Showing 1–50 of 87 results for author: Chang, P

Modelling the onset and evolution of immiscible viscous fingering in porous media

Authors: Paulo L. K. Caetano Chang, Kundan Kumar, Arne Skauge, Kenneth S. Sorbie

Abstract: The simulation of viscous fingering in porous media is of direct relevance to displacement processes in petroleum engineering and hydrogeology. Building on recent work proposing a modelling approach for well-defined fingers at very adverse viscosity ratios, we investigate the physical mechanisms behind viscous fingering and the modelling requirements for capturing the finger scales and saturation… ▽ More The simulation of viscous fingering in porous media is of direct relevance to displacement processes in petroleum engineering and hydrogeology. Building on recent work proposing a modelling approach for well-defined fingers at very adverse viscosity ratios, we investigate the physical mechanisms behind viscous fingering and the modelling requirements for capturing the finger scales and saturation patterns observed in experiments. We simulate and match a viscous fingering experiment at a viscosity ratio of $μ_{o}/μ_{w}{=}2000$, discussing the physical significance of each modelling step. Linear stability analysis is used to characterize the early-stage instability of the displacement. Subsequent numerical simulations show that, for the simulated finger scales to match the experiment, the most unstable wavelength at onset must be several times smaller than the desired finger width---so that, after accounting for shielding and merging in the nonlinear regime, the fingers remain thin. Small-scale channelling effects are also required to disrupt the trailing stable region commonly observed in simulations of viscous fingering in nearly homogeneous media. Finally, we show that including a weakly oil-wet capillary pressure function enables our model to capture the bypassed oil observed in the experiment. △ Less

Submitted 18 August, 2026; originally announced August 2026.

MSC Class: 76Dxx; 76S05; 76Exx

Half a qubit: an algebraic fractionalization

Authors: Po-Yao Chang

Abstract: Fractionalizing a quantum two-level system is usually associated with encodings based on pairs of Majorana fermions---an operational fractionalization. We show an alternative algebraic fractionalization by embedding Székely's classical ``half-coin'' into a non-Hermitian Krein space. The coefficients of $(q+pz)^{1/2}$ define a signed sequence and a normalized, non-Hermitian biorthogonal operator de… ▽ More Fractionalizing a quantum two-level system is usually associated with encodings based on pairs of Majorana fermions---an operational fractionalization. We show an alternative algebraic fractionalization by embedding Székely's classical ``half-coin'' into a non-Hermitian Krein space. The coefficients of $(q+pz)^{1/2}$ define a signed sequence and a normalized, non-Hermitian biorthogonal operator describing a biorthogonal half-qubit. We prove that two such objects fuse into an arbitrary pure qubit through the signed Vandermonde convolution that the collective $N\ge2$ vectors are null in Krein space. $L_1$ norm of the half-qubit follows in closed form, $\lVert p\rVert_1 = 2\sqrt{q}-\sqrt{q-p}$. Its $L_1$ norm increases monotonically with the bias and attains its supremum $\sqrt{2}$ precisely at the unbiased point $p=q=1/2$. Interestingly, we identify two structural results as follows. First, number parity and the $η$-metric generate a distinguished commuting $\mathbb Z_2\times\mathbb Z_2$ subgroup. Second, we find the $η$-metric obstructs any local $η$-self-adjoint partner of the parity, so a half-qubit carries a $\mathbb{Z}_2$ observable but no local $SU(2)$. The full Pauli algebra emerges only upon fusion. We then show that the construction survives truncation of the Fock basis: the fused qubit is exact at every cutoff, and the Vandermonde cancellation is visible in sign-weighted photon-number statistics, and can be tested using existing cavity and trapped-ion state-synthesis methods. Finally, we generalize this algebraic fractionalization to a $1/n$-qubit, which can be achieved by replacing the square root with an $n$th root. △ Less

Submitted 17 August, 2026; originally announced August 2026.

Comments: Comments are welcome

Electrothermal control of spin-reorientation transition in Co/Fe_3GaTe_2 heterostructures

Authors: Po-Wei Chen, Ming-Hsien Hsu, Cheng-Ying Hsiao, Ming-Yang Ho, Masahiro Haze, Yan-Ru Chu, Yu-Cheng Shao, Po-Chun Chang, Chen-Yu Ou, Ruei Chen, Ko-Fan Chen, Chung-Ting Ke, Chao-Hung Du, Yukio Hasegawa, Wen-Chin Lin, .

Abstract: Electrical control of magnetic anisotropy in van der Waals (vdWs) magnets is a key step toward reconfigurable two-dimensional spintronics, yet how a conventional metallic ferromagnet competes with a van der Waals magnet across a direct interface has remained largely unexplored. Here we demonstrate reversible thermal and electrothermal control of a spin-reorientation transition in Co/Fe_3GaTe_2 (FG… ▽ More Electrical control of magnetic anisotropy in van der Waals (vdWs) magnets is a key step toward reconfigurable two-dimensional spintronics, yet how a conventional metallic ferromagnet competes with a van der Waals magnet across a direct interface has remained largely unexplored. Here we demonstrate reversible thermal and electrothermal control of a spin-reorientation transition in Co/Fe_3GaTe_2 (FGaT) heterostructures. As Joule heating weakens the FGaT anisotropy, the heterostructure switches from an out-of-plane- to an in-plane-dominated state at a reorientation temperature of approximately 311 K, well below the Curie temperature, consistent with an exchange-mediated anisotropy competition between the Co overlayer and FGaT. An electrically driven device shows a closely matching loop evolution within an 80-100 mW power window, reversibly over five measurement cycles, consistent with an electrothermal origin. In a Co-free FGaT device, Kerr microscopy traces the switching to a power-tunable domain nucleation barrier and demonstrates power-thresholded, field-assisted magnetization reversal at a threshold near 15 mW. These results demonstrate electrothermal anisotropy competition as a route to heat-assisted and device-level control of vdWs magnetism. △ Less

Submitted 17 July, 2026; originally announced July 2026.

