.2, and 3.4 K, for $M$ = Au, Pd, and Rh, respectively. Using electrical resistivity, magnetization, and specific heat measurements, we establish the bulk nature of the superconducting state and determine the critical fields, characteristic length scales, and anisotropy ratios. All three compounds are moderately anisotropic type-II superconductors, with modest upper critical field anisotropies of $H_{c2}^{\parallel c}/H_{c2}^{\perp c} \approx 1.2$ to $1.5$. These results establish $M$Pb$_{4-x}$Bi$_x$ as a family of anisotropic superconductors and a platform for studying how site disorder and Pb-Bi mixing govern superconductivity in heavy-element intermetallics. △ Less

Submitted 24 June, 2026; originally announced June 2026.

Comments: 22+10 pages, 5+11 figures

Tunable magnetotransport through kinetically hindered first-order phase transitions in an antiferromagnetic metal

Authors: Jaime M. Moya, Scott B. Lee, Sudipta Chatterjee, Nitish Mathur, Grigorii Skorupskii, Connor J. Pollak, Leslie M. Schoop

Abstract: Controllable multilevel resistance states are of interest for memory technologies like neuromorphic computing, but robust materials platforms toward such behavior remain limited. Here, we show that the non-centrosymmetric antiferromagnetic metal CeCoGe$_3$ suggests one such route through a kinetically hindered first-order magnetic transition. Cooling through the kinetically hindered first-order tr… ▽ More Controllable multilevel resistance states are of interest for memory technologies like neuromorphic computing, but robust materials platforms toward such behavior remain limited. Here, we show that the non-centrosymmetric antiferromagnetic metal CeCoGe$_3$ suggests one such route through a kinetically hindered first-order magnetic transition. Cooling through the kinetically hindered first-order transition in an applied magnetic field produces a magnetic glass state in which high- and low-temperature magnetic phases coexist. The relative fraction of these phases can be controlled by the applied field in which the sample is cooled, and the electrical resistance is directly sensitive to that fraction. As a result, it is demonstrated that CeCoGe$_3$ supports stable multilevel resistive states. These results identify kinetically hindered first-order phase transitions as a promising route towards controllable multilevel magnetoresistive states. △ Less

Submitted 19 May, 2026; originally announced May 2026.

Stripping Symmetry: Electrochemical Oxidation to a Superconducting Polar Metal in Au2Pb0.914P2

Authors: Scott B. Lee, Stephanie R. Dulovic, Joseph W. Stiles, Xin Zhang, Fatmagül Katmer, Sudipta Chatterjee, Jaime Moya, Allana G. Iwanicki, Abby N. Neill, Chris Lygouras, Tieyan Chang, Tyrel M. McQueen, Yu-Sheng Chen, Leslie M. Schoop

Abstract: Polar metals and noncentrosymmetric superconductors are exceptionally rare, yet their broken inversion symmetry can give rise to emergent electronic phenomena including mixed singlet-triplet superconducting pairing. As only a few such materials have been found among known compounds, accessing new examples requires synthetic strategies that go beyond conventional crystal growth. Here, we use electr… ▽ More Polar metals and noncentrosymmetric superconductors are exceptionally rare, yet their broken inversion symmetry can give rise to emergent electronic phenomena including mixed singlet-triplet superconducting pairing. As only a few such materials have been found among known compounds, accessing new examples requires synthetic strategies that go beyond conventional crystal growth. Here, we use electrochemical topotactic deintercalation to remove Pb from the centrosymmetric parent compound Au$_2$PbP$_2$, producing the polar metal Au$_2$Pb$_{0.914}$P$_2$. Unlike conventional chemical doping, this transformation actively drives structural symmetry-breaking: the partial removal of Pb triggers a cooperative electronic and geometric rearrangement, mediated by a second-order Jahn-Teller effect and stereochemically active lone pairs, that locks the product into a polar, noncentrosymmetric superspace group Ama2(01g)ss0. We solve the complete (3+1)D modulated structure by synchrotron single-crystal X-ray diffraction and confirm the polar assignment through nonlinear electronic transport. Below T$_c$ = 1.52 K, Au$_2$Pb$_{0.914}$P$_2$ becomes a type-II superconductor whose heat capacity and AC susceptibility both exhibit power-law behavior, suggestive of a gap structure governed by the broken inversion symmetry of the host lattice. This work establishes electrochemical oxidation as a rational route to metastable noncentrosymmetric superconductors through chemically directed symmetry-breaking. △ Less

Submitted 20 April, 2026; originally announced April 2026.

Comments: 105 pages, 5 main figures, 41 supplemental figures, 19 supplemental tables

Collinear spin density wave state in distorted square-lattice GdNiSn$_4$

Authors: Charles C. Tam, Sarah Schwarz, Xin Zhang, Sudipta Chatterjee, Scott B. Lee, Rebecca Scatena, Leslie M. Schoop, Stephen D. Wilson

Abstract: We characterize the magnetic ground state of the newly synthesized lanthanide intermetallic GdNiSn$_4$ via resonant elastic x-ray scattering measurements. This compound forms distorted square nets of Gd that initially order magnetically below 23 K followed by a lower temperature transition at 16 K. Our scattering data identify the ground state order as a single-$q$ incommensurate, collinear order… ▽ More We characterize the magnetic ground state of the newly synthesized lanthanide intermetallic GdNiSn$_4$ via resonant elastic x-ray scattering measurements. This compound forms distorted square nets of Gd that initially order magnetically below 23 K followed by a lower temperature transition at 16 K. Our scattering data identify the ground state order as a single-$q$ incommensurate, collinear order that slides towards a commensurate wave vector above the 16 K transition. Magnetic symmetry analysis combined with azimuthal dependence resolves the ground state magnetic structure as a moment-modulated spin density wave state with Gd moments oriented parallel to the in-plane a-axis. We discuss connections between the observed magnetic order and electronic properties in this square-net compound. △ Less

Submitted 9 March, 2026; originally announced March 2026.

Journal ref: Phys. Rev. Materials 10, 074412 2026

New Crystal Structures Hide in Plain Sight: A Stress Test for AI-Guided Materials Discovery

Authors: Xin Zhang, Scott B. Lee, Sudipta Chatterjee, Hanqi Pi, Yi Jiang, Fatmagül Katmer, Emily G. Ward, Daniel E. Widdowson, Charles C. Tam, Sarah Schwarz, Connor J. Pollak, Jaime M. Moya, Grigorii Skorupskii, Vitaliy A. Kurlin, Stephen D. Wilson, B. Andrei Bernevig, Leslie M. Schoop

Abstract: New types of crystal structures are discovered only rarely, and the artificial intelligence (AI) models now reshaping materials discovery have so far produced new chemical compositions within known structural families rather than genuinely new structures. We report GdNiSn4 and LuNiSn4, intermetallics that adopt a previously unreported structure type, found not by computation but by exploratory syn… ▽ More New types of crystal structures are discovered only rarely, and the artificial intelligence (AI) models now reshaping materials discovery have so far produced new chemical compositions within known structural families rather than genuinely new structures. We report GdNiSn4 and LuNiSn4, intermetallics that adopt a previously unreported structure type, found not by computation but by exploratory synthesis. Single-crystal diffraction shows that the structure is an intergrowth of two known structural units. We then use this system as a benchmark for two leading generative models, MatterGen and DiffCSP++. For DiffCSP++, the benchmark is performed in its crystallographically constrained setting, using the required space-group and Wyckoff-position inputs. Under our sampling budget, neither model recovers the experimentally reported monoclinic structure within the structural-matching tolerance. The generated structures are evaluated without further structural relaxation using the nonmagnetic analog LuNiSn4, where we rule out 4f magnetism as the cause. Because the new structure is built from familiar building blocks, it should be derivable. We argue that encoding chemical reasoning, such as the stacking of known motifs, is a concrete path toward AI that can discover structurally novel materials. △ Less

Submitted 18 June, 2026; v1 submitted 5 March, 2026; originally announced March 2026.

Bosonization Solution to Spin-Valley Kondo Problem: Finite-Size Spectrum and Renormalization Group Analysis

Authors: Yi-Jie Wang, Geng-Dong Zhou, Hyunsung Jung, Seongyeon Youn, Seung-Sup B. Lee, Zhi-Da Song

Abstract: Spin-valley Anderson impurities (SVAIM) with (anti-)Hund's splitting provide a natural explanation to the origin of pairing potential and pseudogap in the magic-angle graphene. In this work, we derive and analytically solve the low-energy Kondo theories for SVAIM at half-filling, with especial focus on the two anti-Hund's regimes: the impurity is either dominated by a valley doublet, or a trivial… ▽ More Spin-valley Anderson impurities (SVAIM) with (anti-)Hund's splitting provide a natural explanation to the origin of pairing potential and pseudogap in the magic-angle graphene. In this work, we derive and analytically solve the low-energy Kondo theories for SVAIM at half-filling, with especial focus on the two anti-Hund's regimes: the impurity is either dominated by a valley doublet, or a trivial singlet. In the doublet regime, we reveal that a novel pair Kondo scattering $λ_x$ is required to flip the valley doublet, which involves a quartic operator of bath electrons. Our renormalization group (RG) calculation based on the Coulomb gas analog shows $λ_x$ drives a phase transition of the Berezinskii-Kosterlitz-Thouless type. One side of the transition is an anisotropic doublet phase, characterized by non-universal phase shifts of bath electrons and non-analytic impurity susceptibilities, while the other is a Fermi liquid formed by pair-Kondo resonance. The finite-size many-body spectrum, thermodynamic quantities, and correlation functions for both phases are analytically solved. Remarkably, the solution in the pair-Kondo Fermi liquid is achieved via the constructive approach of bosonization-refermionization along a solvable fixed line, where the many-body interaction $λ_x$ is mapped into a pseudo-fermion bilinear in a rigorous manner. Finally, we also apply the RG analysis to the singlet regime, and identify a second-order phase transition between the Kondo Fermi liquid and a local singlet phase. △ Less

Submitted 23 January, 2026; originally announced January 2026.