Pauli Spectrum and Stabilizer Rényi Entropy in Gapless Symmetry-Protected Topological Phases

Authors: Ying-Lin Li, Po-Yao Chang

Abstract: Quantum entanglement is widely used as a diagnostic of topological phases of matter. Beyond entanglement, non-stabilizerness captures a distinct aspect of quantum many-body states by quantifying their distance from the manifold of stabilizer states. In this work, we study the stabilizer Rényi entropy in symmetry protected topological (SPT) phases, including both gapped SPT, non-intrinsically gaple… ▽ More Quantum entanglement is widely used as a diagnostic of topological phases of matter. Beyond entanglement, non-stabilizerness captures a distinct aspect of quantum many-body states by quantifying their distance from the manifold of stabilizer states. In this work, we study the stabilizer Rényi entropy in symmetry protected topological (SPT) phases, including both gapped SPT, non-intrinsically gapless SPT, and intrinsically gapless SPT phases. Under symmetry preserving perturbations, we find numerically that the stabilizer Rényi entropy exhibits an extremum near the phase transition. However, the stabilizer Rényi entropy alone cannot distinguish different SPT phases. In contrast, the Pauli spectrum reveals a characteristic crossing structure at the transition point. This crossing reflects the exchange of dominant Pauli-string correlations associated with the non-local string order parameters of the two topological distinct phases. For gapped SPT and non-intrinsically gapless SPT phases, the crossing structure can be understood from a local-unitary duality that maps the Pauli spectrum between the two phases. For intrinsically gapless SPT phases, such a local-unitary mapping is absent. Instead, we find that the Pauli spectrum mapping is generated by a non-invertible duality transformation. These results show that although the stabilizer Rényi entropy provides only a coarse diagnostic of phase transitions, the Pauli spectrum contains finer information about the exchange of string order sectors. Our findings demonstrate that quantum magic offers a complementary perspective for characterizing both gapped and gapless SPT phases. △ Less

Submitted 4 July, 2026; originally announced July 2026.

Comments: 10 pages, 4 figures

Emergent de Sitter Space and Non-Unitary Tensor Networks from Non-Hermitian Quantum Criticality

Authors: Kuang-Hung Chou, Po-Yao Chang

Abstract: Extending the holographic principle to de Sitter (dS) spacetimes remains one of the most vital open frontiers in quantum gravity, where a microscopic, bottom-up tensor-network framework that relates boundary quantum data to emergent de Sitter spacetime is still lacking. In this work, we first show the emergence of de Sitter spacetime from boundary entanglement by formulating a non-unitary continuo… ▽ More Extending the holographic principle to de Sitter (dS) spacetimes remains one of the most vital open frontiers in quantum gravity, where a microscopic, bottom-up tensor-network framework that relates boundary quantum data to emergent de Sitter spacetime is still lacking. In this work, we first show the emergence of de Sitter spacetime from boundary entanglement by formulating a non-unitary continuous multi-scale entanglement renormalization ansatz (cMERA) for a concrete non-Hermitian critical fermion chain. Within this emergent spacetime, we analyze the associated geodesics and show that they act as extremal Ryu-Takayanagi (RT) surfaces undergoing a smooth timelike-to-null transition. Remarkably, we demonstrate that this continuum trajectory dictates a distinct tensor-network architecture in which the bond-counting contribution naturally truncates at the discrete timelike-to-null transition toward the deep infrared. In the resulting architecture, the null ray along the horizon is represented by zero-cost links, since the associated cut severs no tensor legs. This network structure successfully reproduces the logarithmic scaling of non-unitary critical entanglement entropy, offering a bond-counting picture for the de Sitter RT formula. Our results provide the long-sought dS/(c)MERA correspondence at the level of both emergent spacetime and discrete holographic entanglement. △ Less

Submitted 16 June, 2026; originally announced June 2026.

Comments: 13 pages, 4 figures

Magnetic structure in the two-dimensional van der Waals ferromagnet Fe$_3$GaTe$_2$

Authors: Po-Chun Chang, Sabreen Hammouda, Yung-Hsiang Tung, Yishui Zhou, Iurii Kibalin, Bachir Ouladdiaf, Chao-Hung Du, Yixi Su

Abstract: High-quality single crystals of the two-dimensional van der Waals ferromagnet Fe$_3$GaTe$_2$ (FGaT) were successfully grown using the chemical vapour transport method, which effectively reduced surface impurities compared with conventional self-flux growth. Structural and magnetic characterizations were performed using single-crystal X-ray and neutron diffraction. The results confirm that FGaT cry… ▽ More High-quality single crystals of the two-dimensional van der Waals ferromagnet Fe$_3$GaTe$_2$ (FGaT) were successfully grown using the chemical vapour transport method, which effectively reduced surface impurities compared with conventional self-flux growth. Structural and magnetic characterizations were performed using single-crystal X-ray and neutron diffraction. The results confirm that FGaT crystallizes in the hexagonal $P6_3/mmc$ structure, with Fe occupying two inequivalent sites (Fe$^{i}$ and Fe$^{ii}$), where the magnetic moment of Fe$^{i}$ [1.9(2) $μ_B$] is larger than that of Fe$^{ii}$ [1.4(6) $μ_B$]. The magnetic easy axis is oriented along the $c$ axis and the Curie temperature ($T_C$) is approximately 355-360 K. Compared with Fe$_3$GeTe$_2$ (FGT), FGaT exhibits a slightly expanded $a$ axis and a contracted $c$ axis, resulting in a reduction in the Fe$^{i}$-Fe$^{ii}$ interatomic distance along the $c$ axis. This pronounced contraction could strengthen the Fe$-$Fe exchange interaction, which is believed to be the key factor responsible for the significantly higher $T_C$ in FGaT relative to FGT. △ Less

Submitted 10 May, 2026; originally announced May 2026.

Comments: J. Appl. Cryst. (2026)

From Embeddings to Dyson Series: Transformer Mechanics as Non-Hermitian Operator Theory

Authors: Po-Hao Chang

Abstract: Transformer architectures are typically described in algorithmic and statistical terms, leaving their internal mechanics without a familiar structural language for researchers trained in physical theories. To bridge this gap, we develop a complementary operator-theoretic framework that recasts their mechanics in a language familiar to many-body physics. Beginning from the token as a discrete index… ▽ More Transformer architectures are typically described in algorithmic and statistical terms, leaving their internal mechanics without a familiar structural language for researchers trained in physical theories. To bridge this gap, we develop a complementary operator-theoretic framework that recasts their mechanics in a language familiar to many-body physics. Beginning from the token as a discrete index without intrinsic geometry, we show that embedding corresponds to a basis transformation into a continuous representation space. Once such a reference basis is established, self-attention naturally assumes the role of a non-Hermitian interaction operator, and network depth implements an ordered composition of these interactions. Within this formulation, several empirical properties of deep Transformers -- including stability at large depth, representational saturation, and the effectiveness of multi-head decomposition -- find natural structural interpretations as consequences of regulated operator composition. Together, channel factorization and normalization emerge as organizing structural logic rather than isolated architectural choices. This perspective does not rely on post-hoc analogy, but follows a constructive path where each parallel arises from the preceding structural step. By recasting Transformer mechanics in operator language, the framework lowers the conceptual barrier between deep learning and many-body physics through shared mathematical structure, making tools and intuitions from each domain more readily legible to the other. △ Less

Submitted 16 March, 2026; v1 submitted 11 March, 2026; originally announced March 2026.

Comments: 8 pages, 3 figures

Is altermagnetism in vanadium oxychalcogenides a lost cause?