Comments: 40 pages, 5 figures. arXiv admin note: substantial text overlap with arXiv:2510.23604

Measuring the Hall effect in hysteretic materials

Authors: Jaime M. Moya, Anthony Voyemant, Sudipta Chatterjee, Scott B. Lee, Grigorii Skorupskii, Connor J. Pollak, Leslie M. Schoop

Abstract: Measurement of the Hall effect is a ubiquitous probe for materials discovery, characterization, and metrology. Inherent to the Hall measurement geometry, the measured signal is often contaminated by unwanted contributions, so the data must be processed to isolate the Hall response. The standard approach invokes Onsager-Casimir reciprocity and antisymmetrizes the raw signal about zero applied magne… ▽ More Measurement of the Hall effect is a ubiquitous probe for materials discovery, characterization, and metrology. Inherent to the Hall measurement geometry, the measured signal is often contaminated by unwanted contributions, so the data must be processed to isolate the Hall response. The standard approach invokes Onsager-Casimir reciprocity and antisymmetrizes the raw signal about zero applied magnetic field. In hysteretic materials this becomes nontrivial, since Onsager-Casimir relations apply only to microscopically reversible states. Incorrect antisymmetrization can lead to artifacts that mimic anomalous or topological Hall signatures. The situation is especially subtle when hysteresis loops are not centered at zero applied field, as in exchange-biased systems. A practical reference for generically extracting the Hall response in hysteretic materials is lacking. Here, using Co$_3$Sn$_2$S$_2$ as a bulk single-crystal model that can be prepared with or without exchange-biased hysteresis, we demonstrate two procedures that can be used to extract the Hall effect: (1) reverse-magnetic-field reciprocity and (2) antisymmetrization with respect to applied field. We then measure the Hall effect on CeCoGe$_3$, a noncentrosymmetric antiferromagnet which can be prepared to have asymmetric magnetization and magnetoresistance, and demonstrate how improper processing can generate artificial anomalous Hall signals. These methods are generic and can be applied to any conductor. △ Less

Submitted 28 May, 2026; v1 submitted 22 December, 2025; originally announced December 2025.

Journal ref: Advanced Materials, 2026, e23674

Spin-Valley Anderson Impurity for Moiré Systems: Fermi Liquid, Pairing, and Pseudogap

Authors: Yi-Jie Wang, Geng-Dong Zhou, Hyunsung Jung, Seongyeon Youn, Seung-Sup B. Lee, Zhi-Da Song

Abstract: Recent experiments support that the magic-angle graphene can be modeled by a periodic array of correlated quantum impurities, immersed in a Dirac sea. This work analytically tackles a spin-valley Anderson impurity, featuring a general (anti-)Hund's interaction ($J_D, J_S$) that can originate from electron-phonon couplings. We derive its full phase diagram, which encompasses rich continuous local p… ▽ More Recent experiments support that the magic-angle graphene can be modeled by a periodic array of correlated quantum impurities, immersed in a Dirac sea. This work analytically tackles a spin-valley Anderson impurity, featuring a general (anti-)Hund's interaction ($J_D, J_S$) that can originate from electron-phonon couplings. We derive its full phase diagram, which encompasses rich continuous local phase transitions, and presents a unified origin for pairing potential and pseudogap. In particular, $J_D$ favors a valley doublet, and we show it drives a BKT transition out of heavy Fermi liquid, to an anisotropic doublet phase exhibiting a non-analytic zero-energy kink in the impurity spectral function. $J_S$ drives a second-order transition out of heavy Fermi liquid, to a local singlet phase, with a non-Fermi liquid critical point. We analyze the pairing potential across the phase diagram, and unveil their ubiquitous existence triggered by the (anti-)Hund's multiplet splitting. Crucially, we show the pseudogap shoulders in the spectral function represent multiplet excitations induced by an injected electron or hole. All results are obtained analytically, using techniques including bosonization-refermionization, with further verification by numerical renormalization group calculations. Then we derive the correlation self-energy ansatze that account for pseudogap, and apply to the magic-angle graphene lattice. △ Less

Submitted 26 January, 2026; v1 submitted 27 October, 2025; originally announced October 2025.

Comments: 33 pages, 8 figures

Electron affinity difference distributions guide the discovery of the superconductor PtPb$_3$Bi

Authors: Omri Lesser, Yanjun Liu, Natalie Maus, Aaditya Panigrahi, Krishnanand Mallayya, Albert Gong, Anmol Kabra, Scott B. Lee, Sudipta Chatterjee, Amira Merino, Kilian Q. Weinberger, Leslie M. Schoop, Jacob R. Gardner, Eun-Ah Kim

Abstract: Predicting the superconducting transition temperature ($T_c$) from crystal structure and composition remains a central challenge in condensed-matter physics, reflecting the absence of a broadly predictive framework connecting microscopic bonding to macroscopic quantum behavior. Here, we introduce a structure- and chemistry-aware approach implemented in an interpretable Gaussian process model, whic… ▽ More Predicting the superconducting transition temperature ($T_c$) from crystal structure and composition remains a central challenge in condensed-matter physics, reflecting the absence of a broadly predictive framework connecting microscopic bonding to macroscopic quantum behavior. Here, we introduce a structure- and chemistry-aware approach implemented in an interpretable Gaussian process model, which we call GP-$T_c$ (Gaussian Process $T_c$), that enables uncertainty-quantified prediction of superconductivity from experimentally accessible inputs. By encoding local bonding environments and geometry as graphlet histograms and learning within a probabilistic framework, we find that the predictive space collapses to a compact set of descriptors: the distribution of electron-affinity differences between neighboring atoms, together with simple elemental features and interatomic distances, provides an informative basis for predicting $T_c$ across disparate superconducting families. This result identifies an overlooked chemical control parameter while emphasizing the essential role of local structure beyond composition-only approaches. We demonstrate the framework through two complementary tests: validation against a recently established superconducting family and discovery of a previously unknown material. GP-$T_c$ reproduces the experimentally reported $T_c$ range of the infinite-layer nickelate Nd0.8Sr0.2NiO2. We further predict superconductivity in stoichiometric PtPb$_3$Bi and experimentally confirm it through synthesis and bulk measurements, establishing PtPb$_3$Bi as a new superconductor with $T_c$~3 K. GP-$T_c$ identifies additional high-priority superconducting candidates -- including SrNiO2, K(PRh)2, and Ho2C3 -- that provide concrete targets for ongoing and future experimental exploration. △ Less

Submitted 2 April, 2026; v1 submitted 8 October, 2025; originally announced October 2025.

Comments: 9+21 pages, 4+13 figures

The finite-difference parquet method: Enhanced electron-paramagnon scattering opens a pseudogap

Authors: Jae-Mo Lihm, Dominik Kiese, Seung-Sup B. Lee, Fabian B. Kugler

Abstract: We present the finite-difference parquet method that greatly improves the applicability and accuracy of two-particle correlation approaches to interacting electron systems. This method incorporates the nonperturbative local physics from a reference solution and builds all parquet diagrams while circumventing potentially divergent irreducible vertices. Its unbiased treatment of different fluctuatio… ▽ More We present the finite-difference parquet method that greatly improves the applicability and accuracy of two-particle correlation approaches to interacting electron systems. This method incorporates the nonperturbative local physics from a reference solution and builds all parquet diagrams while circumventing potentially divergent irreducible vertices. Its unbiased treatment of different fluctuations is crucial for reproducing the strong-coupling pseudogap in the underdoped Hubbard model, consistent with diagrammatic Monte Carlo calculations. We reveal a strong-coupling spin-fluctuation mechanism of the pseudogap with decisive vertex corrections that encode the enhanced, energy-dependent scattering amplitude between electrons and antiferromagnetic spin fluctuations. △ Less

Submitted 8 March, 2026; v1 submitted 26 May, 2025; originally announced May 2025.

Journal ref: PNAS, 123 (10) e2525308123 (2026)

Hundness in twisted bilayer graphene: correlated gaps and pairing

Authors: Seongyeon Youn, Beomjoon Goh, Geng-Dong Zhou, Zhi-Da Song, Seung-Sup B. Lee

Abstract: We characterize gap-opening mechanisms in the topological heavy fermion (THF) model of magic-angle twisted bilayer graphene (MATBG), with and without electron-phonon coupling, using dynamical mean-field theory (DMFT) with the numerical renormalization group (NRG) impurity solver. In the presence of symmetry breaking associated with valley-orbital ordering (time-reversal-symmetric or Kramers interv… ▽ More We characterize gap-opening mechanisms in the topological heavy fermion (THF) model of magic-angle twisted bilayer graphene (MATBG), with and without electron-phonon coupling, using dynamical mean-field theory (DMFT) with the numerical renormalization group (NRG) impurity solver. In the presence of symmetry breaking associated with valley-orbital ordering (time-reversal-symmetric or Kramers intervalley coherent, or valley polarized), spin anti-Hund and orbital-angular-momentum Hund couplings, induced by the dynamical Jahn-Teller effect, result in a robust pseudogap at filling $2 \lesssim |ν| \lesssim 2.5$. We also find that Hundness enhances the pairing susceptibilities for $1.6 \lesssim |ν| \lesssim 2.8$, which might be a precursor to the superconducting phases neighboring $|ν| = 2$. △ Less

Submitted 4 December, 2024; originally announced December 2024.

Directionally Locked Heteroepitaxy with a Structurally Modulated van der Waals Material

Authors: Nitish Mathur, Guangming Cheng, Francesc Ballester, Gabrielle Carrel, Vincent M. Plisson, Fang Yuan, Jiangchang Zheng, Caiyun Chen, Scott B. Lee, Ratnadwip Singha, Sudipta Chatterjee, Kenji Watanabe, Takashi Taniguchi, Kenneth S. Burch, Berthold Jäck, Ion Errea, Maia G. Vergniory, Nan Yao, Sanfeng Wu, Leslie M. Schoop

Abstract: Precise orientation of symmetry-mismatched epilayers on van der Waals (vdW) substrates via heteroepitaxy has commonly been achieved through surface treatment processes to accommodate weak interlayer registry and bonding strength, thereby limiting the range of material combinations for heterostructure design. In this study, we investigate the influence of lattice instabilities in a structurally mod… ▽ More Precise orientation of symmetry-mismatched epilayers on van der Waals (vdW) substrates via heteroepitaxy has commonly been achieved through surface treatment processes to accommodate weak interlayer registry and bonding strength, thereby limiting the range of material combinations for heterostructure design. In this study, we investigate the influence of lattice instabilities in a structurally modulated vdW TaCo2Te2 substrate on the growth and alignment of a symmetry-mismatched bulk CoxTey epilayer using in situ heating in a transmission electron microscope (TEM). We show that a Peierls-like lattice instability occurs in TaCo2Te2 at a transition temperature of ~523 K, which was corroborated by phonon calculations. Post-heat-treated samples reveal a thermally induced surface diffusion process and the dominant lateral growth of the CoxTey epilayer on the TaCo2Te2 vdW layers, as observed in cross-sectional TEM images. Temperature-dependent selected area electron diffraction (SAED) patterns reveal that the quasi-vdW CoxTey/TaCo2Te2 heterointerface acquires directional locking by aligning larger interlayer lattice mismatch along the lattice instability axis of TaCo2Te2, while preserving a strong lattice matching along the orthogonal direction. This heterostructure exhibits precise interlayer registry with one-dimensional lattice incommensuration along the lattice instability axis, resulting from structural distortion to accommodate lattice-mismatch strain. Moreover, the interfacial reconstruction of TaCo2Te2 back to the distorted phase stabilizes the lattice-locking of the quasi-vdW heterointerface at elevated temperatures. These findings encourage the expansion of material diversity for designing and predicting novel multi-dimensional heterostructures by leveraging lattice instabilities to guide epitaxy. △ Less

Submitted 16 April, 2026; v1 submitted 29 September, 2024; originally announced September 2024.

Comments: 25 pages, 6 figures

Bonding Interactions Can Drive Topological Phase Transitions in a Zintl Antiferromagnetic Insulator

Authors: Tanya Berry, Jaime M. Moya, David Smiadak, Scott B. Lee, Sigalit Aharon, Alexandra Zevalkink, Tyrel M. McQueen, Leslie M. Schoop

Abstract: While $\sim$30% of materials are reported to be topological, topological insulators are rare. Magnetic topological insulators (MTI) are even harder to find. Identifying crystallographic features that can host the coexistence of a topological insulating phase with magnetic order is vital for finding intrinsic MTI materials. Thus far, most materials that are investigated for the determination of an… ▽ More While $\sim$30% of materials are reported to be topological, topological insulators are rare. Magnetic topological insulators (MTI) are even harder to find. Identifying crystallographic features that can host the coexistence of a topological insulating phase with magnetic order is vital for finding intrinsic MTI materials. Thus far, most materials that are investigated for the determination of an MTI are some combination of known topological insulators with a magnetic ion such as MnBi$_2$Te$_4$. Motivated by the recent success of EuIn$_{2}$As$_{2}$, we investigate the role of chemical pressure on topologically trivial insulator, Eu$_5$In$_2$Sb$_6$ via Ga substitution. Eu$_5$Ga$_2$Sb$_6$ is predicted to be topological but is synthetically difficult to stabilize. We look into the intermediate compositions between Eu$_5$In$_2$Sb$_6$ and Eu$_5$Ga$_2$Sb$_6$ through theoretical works to explore a topological phase transition and band inversion mechanism. We attribute the band inversion mechanism to changes in Eu-Sb hybridization as Ga is substituted for In due to chemical pressure. We also synthesize Eu$_{5}$In$_{4/3}$Ga$_{2/3}$Sb$_{6}$, the highest Ga concentration in Eu$_{5}$In$_{2-x}$Ga$_{x}$Sb$_{6}$, and report the thermodynamic, magnetic, transport, and Hall properties. Overall, our work paints a picture of a possible MTI via band engineering and explains why Eu-based Zintl compounds are suitable for the co-existence of magnetism and topology. △ Less

Submitted 31 August, 2024; originally announced September 2024.

Dynamical scaling and Planckian dissipation due to heavy-fermion quantum criticality

Authors: Andreas Gleis, Seung-Sup B. Lee, Gabriel Kotliar, Jan von Delft

Abstract: We study dynamical scaling associated with a Kondo-breakdown quantum critical point (KB-QCP) of the periodic Anderson model, treated by two-site cellular dynamical mean-field theory (2CDMFT). In the quantum critical region, the staggered spin exhibits SYK-like slow dynamics and its dynamical susceptibility shows $ω/T$ scaling. We propose a scaling Ansatz that describes this behavior. It also impli… ▽ More We study dynamical scaling associated with a Kondo-breakdown quantum critical point (KB-QCP) of the periodic Anderson model, treated by two-site cellular dynamical mean-field theory (2CDMFT). In the quantum critical region, the staggered spin exhibits SYK-like slow dynamics and its dynamical susceptibility shows $ω/T$ scaling. We propose a scaling Ansatz that describes this behavior. It also implies Planckian dissipation for the longest-lived excitations. The current susceptibility follows the same scaling ansatz, leading to strange-metal scaling. This demonstrates that the KB-QCP described by 2CDMFT is an intrinsic (i.e., disorder-free) strange-metal fixed point. Surprisingly, the SYK-like dynamics and scaling are driven by strong vertex contributions to the susceptibilities. Our results for the optical conductivity match experimental observations on YbRh${}_2$Si${}_2$ and CeCoIn${}_5$. △ Less

Submitted 22 April, 2024; originally announced April 2024.

Comments: 16 pages, 13 figures, comments are welcome!

Overcomplete intermediate representation of two-particle Green's functions and its relation to partial spectral functions

Authors: Selina Dirnböck, Seung-Sup B. Lee, Fabian B. Kugler, Sebastian Huber, Jan von Delft, Karsten Held, Markus Wallerberger

Abstract: Two-particle response functions are a centerpiece of both experimental and theoretical quantum many-body physics. Yet, due to their size and discontinuity structure, they are challenging to handle numerically. Recently, two advances were made to tackle this problem: first, the overcomplete intermediate representation (OIR), which provides a highly efficient compression of Green's functions in imag… ▽ More Two-particle response functions are a centerpiece of both experimental and theoretical quantum many-body physics. Yet, due to their size and discontinuity structure, they are challenging to handle numerically. Recently, two advances were made to tackle this problem: first, the overcomplete intermediate representation (OIR), which provides a highly efficient compression of Green's functions in imaginary frequency, and second, partial spectral functions (PSFs), which allow for an efficient evaluation in real frequency. We show that there is a two-to-one correspondence between PSFs and OIR coefficients and exploit this fact to construct the OIR for three-or-more-particle propagators. We then use OIR to fit and compress imaginary-frequency data obtained from the numerical renormalization group (NRG), reaching a compression ratio of more than 400. Finally, we attempt to match the OIR data to partial Green's functions from NRG.Due to the overcompleteness, we achieve only qualitative agreement. △ Less

Submitted 8 April, 2024; originally announced April 2024.

Comments: 9 pages, 4 figures

Journal ref: Phys. Rev. Research 6, 043228 (2024)

Analytic continuation of multipoint correlation functions

Authors: Anxiang Ge, Johannes Halbinger, Seung-Sup B. Lee, Jan von Delft, Fabian B. Kugler

Abstract: Conceptually, the Matsubara formalism (MF), using imaginary frequencies, and the Keldysh formalism (KF), formulated in real frequencies, give equivalent results for systems in thermal equilibrium. The MF has less complexity and is thus more convenient than the KF. However, computing dynamical observables in the MF requires the analytic continuation from imaginary to real frequencies. The analytic… ▽ More Conceptually, the Matsubara formalism (MF), using imaginary frequencies, and the Keldysh formalism (KF), formulated in real frequencies, give equivalent results for systems in thermal equilibrium. The MF has less complexity and is thus more convenient than the KF. However, computing dynamical observables in the MF requires the analytic continuation from imaginary to real frequencies. The analytic continuation is well-known for two-point correlation functions (having one frequency argument), but, for multipoint correlators, a straightforward recipe for deducing all Keldysh components from the MF correlator had not been formulated yet. Recently, a representation of MF and KF correlators in terms of formalism-independent partial spectral functions and formalism-specific kernels was introduced by Kugler, Lee, and von Delft [Phys. Rev. X 11, 041006 (2021)]. We use this representation to formally elucidate the connection between both formalisms. We show how a multipoint MF correlator can be analytically continued to recover all partial spectral functions and yield all Keldysh components of its KF counterpart. The procedure is illustrated for various correlators of the Hubbard atom. △ Less

Submitted 6 May, 2024; v1 submitted 19 November, 2023; originally announced November 2023.