Authors: Bishal Thapa, Po-Hao Chang, Kirill Belashchenko, Igor I. Mazin

Abstract: Vanadium-based oxychalcogenide compounds with the inverse Lieb-lattice (ILL) structural pattern have recently been proposed as candidate altermagnets (AM). However, early studies postulated ferromagnetic interlayer coupling, a critical requirement for preserving the bulk AM state. Here we present a systematic survey of the complete AV2Q2O family (A = K, Rb, Cs; Q = S, Se, Te) in terms of their mag… ▽ More Vanadium-based oxychalcogenide compounds with the inverse Lieb-lattice (ILL) structural pattern have recently been proposed as candidate altermagnets (AM). However, early studies postulated ferromagnetic interlayer coupling, a critical requirement for preserving the bulk AM state. Here we present a systematic survey of the complete AV2Q2O family (A = K, Rb, Cs; Q = S, Se, Te) in terms of their magnetic ordering and interlayer coupling. While intralayer exchange interaction favors AM ordering in a single ILL layer across the entire family, the relatively weak interlayer coupling in most cases favors Kramers-degenerate antiferromagnetic order with a doubled magnetic unit cell. This means that most stoichiometric bulk materials, including the previously proposed candidate KV2Se2O, are not altermagnetic, with CsV2Te2O being the only exception. Using hole doping to simulate alkali vacancies, we show that realistic deviations from stoichiometry do not change the magnetic ground state in these compounds. △ Less

Submitted 20 February, 2026; originally announced February 2026.

Comments: 5 pages, 4 figures

Anisotropy, frustration and saddle point in the twisted Kagome antiferromagnet ErPdPb

Authors: Resham Babu Regmi, Sk Jamaluddin, Y. Lee, Hari Bhandari, Po-Hao Chang, Peter E. Siegfried, Abhijeet Nayak, Mohamed El Gazzah, Bence G. Márkus, Anna Nyáry, Zachary T. Messegee, Miya P. Zhao, Xiaoyan Tan, László Forró, Liqin Ke, Igor I. Mazin, Nirmal J. Ghimire

Abstract: The kagome lattice, with its inherent geometric frustration, provides a rich platform for exploring intriguing magnetic phenomena and topological electronic structures. In reduced-symmetry structures, such as twisted kagome systems involving rare earth elements, additional anisotropy can arise, enabling intriguing properties including spin-ice states, magnetocaloric effects, noncollinear magnetic… ▽ More The kagome lattice, with its inherent geometric frustration, provides a rich platform for exploring intriguing magnetic phenomena and topological electronic structures. In reduced-symmetry structures, such as twisted kagome systems involving rare earth elements, additional anisotropy can arise, enabling intriguing properties including spin-ice states, magnetocaloric effects, noncollinear magnetic ordering, and anomalous Hall effect. Here, we report the synthesis of single crystals of ErPdPb, which features a twisted kagome lattice net of Er atoms within the hexagonal ZrNiAl-type structure, and we investigate its magnetic, electronic, and thermal properties. The material exhibits antiferromagnetic ordering below 2.2 K, consistently observed in magnetic, transport, and heat capacity measurements. Magnetization measurements reveal 1/3 metamagnetic steps along the c-axis below the Néel temperature, suggesting an Ising-spin-like state on the twisted kagome lattice. A pronounced anisotropy between in-plane and out-of-plane resistivity is observed throughout the temperature range of 1.8-300 K, and the compound exhibits a significant frustration index of 13.6 (12.7) along the c-axis (ab-plane). Heat capacity measurements show a broad hump at 2.2 K, with an additional increase below 0.5 K. The anisotropic magnetic properties are further explored through density functional theory (DFT) calculations, which suggest strong easy-axis anisotropy, consistent with experimental magnetic measurements and crystal-field model expectations, and quasi-one-dimensional bands and a spin-split saddle point at the zone center. △ Less

Submitted 9 February, 2026; originally announced February 2026.

Dimer-driven multiple reentrant localization with composite potential

Authors: Pei-Jie Chang, Dong Ruan, Gui-Lu Long

Abstract: Recent studies have revealed reentrant localization transitions in quasi-periodic one-dimensional lattices, where the competition between dimerized hopping and staggered disorder plays a central role. Yet the extent to which such reentrant localization persists under more general conditions, such as additional periodic potentials, modified quasi-periodic modulations remains unclear. Here we invest… ▽ More Recent studies have revealed reentrant localization transitions in quasi-periodic one-dimensional lattices, where the competition between dimerized hopping and staggered disorder plays a central role. Yet the extent to which such reentrant localization persists under more general conditions, such as additional periodic potentials, modified quasi-periodic modulations remains unclear. Here we investigate localization phenomena in a one-dimensional lattice subject to a periodic potential and an additional quasi-periodic modulation. Using both eigenstate-based indicators and experimentally accessible dynamical observables, we identify robust reentrant, or multiple, localization transitions. We show that these transitions are uniquely stabilized by the dimer structure of the unit cell, where the competition between the onsite periodic potential and the quasi-periodic modulation becomes most pronounced. By systematically varying the periodicity parameter $α$ and the quasi-periodic frequency $β$, we find that the robust multiple reentrant localization behavior disappears for any deviation from the dimer configuration, confirming its essential role. Our results suggest that the interplay between these competing factors drives the multiple reentrant localization transitions. △ Less

Submitted 28 September, 2025; v1 submitted 12 September, 2025; originally announced September 2025.

PT symmetry-enriched non-unitary criticality

Authors: Kuang-Hung Chou, Xue-Jia Yu, Po-Yao Chang

Abstract: The interplay between topology and quantum criticality has given rise to the notion of symmetry-enriched criticality, which has attracted considerable attention in recent years. In this Letter, we demonstrate that parity time (PT) symmetry enriches non-Hermitian critical points, establishing a topologically distinct class of non unitary criticality. Through the analytic solution of PT symmetric fr… ▽ More The interplay between topology and quantum criticality has given rise to the notion of symmetry-enriched criticality, which has attracted considerable attention in recent years. In this Letter, we demonstrate that parity time (PT) symmetry enriches non-Hermitian critical points, establishing a topologically distinct class of non unitary criticality. Through the analytic solution of PT symmetric free fermion models, we reveal a new family of critical points that are topologically nontrivial and host robust edge modes. Crucially, these points cannot be adiabatically connected to trivial ones without breaking PT symmetry or crossing a multicritical point, and distinct from Hermitian counterparts. We further show that, at these PT symmetry enriched critical points, conformal scaling of the entanglement entropy necessarily comes with a quantized imaginary subleading term, whose quantization is set by the number of boundary modes in the reduced density matrix. This term is robust against PT symmetric disorder and interactions, and admits an interpretation as the Affleck Ludwig g factor associated with the boundary states. These phenomena are shown to arise from a generalized mass inversion unique to non-Hermitian criticality. △ Less

Submitted 14 May, 2026; v1 submitted 11 September, 2025; originally announced September 2025.

Comments: 45 pages, 11 figures

Magic Entropy in Hybrid Spin-Boson Systems

Authors: Samuel Crew, Ying-Lin Li, Heng-Hsi Li, Po-Yao Chang

Abstract: We introduce entropic measures to quantify non-classical resource in hybrid spin-boson systems. We discuss the stabilizer Rényi entropy in the framework of phase space quantisation and define an analogous hybrid magic entropy and a mutual magic entropy that capture the distribution of quantum magic across spin and bosonic subsystems. We use these entropic measures to demonstrate two key phenomena:… ▽ More We introduce entropic measures to quantify non-classical resource in hybrid spin-boson systems. We discuss the stabilizer Rényi entropy in the framework of phase space quantisation and define an analogous hybrid magic entropy and a mutual magic entropy that capture the distribution of quantum magic across spin and bosonic subsystems. We use these entropic measures to demonstrate two key phenomena: the detection of the superradiant phase transition in the Dicke model and the dynamics of magic in the Jaynes-Cummings model following a quench. We develop a Monte Carlo numerical scheme to enable practical computation in many-body examples. △ Less

Submitted 8 August, 2025; originally announced August 2025.

Comments: 15 pages, 3 figures

Inverse Lieb Materials: Altermagnetism and More

Authors: Po-Hao Chang, Igor I. Mazin, Kirill D. Belashchenko

Abstract: The Lieb lattice, originally proposed for cuprate superconductors, has gained new attention in the emerging field of altermagnetism as a minimal analytical model for the latter. While initially the so-called inverse Lieb lattice (ILL) was deemed only a theoretical model, recently several real materials with this crystallographic motif have been found. The unique geometry of ILL can accommodate com… ▽ More The Lieb lattice, originally proposed for cuprate superconductors, has gained new attention in the emerging field of altermagnetism as a minimal analytical model for the latter. While initially the so-called inverse Lieb lattice (ILL) was deemed only a theoretical model, recently several real materials with this crystallographic motif have been found. The unique geometry of ILL can accommodate complex magnetic orderings arising from competing exchange interactions and geometric frustration, offering great tunability for magnetic properties. In this work, we provide comprehensive insights into magnetic phases in ILL materials and establish guidelines for efficient identification of altermagnetic materials within this family. We begin by constructing phase diagrams using a simple Heisenberg model to elucidate the fundamental mechanisms underlying altermagnetism and other complex magnetic phases observed experimentally. To bridge theory with experiment, we systematically investigate a series of existing ILL compounds using density functional theory (DFT) calculations to determine their magnetic ground states. Our computational results are in good agreement with experimental observations. Importantly, we identify a trend linking magnetic ordering to the $d$-shell filling of transition metal ions, with $d^{2-3}$ and $d^{5}$ configurations showing propensity for altermagnetic behavior. Additionally, we identify a promising metallic compound Sr$_{2}$CrO$_{2}$Cr$_{2}$OAs$_{2}$ as an altermagnet that is highly anisotropic in its $J_2$ exchange couplings with large Néel temperature ($\sim 600$ K). Using exchange coupling parameters extracted from DFT calculations, we compute the magnon spectra for altermagnetic systems. As expected, chiral splittings in the magnon dispersion are directly correlated with anisotropy between crystallographically inequivalent $J_{2}$ exchange interactions. △ Less

Submitted 8 August, 2025; v1 submitted 6 August, 2025; originally announced August 2025.

Comments: 11 pages, 8 figures

Biorthogonal quench dynamics of entanglement and quantum geometry in PT-symmetric non-Hermitian systems

Authors: Hsueh-Hao Lu, Po-Yao Chang

Abstract: We explore the quench dynamics of PT-symmetric non-Hermitian systems by utilizing the biorthogonal formalism. We analyze quench dynamics of observable quantities, the quantum geometric tensor, and various entanglement quantities, including the entanglement entropy, the SVD entropy, and the Tu-Tzeng-Chang entropy. Our results show that a sudden quench into a PT-broken phase generally leads to expon… ▽ More We explore the quench dynamics of PT-symmetric non-Hermitian systems by utilizing the biorthogonal formalism. We analyze quench dynamics of observable quantities, the quantum geometric tensor, and various entanglement quantities, including the entanglement entropy, the SVD entropy, and the Tu-Tzeng-Chang entropy. Our results show that a sudden quench into a PT-broken phase generally leads to exponential growth in these quantities, driven by the biorthogonal density matrix's non-positivity. In contrast to generic interacting systems, we observe a surprising linear decay in the TTC entropy for non-interacting fermionic systems. This finding originates from the approximate spectral symmetry of the biorthogonal reduced density matrix, and we confirm our findings using the Yang-Lee and non-Hermitian XXZ models. △ Less

Submitted 27 July, 2025; originally announced July 2025.

Comments: 12 pages, 6 figures, 1 table. Based on Hsueh-Hao Lu's Master's thesis (National Tsing Hua University Library) and preliminary results presented by Hsueh-Hao Lu at APS March Meeting 2025

Modeling of a twisted-Kagome HoAgGe spin ice using Reduced-Configuration-Space Search and Density Functional Theory

Authors: Gunnar F. Schwertfeger, Po-Hao Chang, Predrag Nikolic, Igor I. Mazin

Abstract: The Kagome lattice is a 2D network of corner sharing triangles found in several rare earth materials resulting in a complicated and often frustrated magnetic system. In the last decades, modifications of the motif, such as breathing Kagome, asymmetric Kagome, and twisted Kagome were brought into the limelight. In particular, the latter has lower symmetry than the original Kagome and thus allows im… ▽ More The Kagome lattice is a 2D network of corner sharing triangles found in several rare earth materials resulting in a complicated and often frustrated magnetic system. In the last decades, modifications of the motif, such as breathing Kagome, asymmetric Kagome, and twisted Kagome were brought into the limelight. In particular, the latter has lower symmetry than the original Kagome and thus allows implementations of an "Ising-local" Hamiltonian, leading to a 2D spin ice. One such material implementation, HoAgGe, was recently reported to have an exceptionally rich phase diagram and is a strongly frustrated 2D spin-ice material with a twisted-Kagome geometry. In the presence of an external magnetic field the compound exhibits step-like magnetization plateaus at simple fractions of the saturation magnetization. It is believed that this phenomenon results from strong single-site anisotropy, which in HoAgGe was found to be in-plane and along a high-symmetry direction. Previous Monte Carlo simulations with empirical exchange parameters explain some, but not all experimental observations. In this work we present (a) first-principle calculations of the crucial model parameters and (b) direct energy minimization via a Reduced-Configuration-Space search, as well as Monte-Carlo simulations of the field-dependent phase diagram. We find that for HoAgGe the calculated exchange parameters are very different from the earlier suggested empirical ones, and describe the phase diagram much more accurately. This is likely because the first-principles parameters are, in addition to geometrically, also parametrically frustrated. △ Less

Submitted 9 March, 2026; v1 submitted 5 July, 2025; originally announced July 2025.