Comments: 56 pages, 8 figures

Journal ref: Ann. Phys. (Leipzig) 536, 2300504 (2024)

Emergent Properties of the Periodic Anderson Model: a High-Resolution, Real-Frequency Study of Heavy-Fermion Quantum Criticality

Authors: Andreas Gleis, Seung-Sup B. Lee, Gabriel Kotliar, Jan von Delft

Abstract: We study paramagnetic quantum criticality in the periodic Anderson model (PAM) using cellular dynamical mean-field theory, with the numerical renormalization group (NRG) as an impurity solver. The PAM describes an itinerant $c$ band hybridizing with a localized $f$ band. At $T=0$, it exhibits a hybridization tuned Kondo breakdown quantum critical point (KB-QCP) from a Kondo to an RKKY phase. At th… ▽ More We study paramagnetic quantum criticality in the periodic Anderson model (PAM) using cellular dynamical mean-field theory, with the numerical renormalization group (NRG) as an impurity solver. The PAM describes an itinerant $c$ band hybridizing with a localized $f$ band. At $T=0$, it exhibits a hybridization tuned Kondo breakdown quantum critical point (KB-QCP) from a Kondo to an RKKY phase. At the KB-QCP, the $f$ band changes character from itinerant to mainly localized, while the $c$ band remains itinerant. We elucidate its nature in detail by performing a high-resolution, real-frequency study of dynamical quantities. NRG allows us to study the quantum critical non-Fermi-liquid (NFL) regime located between $T_{FL}<T_{NFL}$. Surprisingly, self-consistency is essential to stabilize the NFL and the QCP. The Fermi-liquid (FL) scale $T_{FL}$ decreases towards and vanishes at the QCP. At $T=0$, we find the following properties. The $f$ quasiparticle (QP) weight $Z_f$ decreases continuously as the QCP is approached from either side, vanishing only at the QCP. Therefore, $Z_f$ is nonzero in both the Kondo and the RKKY phase; hence, the FL QP comprise $c$ and $f$ electrons in both phases. The Fermi surface (FS) volumes in the two phases differ. Whereas the large-FS Kondo phase has a usual two-band structure, the small-FS RKKY phase has an unexpected three-band structure. We provide a detailed analysis of quasiparticle properties of both the Kondo and the RKKY phase. The FS reconstruction is accompanied by the appearance of a Luttinger surface (LS) on which the $f$ self-energy diverges. The FS and LS volumes are related to the density by a generalized Luttinger sum rule. We interpret the small FS volume and the emergent LS as evidence for $f$-electron fractionalization in the RKKY phase. Our Hall coefficient and specific heat are in good qualitative agreement with experiment. △ Less

Submitted 19 October, 2023; originally announced October 2023.

Comments: 38 pages, 26 figures

Symmetric improved estimators for multipoint vertex functions

Authors: Jae-Mo Lihm, Johannes Halbinger, Jeongmin Shim, Jan von Delft, Fabian B. Kugler, Seung-Sup B. Lee

Abstract: Multipoint vertex functions, and the four-point vertex in particular, are crucial ingredients in many-body theory. Recent years have seen significant algorithmic progress toward numerically computing their dependence on multiple frequency arguments. However, such computations remain challenging and are prone to suffer from numerical artifacts, especially in the real-frequency domain. Here, we deri… ▽ More Multipoint vertex functions, and the four-point vertex in particular, are crucial ingredients in many-body theory. Recent years have seen significant algorithmic progress toward numerically computing their dependence on multiple frequency arguments. However, such computations remain challenging and are prone to suffer from numerical artifacts, especially in the real-frequency domain. Here, we derive estimators for multipoint vertices that are numerically more robust than those previously available. We show that the two central steps for extracting vertices from correlators, namely the subtraction of disconnected contributions and the amputation of external legs, can be achieved accurately through repeated application of equations of motion, in a manner that is symmetric with respect to all frequency arguments and involves only fully renormalized objects. The symmetric estimators express the core part of the vertex and all asymptotic contributions through separate expressions that can be computed independently, without subtracting the large-frequency limits of various terms with different asymptotic behaviors. Our strategy is general and applies equally to the Matsubara formalism, the real-frequency zero-temperature formalism, and the Keldysh formalism. We demonstrate the advantages of the symmetric improved estimators by computing the Keldysh four-point vertex of the single-impurity Anderson model using the numerical renormalization group. △ Less

Submitted 22 March, 2024; v1 submitted 18 October, 2023; originally announced October 2023.

Comments: 36 pages, 23 figures

Journal ref: Phys. Rev. B 109, 125138 (2024)

Pb$_9$Cu(PO4)$_6$(OH)$_2$: Phonon bands, Localized Flat Band Magnetism, Models, and Chemical Analysis

Authors: Yi Jiang, Scott B. Lee, Jonah Herzog-Arbeitman, Jiabin Yu, Xiaolong Feng, Haoyu Hu, Dumitru Călugăru, Parker S. Brodale, Eoghan L. Gormley, Maia Garcia Vergniory, Claudia Felser, S. Blanco-Canosa, Christopher H. Hendon, Leslie M. Schoop, B. Andrei Bernevig

Abstract: In a series of recent reports, doped lead apatite (LK-99) has been proposed as a candidate ambient temperature and pressure superconductor. However, from both an experimental and theoretical perspective, these claims are largely unsubstantiated. To this end, our synthesis and subsequent analysis of an LK-99 sample reveals a multiphase material that does not exhibit high-temperature superconductivi… ▽ More In a series of recent reports, doped lead apatite (LK-99) has been proposed as a candidate ambient temperature and pressure superconductor. However, from both an experimental and theoretical perspective, these claims are largely unsubstantiated. To this end, our synthesis and subsequent analysis of an LK-99 sample reveals a multiphase material that does not exhibit high-temperature superconductivity. We study the structure of this phase with single-crystal X-ray diffraction (SXRD) and find a structure consistent with doped $\text{Pb}_{10}(\text{PO}_4)_6(\text{OH})_2$. However, the material is transparent which rules out a superconducting nature. From ab initio defect formation energy calculations, we find that the material likely hosts $\text{OH}^-$ anions, rather than divalent $\text{O}^{2-}$ anions, within the hexagonal channels and that Cu substitution is highly thermodynamically disfavored. Phonon spectra on the equilibrium structures reveal numerous unstable phonon modes. Together, these calculations suggest it is doubtful that Cu enters the structure in meaningful concentrations, despite initial attempts to model LK-99 in this way. However for the sake of completeness, we perform ab initio calculations of the topology, quantum geometry, and Wannier function localization in the Cu-dominated flat bands of four separate doped structures. In all cases, we find they are atomically localized by irreps, Wilson loops, and the Fubini-Study metric. It is unlikely that such bands can support strong superfluidity, and instead are susceptible to ferromagnetism (or out-of-plane antiferromagnetism) at low temperatures, which we find in ab initio studies. In sum, $\text{Pb}_{9}\text{Cu}(\text{PO}_4)_6(\text{OH})_2$ could more likely be a magnet, rather than an ambient temperature and pressure superconductor. △ Less

Submitted 17 August, 2023; v1 submitted 9 August, 2023; originally announced August 2023.

Comments: 39 pages including appendices. Updated defect calculations and energy-dispersive X-ray spectroscopy data

Tuning orbital-selective phase transitions in a two-dimensional Hund's correlated system

Authors: Eun Kyo Ko, Sungsoo Hahn, Changhee Sohn, Sangmin Lee, Seung-Sup B. Lee, Byungmin Sohn, Jeong Rae Kim, Jaeseok Son, Jeongkeun Song, Youngdo Kim, Donghan Kim, Miyoung Kim, Choong H. Kim, Changyoung Kim, Tae Won Noh

Abstract: Hund's rule coupling ($\textit{J}$) has attracted much attention recently for its role in the description of the novel quantum phases of multi orbital materials. Depending on the orbital occupancy, $\textit{J}$ can lead to various intriguing phases. However, experimental confirmation of the orbital occupancy dependency has been difficult as controlling the orbital degrees of freedom normally accom… ▽ More Hund's rule coupling ($\textit{J}$) has attracted much attention recently for its role in the description of the novel quantum phases of multi orbital materials. Depending on the orbital occupancy, $\textit{J}$ can lead to various intriguing phases. However, experimental confirmation of the orbital occupancy dependency has been difficult as controlling the orbital degrees of freedom normally accompanies chemical inhomogeneities. Here, we demonstrate a method to investigate the role of orbital occupancy in $\textit{J}$ related phenomena without inducing inhomogeneities. By growing SrRuO$_3$ monolayers on various substrates with symmetry-preserving interlayers, we gradually tune the crystal field splitting and thus the orbital degeneracy of the Ru $\textit{t$_2$$_g$}$ orbitals. It effectively varies the orbital occupancies of two-dimensional (2D) ruthenates. Via in-situ angle-resolved photoemission spectroscopy, we observe a progressive metal-insulator transition (MIT). It is found that the MIT occurs with orbital differentiation: concurrent opening of a band insulating gap in the $\textit{d$_x$$_y$}$ band and a Mott gap in the $\textit{d$_x$$_z$$_/$$_y$$_z$}$ bands. Our study provides an effective experimental method for investigation of orbital-selective phenomena in multi-orbital materials. △ Less

Submitted 18 October, 2022; v1 submitted 11 October, 2022; originally announced October 2022.

Reply to: "Extracting Kondo temperature of strongly-correlated systems from the inverse local magnetic susceptibility"

Authors: Xiaoyu Deng, Katharina M. Stadler, Kristjan Haule, Seung-Sup B. Lee, Andreas Weichselbaum, Jan von Delft, Gabriel Kotliar

Abstract: This is a reply to A. A. Katanin's comment [A. A. Katanin, Nat. Commun. 12, 1433 (2021); arXiv:2103.02966] on our paper [X. Deng et al., Nat. Commun. 10, 2721 (2019); arXiv:1708.05752]. This is a reply to A. A. Katanin's comment [A. A. Katanin, Nat. Commun. 12, 1433 (2021); arXiv:2103.02966] on our paper [X. Deng et al., Nat. Commun. 10, 2721 (2019); arXiv:1708.05752]. △ Less

Submitted 11 March, 2021; originally announced March 2021.

Journal ref: Nat. Commun. 12, 1445 (2021)

Computing local multipoint correlators using the numerical renormalization group

Authors: Seung-Sup B. Lee, Fabian B. Kugler, Jan von Delft

Abstract: Local three- and four-point correlators yield important insight into strongly correlated systems and have many applications. However, the nonperturbative, accurate computation of multipoint correlators is challenging, particularly in the real-frequency domain for systems at low temperatures. In the accompanying paper, we introduce generalized spectral representations for multipoint correlators. He… ▽ More Local three- and four-point correlators yield important insight into strongly correlated systems and have many applications. However, the nonperturbative, accurate computation of multipoint correlators is challenging, particularly in the real-frequency domain for systems at low temperatures. In the accompanying paper, we introduce generalized spectral representations for multipoint correlators. Here, we develop a numerical renormalization group (NRG) approach, capable of efficiently evaluating these spectral representations, to compute local three- and four-point correlators of quantum impurity models. The key objects in our scheme are partial spectral functions, encoding the system's dynamical information. Their computation via NRG allows us to simultaneously resolve various multiparticle excitations down to the lowest energies. By subsequently convolving the partial spectral functions with appropriate kernels, we obtain multipoint correlators in the imaginary-frequency Matsubara, the real-frequency zero-temperature, and the real-frequency Keldysh formalisms. We present exemplary results for the connected four-point correlators of the Anderson impurity model, and for resonant inelastic x-ray scattering (RIXS) spectra of related impurity models. Our method can treat temperatures and frequencies -- imaginary or real -- of all magnitudes, from large to arbitrarily small ones. △ Less

Submitted 13 October, 2021; v1 submitted 3 January, 2021; originally announced January 2021.