Journal ref: Phys. Rev. B112, 214411 (2025)

Nodal-line semimetals and their variance

Authors: Po-Yao Chang

Abstract: Topological nodal-line semimetals (NLSMs) are a new family of topological materials characterized by electronic band crossings that form lines in the Brillouin zone. These NLSMs host exotic nodal-line structures and exhibit distinct features such as drumhead surface states and unique electromagnetic responses. This review classifies various NLSM types based on their nodal structures and protecting… ▽ More Topological nodal-line semimetals (NLSMs) are a new family of topological materials characterized by electronic band crossings that form lines in the Brillouin zone. These NLSMs host exotic nodal-line structures and exhibit distinct features such as drumhead surface states and unique electromagnetic responses. This review classifies various NLSM types based on their nodal structures and protecting symmetries, highlighting that these nodal-line structures can form links, knots, and chains. We discuss their characteristic electromagnetic responses, including Landau level spectroscopy, optical conductivity, and permittivity. Furthermore, the strong correlation effects in these NLSMs modify their semimetallic phases and lead to novel quantum phases where magnetism and superconductivity intertwine. △ Less

Submitted 6 October, 2025; v1 submitted 3 July, 2025; originally announced July 2025.

Comments: resubmit a review to the special issue "Topological Quantum Materials" in Materials Today Quantum. Comments are welcome

Journal ref: Materials Today Quantum Volume 8, December 2025, 100057

Doping-induced Spin Reorientation in Kagome Magnet TmMn6Sn6

Authors: Mohamed El Gazzah, Po-Hao Chang, Y. Lee, Hari Bhandari, Resham Regmi, Xiuqan Zhou, John F. Mitchell, Liqin Ke, Igor I. Mazin, Nirmal J. Ghimire

Abstract: The kagome-lattice compounds RMn6Sn6 (R is a rare earth element), where the Mn atoms form a kagome net in the basal plane, are currently attracting a great deal of attention as they have been shown to host complex magnetic textures and electronic topological states strongly sensitive to the choice of the R atom. Among the magnetic R atoms, TmMn6Sn6 orders with the easy-plane magnetization forming… ▽ More The kagome-lattice compounds RMn6Sn6 (R is a rare earth element), where the Mn atoms form a kagome net in the basal plane, are currently attracting a great deal of attention as they have been shown to host complex magnetic textures and electronic topological states strongly sensitive to the choice of the R atom. Among the magnetic R atoms, TmMn6Sn6 orders with the easy-plane magnetization forming a complex magnetic spiral along the c-axis. Previous neutron studies, carried on polycrystalline, samples found that Ga doping changes the magnetic anisotropy from easy-plane to easy-axis. Here we present magnetic and magnetotransport measurements on a single crystal and first principles calculations in the doping series of TmMn6Sn6-xGax. We find that the magnetic properties are highly sensitive even to a small concentration of Ga. With minimal Ga substitution, the easy-plane anisotropy is maintained, which gradually changes to the easy-axis anisotropy with increasing Ga. We discuss these observations with respect to the effect of Ga doping on magnetocrystalline anisotropy and Tm crystal field △ Less

Submitted 5 May, 2025; originally announced May 2025.

Impurity-induced non-unitary criticality

Authors: Heng-Hsi Li, Kuang-Hung Chou, Xueda Wen, Po-Yao Chang

Abstract: Quantum impurities give rise to rich physical phenomena, with some exhibiting critical behavior described by conformal field theories (CFTs) in the low-energy limit. In parallel, party-time ($\mathcal{PT}$) symmetric non-Hermitian systems host exceptional points (EPs) at criticality, leading to exotic features governed by non-unitary CFTs. Here, we establish a connection between non-Hermitian impu… ▽ More Quantum impurities give rise to rich physical phenomena, with some exhibiting critical behavior described by conformal field theories (CFTs) in the low-energy limit. In parallel, party-time ($\mathcal{PT}$) symmetric non-Hermitian systems host exceptional points (EPs) at criticality, leading to exotic features governed by non-unitary CFTs. Here, we establish a connection between non-Hermitian impurities and CFTs by demonstrating that the critical properties of a (1+1)-dimensional free-fermion chain with central charge $c=1$ can be drastically altered by the presence of a local non-Hermitian impurity. Through a systematic analysis of entanglement/Rényi entropy, the finite-size scaling of the many-body spectrum, and fidelity susceptibility, we identify that this impurity-induced non-Hermitian criticality is characterized by a non-unitary CFT with central charge $c=-2$. Furthermore, we find that these non-unitary critical properties exhibit strong sensitivity to boundary conditions. △ Less

Submitted 17 February, 2025; originally announced February 2025.

Comments: 9 pages, 6 figures

Investigation of reentrant localization transition in one-dimensional quasi-periodic lattice with long-range hopping

Authors: Pei-Jie Chang, Qi-Bo Zeng, Jinghui Pi, Dong Ruan, Gui-Lu Long

Abstract: Reentrant localization has recently been observed in systems with quasi-periodic nearest-neighbor hopping, where the interplay between dimerized hopping and staggered disorder is identified as the driving mechanism. However, the robustness of reentrant localization in the presence of long-range hopping remains an open question. In this work, we investigate the phenomenon of reentrant localization… ▽ More Reentrant localization has recently been observed in systems with quasi-periodic nearest-neighbor hopping, where the interplay between dimerized hopping and staggered disorder is identified as the driving mechanism. However, the robustness of reentrant localization in the presence of long-range hopping remains an open question. In this work, we investigate the phenomenon of reentrant localization in systems incorporating long-range hopping. Our results reveal that long-range hopping induces reentrant localization regardless of whether the disorder is staggered or uniform. We demonstrate that long-range hopping does not inherently disrupt localization; instead, under specific conditions, it facilitates the emergence of reentrant localization. Furthermore, by analyzing critical exponents, we show that the inclusion of long-range hopping modifies the critical behavior, leading to transitions that belong to distinct universality classes. △ Less

Submitted 18 December, 2024; originally announced December 2024.