Comments: See also the jointly published paper [F. B. Kugler, S.-S. B. Lee, and J. von Delft, Phys. Rev. X 11, 041006 (2021); arXiv:2101.00707]

Journal ref: Phys. Rev. X 11, 041007 (2021)

Multipoint correlation functions: spectral representation and numerical evaluation

Authors: Fabian B. Kugler, Seung-Sup B. Lee, Jan von Delft

Abstract: The many-body problem is usually approached from one of two perspectives: the first originates from an action and is based on Feynman diagrams, the second is centered around a Hamiltonian and deals with quantum states and operators. The connection between results obtained in either way is made through spectral (or Lehmann) representations, well known for two-point correlation functions. Here, we c… ▽ More The many-body problem is usually approached from one of two perspectives: the first originates from an action and is based on Feynman diagrams, the second is centered around a Hamiltonian and deals with quantum states and operators. The connection between results obtained in either way is made through spectral (or Lehmann) representations, well known for two-point correlation functions. Here, we complete this picture by deriving generalized spectral representations for multipoint correlation functions that apply in all of the commonly used many-body frameworks: the imaginary-frequency Matsubara and the real-frequency zero-temperature and Keldysh formalisms. Our approach separates spectral from time-ordering properties and thereby elucidates the relation between the three formalisms. The spectral representations of multipoint correlation functions consist of partial spectral functions and convolution kernels. The former are formalism independent but system specific; the latter are system independent but formalism specific. Using a numerical renormalization group (NRG) method described in the accompanying paper, we present numerical results for selected quantum impurity models. We focus on the four-point vertex (effective interaction) obtained for the single-impurity Anderson model and for the dynamical mean-field theory (DMFT) solution of the one-band Hubbard model. In the Matsubara formalism, we analyze the evolution of the vertex down to very low temperatures and describe the crossover from strongly interacting particles to weakly interacting quasiparticles. In the Keldysh formalism, we first benchmark our results at weak and infinitely strong interaction and then reveal the rich real-frequency structure of the DMFT vertex in the coexistence regime of a metallic and insulating solution. △ Less

Submitted 13 October, 2021; v1 submitted 3 January, 2021; originally announced January 2021.

Comments: See also the jointly published paper [S.-S. B. Lee, F. B. Kugler, and J. von Delft, Phys. Rev. X 11, 041007 (2021); arXiv:2101.00708]

Journal ref: Phys. Rev. X 11, 041006 (2021)

Strongly correlated materials from a numerical renormalization group perspective: How the Fermi-liquid state of Sr$_2$RuO$_4$ emerges

Authors: Fabian B. Kugler, Manuel Zingl, Hugo U. R. Strand, Seung-Sup B. Lee, Jan von Delft, Antoine Georges

Abstract: The crossover from fluctuating atomic constituents to a collective state as one lowers temperature or energy is at the heart of the dynamical mean-field theory description of the solid state. We demonstrate that the numerical renormalization group is a viable tool to monitor this crossover in a real-materials setting. The renormalization group flow from high to arbitrarily small energy scales clea… ▽ More The crossover from fluctuating atomic constituents to a collective state as one lowers temperature or energy is at the heart of the dynamical mean-field theory description of the solid state. We demonstrate that the numerical renormalization group is a viable tool to monitor this crossover in a real-materials setting. The renormalization group flow from high to arbitrarily small energy scales clearly reveals the emergence of the Fermi-liquid state of Sr$_2$RuO$_4$. We find a two-stage screening process, where orbital fluctuations are screened at much higher energies than spin fluctuations, and Fermi-liquid behavior, concomitant with spin coherence, below a temperature of 25 K. By computing real-frequency correlation functions, we directly observe this spin--orbital scale separation and show that the van Hove singularity drives strong orbital differentiation. We extract quasiparticle interaction parameters from the low-energy spectrum and find an effective attraction in the spin-triplet sector. △ Less

Submitted 7 January, 2020; v1 submitted 5 September, 2019; originally announced September 2019.

Journal ref: Phys. Rev. Lett. 124, 016401 (2020)

Mott quantum criticality in the one-band Hubbard model: Dynamical mean-field theory, power-law spectra, and scaling

Authors: Heike Eisenlohr, Seung-Sup B. Lee, Matthias Vojta

Abstract: Recent studies of electrical transport, both theoretical and experimental, near the bandwidth-tuned Mott metal-insulator transition have uncovered apparent quantum critical scaling of the electrical resistivity at elevated temperatures, despite the fact that the actual low-temperature phase transition is of first order. This raises the question whether there is a hidden Mott quantum critical point… ▽ More Recent studies of electrical transport, both theoretical and experimental, near the bandwidth-tuned Mott metal-insulator transition have uncovered apparent quantum critical scaling of the electrical resistivity at elevated temperatures, despite the fact that the actual low-temperature phase transition is of first order. This raises the question whether there is a hidden Mott quantum critical point. Here we argue that the dynamical mean-field theory of the Hubbard model admits, in the low-temperature limit, asymptotically scale-invariant (i.e. power-law) solutions, corresponding to the metastable insulator at the boundary of metal-insulator coexistence region, which can be linked to the physics of the pseudogap Anderson model. While our state-of-the-art numerical renormalization group calculations reveal that this asymptotic regime is restricted to very small energies and temperatures and hence difficult to access numerically, we uncover the existence of a wide crossover regime where the single-particle spectrum displays a \emph{different} power law. We show that it is this power-law regime, corresponding to approximate local quantum criticality, which is continuously connected to and responsible for the apparent quantum critical scaling above the classical critical end point. We connect our findings to experiments on tunable Mott materials. △ Less

Submitted 12 September, 2019; v1 submitted 12 June, 2019; originally announced June 2019.

Comments: 12 pages, 14 figures

Journal ref: Phys. Rev. B 100, 155152 (2019)

Non-Fermi-liquid Kondo screening under Rabi driving

Authors: Seung-Sup B. Lee, Jan von Delft, Moshe Goldstein

Abstract: We investigate a Rabi-Kondo model describing an optically driven two-channel quantum dot device featuring a non-Fermi-liquid Kondo effect. Optically induced Rabi oscillation between the valence and conduction levels of the dot gives rise to a two-stage Kondo effect: Primary screening of the local spin is followed by secondary nonequilibrium screening of the local orbital degree of freedom. Using b… ▽ More We investigate a Rabi-Kondo model describing an optically driven two-channel quantum dot device featuring a non-Fermi-liquid Kondo effect. Optically induced Rabi oscillation between the valence and conduction levels of the dot gives rise to a two-stage Kondo effect: Primary screening of the local spin is followed by secondary nonequilibrium screening of the local orbital degree of freedom. Using bosonization arguments and the numerical renormalization group, we compute the dot emission spectrum and residual entropy. Remarkably, both exhibit two-stage Kondo screening with non-Fermi-liquid properties at both stages. △ Less

Submitted 10 February, 2020; v1 submitted 29 May, 2019; originally announced May 2019.

Journal ref: Phys. Rev. B 101, 085110 (2020)

Orbital differentiation in Hund metals

Authors: Fabian B. Kugler, Seung-Sup B. Lee, Andreas Weichselbaum, Gabriel Kotliar, Jan von Delft

Abstract: Orbital differentiation is a common theme in multiorbital systems, yet a complete understanding of it is still missing. Here, we consider a minimal model for orbital differentiation in Hund metals with a highly accurate method: We use the numerical renormalization group as a real-frequency impurity solver for a dynamical mean-field study of three-orbital Hubbard models, where a crystal field shift… ▽ More Orbital differentiation is a common theme in multiorbital systems, yet a complete understanding of it is still missing. Here, we consider a minimal model for orbital differentiation in Hund metals with a highly accurate method: We use the numerical renormalization group as a real-frequency impurity solver for a dynamical mean-field study of three-orbital Hubbard models, where a crystal field shifts one orbital in energy. The individual phases are characterized with dynamic correlation functions and their relation to diverse Kondo temperatures. Upon approaching the orbital-selective Mott transition, we find a strongly suppressed spin coherence scale and uncover the emergence of a singular Fermi liquid and interband doublon-holon excitations. Our theory describes the diverse polarization-driven phenomena in the $t_{2g}$ bands of materials such as ruthenates and iron-based superconductors, and our methodological advances pave the way towards real-frequency analyses of strongly correlated materials. △ Less

Submitted 7 January, 2020; v1 submitted 24 April, 2019; originally announced April 2019.

Journal ref: Phys. Rev. B 100, 115159 (2019)

Quadrupolar ordering and exotic magnetocaloric effect in RB4 (R = Dy, Ho)

Authors: M. S. Song, K. K. Cho, B. Y. Kang, S. B. Lee, B. K. Cho

Abstract: The interplay of charge, spin, orbital and lattice degrees of freedom has recently received great interest due to its potential to improve the magnetocaloric effect (MCE) for the purpose of magnetic cooling applications. Here we propose a new mechanism for a giant inverse MCE in rare-earth tetraborides, especially for Ho1-xDyxB4 (x = 0.0, 0.5, and 1.0). For x = 0.0, 0.5, and 1.0, the maximum entro… ▽ More The interplay of charge, spin, orbital and lattice degrees of freedom has recently received great interest due to its potential to improve the magnetocaloric effect (MCE) for the purpose of magnetic cooling applications. Here we propose a new mechanism for a giant inverse MCE in rare-earth tetraborides, especially for Ho1-xDyxB4 (x = 0.0, 0.5, and 1.0). For x = 0.0, 0.5, and 1.0, the maximum entropy changes of the giant inverse MCE are found to be 22.7 J/kgK, 19.6 J/kgK, and 19.0 J/kgK with critical fields of 25 kOe, 40 kOe, and 50 kOe, respectively. It is remarkable that such a giant MCE is realized, even when applying a low magnetic field, which enables a field-tuned entropy change and brings about a significant advantage for several applications. For all compounds, we have systematically studied how the entropy changes as a function of the field and temperature and investigated their correlation with consecutive double transitions, i.e., the magnetic dipolar order at T = TN and the quadrupolar order at T = TQ (TQ < TN). We found that the maximum entropy change occurs at T = TQ and the critical field associated with the meta-magnetic transition, which is in good agreement with the experimental data. Thus, we elucidate that this unique behaviour is attributed to the strong coupling between magnetic dipoles and quadrupoles in the presence of strong spin-orbit coupling and geometric frustration. Our work offers new insights into both the academic interest of multipolar degrees of freedom in magnetic materials and the discovery of giant MCE with various applications for magnetic cooling systems. △ Less

Submitted 7 February, 2019; originally announced February 2019.

Comments: 25 pages, 12 figures

Detecting Kondo Entanglement by Electron Conductance

Authors: Gwangsu Yoo, Seung-Sup B. Lee, H. -S. Sim

Abstract: Quantum entanglement between an impurity spin and electrons nearby is a key property of the single-channel Kondo effects. We show that the entanglement can be detected by measuring electron conductance through a double quantum dot in an orbital Kondo regime. We derive a relation between the entanglement and the conductance, when the SU(2) spin symmetry of the regime is weakly broken. The relation… ▽ More Quantum entanglement between an impurity spin and electrons nearby is a key property of the single-channel Kondo effects. We show that the entanglement can be detected by measuring electron conductance through a double quantum dot in an orbital Kondo regime. We derive a relation between the entanglement and the conductance, when the SU(2) spin symmetry of the regime is weakly broken. The relation reflects the universal form of many-body states near the Kondo fixed point. Using it, the spatial distribution of the entanglement, hence, the Kondo cloud, can be detected, with breaking the symmetry spatially nonuniformly by electrical means. △ Less

Submitted 27 August, 2018; originally announced August 2018.

Journal ref: Phys. Rev. Lett. 120, 146801 (2018)

Numerical renormalization group method for entanglement negativity at finite temperature

Authors: Jeongmin Shim, H. -S. Sim, Seung-Sup B. Lee

Abstract: We develop a numerical method to compute the negativity, an entanglement measure for mixed states, between the impurity and the bath in quantum impurity systems at finite temperature. We construct a thermal density matrix by using the numerical renormalization group (NRG), and evaluate the negativity by implementing the NRG approximation that reduces computational cost exponentially. We apply the… ▽ More We develop a numerical method to compute the negativity, an entanglement measure for mixed states, between the impurity and the bath in quantum impurity systems at finite temperature. We construct a thermal density matrix by using the numerical renormalization group (NRG), and evaluate the negativity by implementing the NRG approximation that reduces computational cost exponentially. We apply the method to the single-impurity Kondo model and the single-impurity Anderson model. In the Kondo model, the negativity exhibits a power-law scaling at temperature much lower than the Kondo temperature and a sudden death at high temperature. In the Anderson model, the charge fluctuation of the impurity contribute to the negativity even at zero temperature when the on-site Coulomb repulsion of the impurity is finite, while at low temperature the negativity between the impurity spin and the bath exhibits the same power-law scaling behavior as in the Kondo model. △ Less

Submitted 26 August, 2018; originally announced August 2018.

Journal ref: Physical Review B 97, 155123 (2018)

Filling-driven Mott transition in SU(N) Hubbard models

Authors: Seung-Sup B. Lee, Jan von Delft, Andreas Weichselbaum

Abstract: We study the filling-driven Mott transition involving the metallic and paramagnetic insulating phases in SU(N) Fermi-Hubbard models, using dynamical mean-field theory (DMFT) and the numerical renormalization group (NRG) as impurity solver. The compressibility shows a striking temperature dependence: near the critical temperature, it is strongly enhanced in the metallic phase close to the insulatin… ▽ More We study the filling-driven Mott transition involving the metallic and paramagnetic insulating phases in SU(N) Fermi-Hubbard models, using dynamical mean-field theory (DMFT) and the numerical renormalization group (NRG) as impurity solver. The compressibility shows a striking temperature dependence: near the critical temperature, it is strongly enhanced in the metallic phase close to the insulating phase. We demonstrate that this compressibility enhancement is associated with the thermal suppression of the quasiparticle peak in the local spectral functions. We also explain that the asymmetric shape of the quasiparticle peak originates from the asymmetry in the underlying doublon-holon dynamics. △ Less

Submitted 11 October, 2017; originally announced October 2017.

Journal ref: Phys. Rev. B 97, 165143 (2018)

Generalized Schrieffer-Wolff transformation of multi-flavor Hubbard models

Authors: Seung-Sup B. Lee, Jan von Delft, Andreas Weichselbaum

Abstract: We give a self-contained derivation of the low-energy effective interactions of the SU($N$) Hubbard model, a multiflavor generalization of the one-band Hubbard model, by using a generalized Schrieffer-Wolff transformation (SWT). The effective interaction of doublons and holons, which has been largely ignored in previous SWT studies (e.g., the $t$-$J$ model), leads to distinct peaks in the local de… ▽ More We give a self-contained derivation of the low-energy effective interactions of the SU($N$) Hubbard model, a multiflavor generalization of the one-band Hubbard model, by using a generalized Schrieffer-Wolff transformation (SWT). The effective interaction of doublons and holons, which has been largely ignored in previous SWT studies (e.g., the $t$-$J$ model), leads to distinct peaks in the local density of states. As shown by Lee et al. [Phys. Rev. Lett. 119, 236402 (2017)], this underlying effective doublon-holon interaction explains the numerical observation of the subpeaks at the inner edges of the Hubbard bands in the metallic phase close to the Mott transition. △ Less

Submitted 6 December, 2017; v1 submitted 3 October, 2017; originally announced October 2017.

Comments: Published version. This work was formerly a part of Supplementary Material of arXiv:1705.03910v1, it is now a separate paper

Journal ref: Phys. Rev. B 96, 245106 (2017)

Doublon-holon origin of the subpeaks at the Hubbard band edges

Authors: Seung-Sup B. Lee, Jan von Delft, Andreas Weichselbaum

Abstract: Dynamical mean-field theory (DMFT) studies frequently observe a fine structure in the local spectral function of the SU(2) Fermi-Hubbard model at half filling: In the metallic phase close to the Mott transition, subpeaks emerge at the inner edges of the Hubbard bands. Here we demonstrate that these subpeaks originate from the low-energy effective interaction of doublon-holon pairs, by investigatin… ▽ More Dynamical mean-field theory (DMFT) studies frequently observe a fine structure in the local spectral function of the SU(2) Fermi-Hubbard model at half filling: In the metallic phase close to the Mott transition, subpeaks emerge at the inner edges of the Hubbard bands. Here we demonstrate that these subpeaks originate from the low-energy effective interaction of doublon-holon pairs, by investigating how the correlation functions of doublon and holon operators contribute to the subpeaks. A mean-field analysis of the low-energy effective Hamiltonian provides results consistent with our DMFT calculation using the numerical renormalization group as an impurity solver. In the SU(3) and SU(4) Hubbard models, the subpeaks become more pronounced due to the increased degeneracy of doublon-holon pair excitations. △ Less

Submitted 6 December, 2017; v1 submitted 10 May, 2017; originally announced May 2017.

Comments: Published version. For the detail of analytical part, refer to arXiv:1710.01171

Journal ref: Phys. Rev. Lett. 119, 236402 (2017)

Open Wilson chains for quantum impurity models: Keeping track of all bath modes

Authors: Benedikt Bruognolo, Nils-Oliver Linden, Frauke Schwarz, Seung-Sup B. Lee, Katharina Stadler, Andreas Weichselbaum, Matthias Vojta, Frithjof B. Anders, Jan von Delft

Abstract: When constructing a Wilson chain to represent a quantum impurity model, the effects of truncated bath modes are neglected. We show that their influence can be kept track of systematically by constructing an "open Wilson chain" in which each site is coupled to a separate effective bath of its own. As a first application, we use the method to cure the so-called mass-flow problem that can arise when… ▽ More When constructing a Wilson chain to represent a quantum impurity model, the effects of truncated bath modes are neglected. We show that their influence can be kept track of systematically by constructing an "open Wilson chain" in which each site is coupled to a separate effective bath of its own. As a first application, we use the method to cure the so-called mass-flow problem that can arise when using standard Wilson chains to treat impurity models with asymmetric bath spectral functions at finite temperature. We demonstrate this for the strongly sub-Ohmic spin-boson model at quantum criticality where we directly observe the flow towards a Gaussian critical fixed point. △ Less

Submitted 3 April, 2017; v1 submitted 16 November, 2016; originally announced November 2016.

Comments: 4 + 12 pages, 4 + 6 figures (published version)

Journal ref: Phys. Rev. B 95, 121115 (2017)

Adaptive broadening to improve spectral resolution in the numerical renormalization group

Authors: Seung-Sup B. Lee, Andreas Weichselbaum

Abstract: We propose an adaptive scheme of broadening the discrete spectral data from numerical renormalization group (NRG) calculations to improve the resolution of dynamical properties at finite energies. While the conventional scheme overbroadens narrow features at large frequency by broadening discrete weights with constant width in log-frequency, our scheme broadens each discrete contribution individua… ▽ More We propose an adaptive scheme of broadening the discrete spectral data from numerical renormalization group (NRG) calculations to improve the resolution of dynamical properties at finite energies. While the conventional scheme overbroadens narrow features at large frequency by broadening discrete weights with constant width in log-frequency, our scheme broadens each discrete contribution individually based on its sensitivity to a z-shift in the logarithmic discretization intervals. We demonstrate that the adaptive broadening better resolves various features in non-interacting and interacting models at comparable computational cost. The resolution enhancement is more significant for coarser discretization as typically required in multi-band calculations. At low frequency below the energy scale of temperature, the discrete NRG data necessarily needs to be broadened on a linear scale. Here we provide a method that minimizes transition artefacts in between these broadening kernels. △ Less

Submitted 22 April, 2017; v1 submitted 5 September, 2016; originally announced September 2016.