Ground-state magnetic structures of topological kagome metals RV$_6$Sn$_6$ (R = Tb, Dy, Ho, Er)

Authors: Yishui Zhou, Min-Kai Lee, Sabreen Hammouda, Sheetal Devi, Shin-Ichiro Yano, Romain Sibille, Oksana Zaharko, Wolfgang Schmidt, Karin Schmalzl, Ketty Beauvois, Eric Ressouche, Po-Chun Chang, Chun-Hao Huang, Lieh-Jeng Chang, Thomas Brückel, Yixi Su

Abstract: Magnetic kagome metals have attracted tremendous research interests recently, because they represent an ideal playground for exploring the fascinating interplay between their intrinsically inherited topologically non-trivial electron band structures, magnetism and electronic correlation effects, and the resultant novel electronic/magnetic states and emergent excitations. In this work, we report a… ▽ More Magnetic kagome metals have attracted tremendous research interests recently, because they represent an ideal playground for exploring the fascinating interplay between their intrinsically inherited topologically non-trivial electron band structures, magnetism and electronic correlation effects, and the resultant novel electronic/magnetic states and emergent excitations. In this work, we report a comprehensive single-crystal neutron diffraction investigation of the ground-state magnetic structures of the recently discovered V-based topological kagome metals RV$_6$Sn$_6$ (R = Tb, Dy, Ho, Er). Furthermore, the sample synthesis details and our systematic studies of crystal structure, low-temperature magnetic and thermodynamic properties of these compounds via various in-house characterization techniques are also reported. It can be revealed that RV$_6$Sn$_6$ (R = Tb, Dy, Ho) have a collinear ferromagnetic order in the ground state, with the ordered magnetic moment aligned along the c axis for R = Tb, Ho, while approximately 20${^\circ}$ tilted off from the c axis for R = Dy. In contrast, ErV$_6$Sn$_6$ shows an A-type antiferromagnetic structure with a magnetic propagation vector k = (0, 0, 0.5), and with the ordered magnetic moment aligned in the ab plane. A comparison of the low-temperature magnetic structures for both the extensively investigated topological kagome metal series of RV$_6$Sn$_6$ and RMn$_6$Sn$_6$ is given in details. This allows to gain new insights into the complex magnetic interactions, diverse single-ion magnetic anisotropies and spin dynamics in these compounds. The reported ground-state magnetic structures in RV$_6$Sn$_6$ (R = Tb, Dy, Ho, Er) can pave the way for further explorations of the possible interplay between magnetism and topologically non-trivial electron band structures in the magnetically ordered phase regime. △ Less

Submitted 21 November, 2024; originally announced November 2024.

Comments: To appear in Phys. Rev. Research

Journal ref: Phys. Rev. Research 6, 043291 (2024)

Three-dimensional nature of anomalous Hall conductivity in YMn6Sn6-xGax, x ~ 0.55

Authors: Hari Bhandari, Zhenhua Ning, Po-Hao Chang, Peter E. Siegfried, Resham B. Regmi, Mohamed El. Gazzah, Albert V. Davydov, Allen G. Oliver, Liqin Ke, Igor I. Mazin, Nirmal J. Ghimire

Abstract: The unique connectivity of kagome lattices gives rise to topological properties, such as flat bands and Dirac cones. When combined with ferromagnetism and a chemical potential near the 2D Dirac points, this structure offers the potential to realize the highly sought-after topological Chern magnetotransport. Recently, there was considerable excitement surrounding this possibility in the ferrimagnet… ▽ More The unique connectivity of kagome lattices gives rise to topological properties, such as flat bands and Dirac cones. When combined with ferromagnetism and a chemical potential near the 2D Dirac points, this structure offers the potential to realize the highly sought-after topological Chern magnetotransport. Recently, there was considerable excitement surrounding this possibility in the ferrimagnetic kagome metal TbMn$_\mathbf{6}$Sn$_\mathbf{6}$. However, density functional theory (DFT) calculations reveal that the 2D Chern gap lies well above the Fermi energy, challenging its relevance in the observed anomalous Hall conductivity. Here, we investigate YMn$_\mathbf{6}$Sn$_\mathbf{5.45}$Ga$_\mathbf{0.55}$, a compound with similar crystallographic, magnetic, and electronic properties to TbMn$_\mathbf{6}$Sn$_\mathbf{6}$. Our findings show that the intrinsic anomalous Hall conductivity in this material, while comparable in magnitude to that in TbMn$_\mathbf{6}$Sn$_\mathbf{6}$, is fully three-dimensional, thus providing experimental evidence that Hall conductivity in this class of materials does not originate from 2D Chern gaps. Additionally, we confirm that the newly proposed empirical scaling relation for extrinsic Hall conductivity is universally governed by spin fluctuations. △ Less

Submitted 18 November, 2024; originally announced November 2024.

Chaotic-Integrable Transition for Disordered Orbital Hatsugai-Kohmoto Model

Authors: Ying-Lin Li, Chen-Te Ma, Po-Yao Chang

Abstract: We have drawn connections between the Sachdev-Ye-Kitaev model and the multi-orbit Hatsugai-Kohmoto model, emphasizing their similarities and differences regarding chaotic behaviors. The features of the spectral form factor, such as the dip-ramp-plateau structure and the adjacent gap ratio, indicate chaos in the disordered orbital Hatsugai-Kohmoto model. One significant conclusion is that the plate… ▽ More We have drawn connections between the Sachdev-Ye-Kitaev model and the multi-orbit Hatsugai-Kohmoto model, emphasizing their similarities and differences regarding chaotic behaviors. The features of the spectral form factor, such as the dip-ramp-plateau structure and the adjacent gap ratio, indicate chaos in the disordered orbital Hatsugai-Kohmoto model. One significant conclusion is that the plateau value of the out-of-time-order correlator, whether in the Hatsugai-Kohmoto model, Sachdev-Ye-Kitaev model with two- or four-body interactions, or a disorder-free Sachdev-Ye-Kitaev model, does not effectively differentiate between integrable and chaotic phases in many-body systems. This observation suggests a limitation in using out-of-time-order correlator plateau values as a diagnostic tool for chaos. Our exploration of these ideas provides a deeper understanding of how chaos arises in non-Fermi liquid systems and the tools we use to study it. It opens the door to further questions, particularly about whether there are more effective ways to distinguish between chaotic and integrable phases in these complex systems. △ Less

Submitted 12 May, 2025; v1 submitted 13 November, 2024; originally announced November 2024.

Comments: 19 pages, 4 figures, 1 table, minor changes, reference added

Towards a Topological Proof of the Strong Subadditivity

Authors: Chih-Yu Lo, Po-Yao Chang

Abstract: Topological entanglement entropy (TEE) represents an intrinsic contribution to the entanglement entropy (EE) in topologically ordered systems. In quantum information theory, strong subadditivity (SSA) is a fundamental property of EE, reflecting the non-negativity of conditional mutual information. TEE was originally believed to be a universal correction to the area law of EE, suggesting that its S… ▽ More Topological entanglement entropy (TEE) represents an intrinsic contribution to the entanglement entropy (EE) in topologically ordered systems. In quantum information theory, strong subadditivity (SSA) is a fundamental property of EE, reflecting the non-negativity of conditional mutual information. TEE was originally believed to be a universal correction to the area law of EE, suggesting that its SSA would directly follow from the SSA of EE. However, due to spurious contributions, the correction term is not universal; consequently, the value predicted by topological quantum field theory (TQFT) provides only a lower bound. In this work, we present a topological analysis showing that the SSA for TEE is equivalent to a specific inequality within the TQFT framework. We further verify that this inequality holds for all known unitary modular tensor categories (UMTCs) up to rank 11, supporting the conjecture that SSA holds universally in the TQFT framework. Conversely, assuming the validity of the SSA condition, the inequality can be interpreted as a consistency criterion for candidate UMTCs. △ Less

Submitted 22 July, 2025; v1 submitted 7 November, 2024; originally announced November 2024.

Tunable topological transitions in the frustrated magnet HoAgGe

Authors: Hari Bhandari, Po-Hao Chang, Resham Babu Regmi, Bence Gábor Márkus, László Forró, J. F. Mitchell, I. I. Mazin, Nirmal J. Ghimire

Abstract: The kagome lattice, known for its strong frustration in two dimensions, hosts a variety of exotic magnetic and electronic states. A variation of this geometry, where the triangular motifs are twisted to further reduce symmetry, has recently revealed even more complex physics. HoAgGe exemplifies such a structure, with magnetic and electronic properties believed to be driven by strong in-plane aniso… ▽ More The kagome lattice, known for its strong frustration in two dimensions, hosts a variety of exotic magnetic and electronic states. A variation of this geometry, where the triangular motifs are twisted to further reduce symmetry, has recently revealed even more complex physics. HoAgGe exemplifies such a structure, with magnetic and electronic properties believed to be driven by strong in-plane anisotropy of the Ho spins, effectively acting as a two-dimensional spin ice. In this study, using a combination of magnetization, Hall conductivity measurements, and density functional theory calculations, we demonstrate how various spin-ice states, stabilized by external magnetic fields, influence the Fermi surface topology. More interestingly, we observe sharp transitions in Hall conductivity without concurrent changes in magnetization when an external magnetic field is applied along a particular crystallographic direction, underscoring the role of strong magnetic frustration and providing a new platform for exploring the interplay between magnetic frustration, electronic topology, and crystalline symmetry. These results also highlight the limitations of a simple spin-ice model, suggesting that a more sophisticated framework is necessary to capture the subtle experimental nuances observed. △ Less

Submitted 27 March, 2025; v1 submitted 15 October, 2024; originally announced October 2024.

Journal ref: Commun Mater 6, 52 (2025).

The mysterious magnetic ground state of Ba14MnBi11 is likely altermagnetic

Authors: Po-Hao Chang, Igor I. Mazin

Abstract: Mn-based transition metal Zintl compounds in the 14-1-11 phase are known to host complex atomic and magnetic structures owing to their intricate crystal structure. Among them, Ba14MnBi11 stands out as one of the least understood compounds, with experimental measurements and theoretical findings largely inconsistent. Following up on the earlier attempt [D. Sanchez-Portal et al., PRB 65, 144414 (200… ▽ More Mn-based transition metal Zintl compounds in the 14-1-11 phase are known to host complex atomic and magnetic structures owing to their intricate crystal structure. Among them, Ba14MnBi11 stands out as one of the least understood compounds, with experimental measurements and theoretical findings largely inconsistent. Following up on the earlier attempt [D. Sanchez-Portal et al., PRB 65, 144414 (2002)] at establishing a connection between metallicity and magnetism through a DFT-based analysis, our work aims to provide additional insights to resolve the existing contradictions. Our key finding is that the magnetic ground state is very susceptible to charge doping. DFT calculations for stoichiometric Ba14MnBi11 give a rather stable ferromagnetic metallic ground state. However, by adding exactly one additional electron per Mn, the system becomes semiconducting and the magnetic ground state becomes weakly antiferromagnetic (AF). On the other hand, upon small hole doping the system transitions to a special type of AF state known as altermagnetic ordering. The observed trends suggest that hole and electron doping-induced phase transitions likely result from different underlying mechanisms, influencing various exchange pathways. Additionally, our projected density-of-states along with bandstructure analyses indicate that, besides the largest hole contribution coming from the tetrahedral unit of Bi, the isolated Bi sites also play a substantial role and the dispersive bands near VBM suggest a rather complex hybridization network involving both Bi band characters. Through a comprehensive comparison of available data and our analysis, we propose that the inconsistency in magnetic states between experimental findings and DFT calculations is due to nonstoichiometric effects, likely impurities or defects. △ Less

Submitted 22 July, 2024; originally announced July 2024.

Comments: 9 pages, 9 figures

Phase transitions from Heating to non-heating in SU(1, 1) quantum dynamics: applications to Bose-Einstein condensates and periodically driven coupled oscillators

Authors: Heng-Hsi Li, Po-Yao Chang

Abstract: We study the entanglement properties in non-equilibrium quantum systems with the SU(1, 1) structure. Through Möbius transformation, we map the dynamics of these systems following a sudden quench or a periodic drive onto three distinct trajectories on the Poincaré disc, corresponding the heating, non-heating, and a phase boundary describing these non-equilibrium quantum states. We consider two expe… ▽ More We study the entanglement properties in non-equilibrium quantum systems with the SU(1, 1) structure. Through Möbius transformation, we map the dynamics of these systems following a sudden quench or a periodic drive onto three distinct trajectories on the Poincaré disc, corresponding the heating, non-heating, and a phase boundary describing these non-equilibrium quantum states. We consider two experimentally feasible systems where their quantum dynamics exhibit the SU(1, 1) structure: the quench dynamics of the Bose-Einstein condensates and the periodically driven coupled oscillators. In both cases, the heating, non-heating phases, and their boundary manifest through distinct signatures in the phonon population where exponential, oscillatory, and linear growths classify these phases. Similarly, the entanglement entropy and negativity also exhibit distinct behaviors (linearly, oscillatory, and logarithmic growths) characterizing these phases, respectively. Notibly, for the periodically driven coupled oscillators, the non-equilibrium properties are characterized by two sets of SU(1, 1) generators. The corresponding two sets of the trajectories on two Poincaré discs lead to a more complex phase diagram. We identify two distinct phases within the heating region discernible solely by the growth rate of the entanglement entropy, where a discontinuity is observed when varying the parameters across the phase boundary within in heating region. This discontinuity is not observed in the phonon population. △ Less

Submitted 16 December, 2024; v1 submitted 21 May, 2024; originally announced May 2024.

Comments: 28 pages, 6 figures

Journal ref: SciPost Phys. Core 8, 018 (2025)

Topological phases of extended Su-Schrieffer-Heeger-Hubbard model

Authors: Pei-Jie Chang, Jinghui Pi, Muxi Zheng, Yu-Ting Lei, Dong Ruan, Gui-Lu Long

Abstract: Despite extensive studies on the one-dimensional Su-Schrieffer-Heeger-Hubbard (SSHH) model, the variant incorporating next-nearest neighbour hopping remains largely unexplored. Here, we investigate the ground-state properties of this extended SSHH model using the constrained-path auxiliary-field quantum Monte Carlo (CP-AFQMC) method. We show that this model exhibits rich topological phases, charac… ▽ More Despite extensive studies on the one-dimensional Su-Schrieffer-Heeger-Hubbard (SSHH) model, the variant incorporating next-nearest neighbour hopping remains largely unexplored. Here, we investigate the ground-state properties of this extended SSHH model using the constrained-path auxiliary-field quantum Monte Carlo (CP-AFQMC) method. We show that this model exhibits rich topological phases, characterized by robust edge states against interaction. We quantify the properties of these edge states by analyzing spin correlation and second-order Rényi entanglement entropy. The system exhibits long-range spin correlation and near-zero Rényi entropy at half-filling. Besides, there is a long-range anti-ferromagnetic order at quarter-filling. Interestingly, an external magnetic field disrupts this long-range anti-ferromagnetic order, restoring long-range spin correlation and near-zero Rényi entropy. Furthermore, our work provides a paradigm studying topological properties in large interacting systems via the CP-AFQMC algorithm. △ Less

Submitted 19 June, 2024; v1 submitted 16 May, 2024; originally announced May 2024.