Journal ref: Phys. Rev. B 94, 235127 (2016)

Linkage-length dependent structuring behaviour of bent-core molecules in helical nanostructures

Authors: Hanim Kim, Anna Zep, Seong Ho Ryu, Hyungju Ahn, Tae Joo Shin, Sang Bok Lee, Damian Pociecha, Ewa Gorecka, Dong Ki Yoon

Abstract: We studied the correlation between the molecular structure and the formation of helical nanofilaments (HNFs) of bent-core dimeric molecules with varying linkage lengths. To obtain precise structural data, a single domain of HNFs was prepared under physical confinement using porous 1D nanochannels, made up of anodic aluminium oxide films. Electron microscopy and grazing incidence X-ray diffraction… ▽ More We studied the correlation between the molecular structure and the formation of helical nanofilaments (HNFs) of bent-core dimeric molecules with varying linkage lengths. To obtain precise structural data, a single domain of HNFs was prepared under physical confinement using porous 1D nanochannels, made up of anodic aluminium oxide films. Electron microscopy and grazing incidence X-ray diffraction were used to elucidate the linkage length-dependent formation of HNFs. △ Less

Submitted 16 February, 2016; originally announced February 2016.

Comments: published in Soft Matter (2016). 17 pages in total, 5 figures, 2 figures in Supporting materials

Low Temperature metamagnetism and Hall effect anomaly in Kondo compound CeAgBi2

Authors: S. M. Thomas, P. F. S. Rosa, S. B. Lee, S. A. Parameswaran, Z. Fisk, J. Xia

Abstract: Heavy fermion (HF) materials exhibit a rich array of phenomena due to the strong Kondo coupling between their localized moments and itinerant electrons. A central question in their study is to understand the interplay between magnetic order and charge transport, and its role in stabilizing new quantum phases of matter. Particularly promising in this regard is a family of tetragonal intermetallic c… ▽ More Heavy fermion (HF) materials exhibit a rich array of phenomena due to the strong Kondo coupling between their localized moments and itinerant electrons. A central question in their study is to understand the interplay between magnetic order and charge transport, and its role in stabilizing new quantum phases of matter. Particularly promising in this regard is a family of tetragonal intermetallic compounds Ce{$TX$}$_2$ ($T=$ transition metal, $X=$ pnictogen), that includes a variety of HF compounds showing $T$-linear electronic specific heat $\bf{C_e \sim γT}$, with $γ\sim$ 20-500 mJ$\cdot$mol$^{-1}$~K$^{-2}$, reflecting an effective mass enhancement ranging from small to modest. Here, we study the low-temperature field-tuned phase diagram of high-quality CeAgBi$_2$ using magnetometry and transport measurements. We find an antiferromagnetic transition at ${T_{N} = 6.4}$~K with weak magnetic anisotropy and the easy axis along the $c$-axis, similar to previous reports (${T_{N} = 6.1}$~K). This scenario, along with the presence of two anisotropic Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions, leads to a rich field-tuned magnetic phase diagram, consisting of five metamagnetic transitions of both first and second order. In addition, we unveil an anomalous Hall contribution for fields $H<54$ kOe which is drastically altered when $H$ is tuned through a trio of transitions at 57, 78, and 84~kOe, suggesting that the Fermi surface is reconstructed in a subset of the metamagnetic transitions. △ Less

Submitted 29 February, 2016; v1 submitted 21 August, 2015; originally announced August 2015.

Comments: (*equal contribution)

Journal ref: Phys. Rev. B 93, 075149 (2016)

Non-filamentary memristive switching in Pt/CuOx/Si/Pt systems

Authors: L. L. Wei, D. S. Shang, J. R. Sun, S. B. Lee, Z. G. Sun, B. G. Shen

Abstract: We report a memristive switching effect in the Pt/CuOx/Si/Pt devices prepared by rf sputtering technique at room temperature. Different from other Cu-based switching systems, the devices show a non-filamentary switching effect. A gradual electroforming marked by resistance increasing and capacitance decreasing is observed in current-voltage and capacitance characteristics. By the Auger electron sp… ▽ More We report a memristive switching effect in the Pt/CuOx/Si/Pt devices prepared by rf sputtering technique at room temperature. Different from other Cu-based switching systems, the devices show a non-filamentary switching effect. A gradual electroforming marked by resistance increasing and capacitance decreasing is observed in current-voltage and capacitance characteristics. By the Auger electron spectroscopy analysis, a model based on Cu ion and oxygen vacancy drift, and thickness change of the SiOx layer at the CuOx/Si interface was proposed for the memristive switching and gradual electroforming, respectively. The present work would be meaningful for the preparation of forming-free and homogeneous memristive devices. △ Less

Submitted 15 April, 2013; originally announced April 2013.

Comments: 17 pages, 8 figures

Journal ref: Nanotechnology 24, 325202 2013

Two opposite hysteresis curves in semiconductors with mobile dopants

Authors: Jae Sung Lee, Shin Buhm Lee, Byungnam Kahng, Tae Won Noh

Abstract: Recent experimental researches on semiconductors with mobile dopants (SMD) have reported unconventional hysteretic current-voltage (I-V) curves, which form dynamically in either one of the two opposite directions, the counter-figure-eight and figure-eight ways. However the fundamental theory for the formation of the two directions is still absent, and this poses a major barrier for researches orie… ▽ More Recent experimental researches on semiconductors with mobile dopants (SMD) have reported unconventional hysteretic current-voltage (I-V) curves, which form dynamically in either one of the two opposite directions, the counter-figure-eight and figure-eight ways. However the fundamental theory for the formation of the two directions is still absent, and this poses a major barrier for researches oriented to applications. Here, we introduce a theoretical model to explain the origin of the two directions, and find that the two ways originate from the spatial inhomogeneity of the dopant distribution in SMD. The counter-figure-eight (figure-eight) way of the hysteretic curve is obtained when dopants are driven from far from (near) the Schottky interface to the opposite side in the SMD. This finding indicates that the directions of hysteretic curve in SMD can be controlled. △ Less

Submitted 12 August, 2012; originally announced August 2012.

Comments: 5 pages, 4 figures

Journal ref: Appl. Phys. Lett. 102, 253503 (2013)

Stabilizing the forming process in unipolar resistance switching using an improved compliance current limiter

Authors: S. B. Lee, S. H. Chang, H. K. Yoo, B. S. Kang

Abstract: The high reset current IR in unipolar resistance switching now poses major obstacles to practical applications in memory devices. In particular, the first IR-value after the forming process is so high that the capacitors sometimes do not exhibit reliable unipolar resistance switching. We found that the compliance current Icomp is a critical parameter for reducing IR-values. We therefore introduced… ▽ More The high reset current IR in unipolar resistance switching now poses major obstacles to practical applications in memory devices. In particular, the first IR-value after the forming process is so high that the capacitors sometimes do not exhibit reliable unipolar resistance switching. We found that the compliance current Icomp is a critical parameter for reducing IR-values. We therefore introduced an improved, simple, easy to use Icomp-limiter that stabilizes the forming process by drastically decreasing current overflow, in order to precisely control the Icomp- and subsequent IR-values. △ Less

Submitted 26 June, 2010; originally announced June 2010.

Comments: 15 pages, 4 figures

Reduction of high reset currents in unipolar resistance switching Pt/SrTiOx/Pt capacitors using acceptor doping

Authors: S. B. Lee, A. Kim, J. S. Lee, S. H. Chang, H. K. Yoo, T. W. Noh, B. Kahng, M. -J. Lee, C. J. Kim, B. S. Kang

Abstract: The high reset current, IR, in unipolar resistance switching is an important issue which should be resolved for practical applications in nonvolatile memories. We showed that,during the forming and set processes, the compliance current, Icomp, can work as a crucial parameter to reduce IR. Doping with Co or Mn can significantly reduce the leakage current in capacitors made using SrTiOx film, openin… ▽ More The high reset current, IR, in unipolar resistance switching is an important issue which should be resolved for practical applications in nonvolatile memories. We showed that,during the forming and set processes, the compliance current, Icomp, can work as a crucial parameter to reduce IR. Doping with Co or Mn can significantly reduce the leakage current in capacitors made using SrTiOx film, opening a larger operation window for Icomp. By decreasing Icomp with acceptor doping, we could reduce IR in SrTiOx films by a factor of approximately 20. Our work suggests that the decrease of Icomp by carrier doping could be a viable alternative for reducing IR in unipolar resistance switching. △ Less

Submitted 6 March, 2010; originally announced March 2010.

Comments: 14 pages, 3 figures

Large 1/f noise of unipolar resistance switching and its percolating nature

Authors: S. B. Lee, S. Park, J. S. Lee, S. C. Chae, S. H. Chang, M. H. Jung, Y. Jo, B. Kahng, B. S. Kang, M. -J. Lee, T. W. Noh

Abstract: We investigated the 1/f noise of Pt/NiO/Pt capacitors that show unipolar resistance switching. When they were switched from the low to high resistance states, the power spectral density of the voltage fluctuation was increased by approximately five orders of magnitude. At 100 K, the relative resistance fluctuation, SR/R2, in the low resistance state displayed a power law dependence on the resist… ▽ More We investigated the 1/f noise of Pt/NiO/Pt capacitors that show unipolar resistance switching. When they were switched from the low to high resistance states, the power spectral density of the voltage fluctuation was increased by approximately five orders of magnitude. At 100 K, the relative resistance fluctuation, SR/R2, in the low resistance state displayed a power law dependence on the resistance R with exponent w = 1.6. This behavior can be explained by percolation theory; however, at higher temperatures or near the switching voltage, SR/R2 becomes enhanced further. This large 1/f noise can be therefore an important problem in the development of resistance random access memory devices. △ Less

Submitted 11 August, 2009; originally announced August 2009.

Comments: 15 pages, 3 figures

Predictability of reset switching voltages in unipolar resistance switching

Authors: S. B. Lee, S. C. Chae, S. H. Chang, T. W. Noh

Abstract: In unipolar resistance switching of NiO capacitors, Joule heating in the conducting channels should cause a strong nonlinearity in the low resistance state current-voltage (I-V) curves. Due to the percolating nature of the conducting channels, the reset current IR, can be scaled to the nonlinear coefficient Bo of the I-V curves. This scaling relationship can be used to predict reset voltages, in… ▽ More In unipolar resistance switching of NiO capacitors, Joule heating in the conducting channels should cause a strong nonlinearity in the low resistance state current-voltage (I-V) curves. Due to the percolating nature of the conducting channels, the reset current IR, can be scaled to the nonlinear coefficient Bo of the I-V curves. This scaling relationship can be used to predict reset voltages, independent of NiO capacitor size; it can also be applied to TiO2 and FeOy capacitors. Using this relation, we developed an error correction scheme to provide a clear window for separating reset and set voltages in memory operations. △ Less

Submitted 9 March, 2009; originally announced March 2009.

Scaling behaviors of RESET voltages and currents in unipolar resistance switching

Authors: S. B. Lee, S. C. Chae, S. H. Chang, J. S. Lee, S. Seo, B. Kahng, T. W. Noh

Abstract: Unipolar switching phenomena have attracted a great deal of recent attention, but the wide distributions of switching voltages still pose major obstacles for scientific advancement and practical applications. Using NiO capacitors, we investigated the distributions of the RESET voltage and current. We found that they scaled with the resistance value Ro in the low resistance state, and that the sc… ▽ More Unipolar switching phenomena have attracted a great deal of recent attention, but the wide distributions of switching voltages still pose major obstacles for scientific advancement and practical applications. Using NiO capacitors, we investigated the distributions of the RESET voltage and current. We found that they scaled with the resistance value Ro in the low resistance state, and that the scaling exponents varied at Ro = 30 Ohm. We explain these intriguing scaling behaviors and their crossovers by analogy with percolation theory. We show that the connectivity of conducting filaments plays a crucial role in the RESET process. △ Less

Submitted 22 October, 2008; originally announced October 2008.

Comments: 17 pages, 4 figures

Strong resistance nonlinearity and third harmonic generation in the unipolar resistance switching of NiO thin films

Authors: S. B. Lee, S. C. Chae, S. H. Chang, J. S. Lee, S. Park, Y. Jo, S. Seo, B. Kahng, T. W. Noh

Abstract: We investigated third harmonic generation in NiO thin films, which exhibit unipolar resistance switching behavior. We found that the low resistance states of the films were strongly nonlinear, with variations in the resistance R as large as 60%. This strong nonlinear behavior was most likely caused by Joule heating of conducting filaments inside the films. By carefully controlling the applied dc… ▽ More We investigated third harmonic generation in NiO thin films, which exhibit unipolar resistance switching behavior. We found that the low resistance states of the films were strongly nonlinear, with variations in the resistance R as large as 60%. This strong nonlinear behavior was most likely caused by Joule heating of conducting filaments inside the films. By carefully controlling the applied dc bias, we obtained several low resistance states, whose values of the third harmonic coefficient B3f were proportional to R2+w (with w = 2.07). This suggested that the resistance changes of the NiO films were accompanied by connectivity changes of the conducting filaments, as observed in classical percolating systems. △ Less

Submitted 6 October, 2008; originally announced October 2008.

Comments: 17 pages, 3 figures

Percolation Model Explaining Both Unipolar Memory and Threshold Resistance Switchings in NiO Film

Authors: S. H. Chang, J. S. Lee, S. C. Chae, S. B. Lee, C. Liu, B. Kahng, D. -W. Kim, T. W. Noh

Abstract: We observed two types of unipolar resistance switching (RS) in NiO film: memory RS at low temperature and threshold RS at high temperature. We explain these phenomena using a bond percolation model that describes the forming and rupturing of conducting filaments. Assuming Joule heating and thermal dissipation processes in the bonds, we explain how both RS types could occur and be controlled by t… ▽ More We observed two types of unipolar resistance switching (RS) in NiO film: memory RS at low temperature and threshold RS at high temperature. We explain these phenomena using a bond percolation model that describes the forming and rupturing of conducting filaments. Assuming Joule heating and thermal dissipation processes in the bonds, we explain how both RS types could occur and be controlled by temperature. We show that these unipolar RS are closely related and can be explained by a simple unified percolation picture. △ Less

Submitted 30 March, 2008; originally announced March 2008.

Comments: 16 pages, 5 figures

Effects of heat dissipation on unipolar resistance switching in Pt/NiO/Pt capacitors

Authors: S. H. Chang, S. C. Chae, S. B. Lee, C. Liu, T. W. Noh, J. S. Lee, B. Kahng, J. H. Jang, M. Y. Kim, D. -W. Kim, C. U. Jung

Abstract: We fabricated Pt/NiO/Pt capacitor structures with various bottom electrode thicknesses, $t_{BE}$, and investigated their resistance switching behaviors. The capacitors with $t_{BE} \geq 50$ nm exhibited typical unipolar resistance memory switching, while those with $t_{BE} \leq 30$ nm showed threshold switching. This interesting phenomenon can be explained in terms of the temperature-dependent s… ▽ More We fabricated Pt/NiO/Pt capacitor structures with various bottom electrode thicknesses, $t_{BE}$, and investigated their resistance switching behaviors. The capacitors with $t_{BE} \geq 50$ nm exhibited typical unipolar resistance memory switching, while those with $t_{BE} \leq 30$ nm showed threshold switching. This interesting phenomenon can be explained in terms of the temperature-dependent stability of conducting filaments. In particular, the thinner $t_{BE}$ makes dissipation of Joule heat less efficient, so the filaments will be at a higher temperature and become less stable. This study demonstrates the importance of heat dissipation in resistance random access memory. △ Less

Submitted 25 February, 2008; originally announced February 2008.

Comments: 14 pages, 3 figures

Abnormal Resistance Switching Behaviors of NiO Thin Films: Possible Occurrence of Simultaneous Formation and Rupture of Conducting Channels

Authors: Chunli Liu, S. C. Chae, S. H. Chang, S. B. Lee, T. W. Noh, J. S. Lee, B. Kahng, D. -W. Kim, C. U. Jung, S. Seo, Seung-Eon Ahn

Abstract: We report the detailed current-voltage (I-V) characteristics of resistance switching in NiO thin films. In unipolar resistance switching, it is commonly believed that conducting filaments will rupture when NiO changes from a low resistance to a high resistance state. However, we found that this resistance switching can sometimes show abnormal behavior during voltage- and current-driven I-V measu… ▽ More We report the detailed current-voltage (I-V) characteristics of resistance switching in NiO thin films. In unipolar resistance switching, it is commonly believed that conducting filaments will rupture when NiO changes from a low resistance to a high resistance state. However, we found that this resistance switching can sometimes show abnormal behavior during voltage- and current-driven I-V measurements. We used the random circuit breaker network model to explain how abnormal switching behaviors could occur. We found that this resistance change can occur via a series of avalanche processes, where conducting filaments could be formed as well as ruptured. △ Less

Submitted 22 January, 2008; originally announced January 2008.

Comments: 12 pages, 5 figures

Electron orbital valves made of multiply connected armchair carbon nanotubes with mirror-reflection symmetry: tight-binding study

Authors: Gunn Kim, Sang Bong Lee, Hoonkyung Lee, Jisoon Ihm

Abstract: Using the tight-binding method and the Landauer-Büttiker conductance formalism, we demonstrate that a multiply connected armchair carbon nanotube with a mirror-reflection symmetry can sustain an electron current of the $π$-bonding orbital while suppress that of the $π$-antibonding orbital over a certain energy range. Accordingly, the system behaves like an electron orbital valve and may be used… ▽ More Using the tight-binding method and the Landauer-Büttiker conductance formalism, we demonstrate that a multiply connected armchair carbon nanotube with a mirror-reflection symmetry can sustain an electron current of the $π$-bonding orbital while suppress that of the $π$-antibonding orbital over a certain energy range. Accordingly, the system behaves like an electron orbital valve and may be used as a scanning tunneling microscope to probe pairing symmetry in d-wave superconductors or even orbital ordering in solids which is believed to occur in some transition-metal oxides. △ Less

Submitted 18 December, 2006; v1 submitted 26 June, 2006; originally announced June 2006.

Comments: 4 figures, 12 pages

Journal ref: Journal of Physics: Condensed Matter v.19, p.26217 (2007)

Fano Resonance and Orbital Filtering in Multiply Connected Carbon Nanotubes

Authors: Gunn Kim, Sang Bong Lee, Tae-Suk Kim, Jisoon Ihm

Abstract: We investigate the electron transport in multiply connected metallic carbon nanotubes within the Landauer-Büttiker formalism. Quasibound states coupled to the incident $π^{*}$ states give rise to energy levels of different widths depending on the coupling strength. In particular, donorlike states originating from heptagonal rings are found to give a very narrow level. Interference between broad… ▽ More We investigate the electron transport in multiply connected metallic carbon nanotubes within the Landauer-Büttiker formalism. Quasibound states coupled to the incident $π^{*}$ states give rise to energy levels of different widths depending on the coupling strength. In particular, donorlike states originating from heptagonal rings are found to give a very narrow level. Interference between broad and narrow levels produces Fano-type resonant backscattering as well as resonant tunneling. Over a significantly wide energy range, almost perfect suppression of the conduction of $π^{*}$ electrons occurs, which may be regarded as filtering of particular electrons ($π$-pass filter). △ Less

Submitted 29 May, 2005; v1 submitted 25 February, 2004; originally announced February 2004.

Journal ref: Phys. Rev. B 71, 205415 (2005)