Comments: 16 pages, 11 figures

Topological entanglement entropy for torus knot bipartitions and the Verlinde-like formulas

Authors: Chih-Yu Lo, Po-Yao Chang

Abstract: The topological Rényi and entanglement entropies depend on the bipartition of the manifold and the choice of the ground states. However, these entanglement quantities remain invariant under a coordinate transformation when the bipartition also undergoes the same transformation. In the context of topological quantum field theories, these coordinate transformations reduce to representations of the m… ▽ More The topological Rényi and entanglement entropies depend on the bipartition of the manifold and the choice of the ground states. However, these entanglement quantities remain invariant under a coordinate transformation when the bipartition also undergoes the same transformation. In the context of topological quantum field theories, these coordinate transformations reduce to representations of the mapping class group on the manifold of the Hilbert space. We employ this invariant property of the Rényi and entanglement entropies under coordinate transformations for TQFTs in (2 + 1) dimensions on a torus with various bipartitions. By utilizing the replica trick and the surgery method to compute the topological Rényi and entanglement entropies, the invariant property results in Verlinde-like formulas. Furthermore, for the bipartition with interfaces as two non-intersecting torus knots, an $SL(2, \mathbb{Z})$ transformation can untwist the torus knots, leading to a simple bipartition with an effective ground state. This invariant property allows us to demonstrate that the topological entanglement entropy has a lower bound $-2 \ln D$, where $D$ is the total quantum dimensions of the system. △ Less

Submitted 19 February, 2024; v1 submitted 13 December, 2023; originally announced December 2023.

Comments: ref.[20] is corrrected to Phys. Rev. Lett. 98, 060401 (2007); Fig. 3.15 is also modified

Journal ref: J. High Energ. Phys. 2024, 117 (2024)

Exploring Metamagnetism in Triangular Ising Networks: Insights from Further-Neighbor Interactions with a Case Study on ErGa2

Authors: Po-Hao Chang, Igor I. Mazin

Abstract: The classical Ising model on the triangular lattice (we will call it I-3 model below), while simple in the nearest-neighbors (NN) only approximation, becomes increasingly richer and more complex when further interactions are incorporated. However, the studies so far have not been exhaustive, nor have any attempts been made to estimate how realistic are the parameter ranges that generate strong met… ▽ More The classical Ising model on the triangular lattice (we will call it I-3 model below), while simple in the nearest-neighbors (NN) only approximation, becomes increasingly richer and more complex when further interactions are incorporated. However, the studies so far have not been exhaustive, nor have any attempts been made to estimate how realistic are the parameter ranges that generate strong metamagnetism with a large number of magnetization steps. In this study, we identify one such candidate, ErGa$_{2}$, a material known to have one strong magnetization step, albeit some narrow steps below and above cannot be confidently excluded. It has been established, and we can confirm the same computationally, to have an easy axis perpendicular to the triangular Er plane, with a strong anisotropy and with a large magnetic moment of $9.5\:μ_{B}$, making it a perfect implementation of the classical I-3 model. In the first part of the analysis, we present the I-3 model with up to the third nearest-neighbors in a range of parameters $J_{2}$ and $J_{3}$ ($J_{1}$ in this part is set to 1), and in some cases adding a rather small $J_{4}$ in order to reveal new phases otherwise degenerate with some others. The richest phase diagram is, not surprisingly, observed when all interactions are antiferromagnetic (AF). Subsequently, a more realistic case, inspired by RKKY and by our calculations for ErGa$_{2}$, where $J_{1},J_{2}>0$ (antiferromagnetic) and $J_{3},J_{4}<0$ (ferromagnetic), is presented. Finally, we report our first-principles calculations of $J_{1-4}$ in ErGa$_{2}$ and compared the phase diagram in the regime corresponding to the calculated values with the experiment. △ Less

Submitted 11 December, 2023; originally announced December 2023.

Comments: 7 pages, 5 figures

Dual-species Bose-Einstein condensates of $^{7}$Li and $^{133}$Cs

Authors: Y. -D. Chen, W. -X. Li, Y. -T. Sun, Q. -C. Chen, P. -Y. Chang, S. Tung

Abstract: We report the creation of dual-species Bose-Einstein condensates (BECs) of $^{7}$Li and $^{133}$Cs. These BECs are formed in a bichromatic optical dipole trap created with 1550-nm and 780-nm laser beams. During the production process, an external magnetic field of 886~G is applied to adjust the scattering lengths to $a_{\rm{Cs}} = 123a_0$, $a_{\rm{Li}} = 484a_0$, and $a_{\rm{LiCs}} = 248a_0$. Thes… ▽ More We report the creation of dual-species Bose-Einstein condensates (BECs) of $^{7}$Li and $^{133}$Cs. These BECs are formed in a bichromatic optical dipole trap created with 1550-nm and 780-nm laser beams. During the production process, an external magnetic field of 886~G is applied to adjust the scattering lengths to $a_{\rm{Cs}} = 123a_0$, $a_{\rm{Li}} = 484a_0$, and $a_{\rm{LiCs}} = 248a_0$. These scattering lengths allow for efficient evaporation and sympathetic cooling. The dual-species BECs are typically produced with $1.5\times 10^4$ Cs atoms and $6.0\times 10^3$ Li atoms. This quantum degenerate mixture of Li and Cs provides an ideal platform for exploring phenomena such as polarons and Efimov trimers, as well as for creating ground-state LiCs molecules. △ Less

Submitted 8 September, 2023; v1 submitted 5 June, 2023; originally announced June 2023.

Comments: 9 pages, 9 figures

Journal ref: Phys. Rev. A 108, 033301 (2023)

Hyperbolic polaritons in topological nodal ring semimetals

Authors: Ashutosh Singh, Maria Sebastian, Yuanping Chen, Po-Yao Chang, Alexey Belyanin

Abstract: In mirror-symmetric systems, there is a possibility of the realization of extended gapless electronic states characterized as nodal lines or rings. Strain induced modifications to these states lead to emergence of different classes of nodal rings with qualitatively different physical properties. Here we study optical response and the electromagnetic wave propagation in type I nodal ring semimetals… ▽ More In mirror-symmetric systems, there is a possibility of the realization of extended gapless electronic states characterized as nodal lines or rings. Strain induced modifications to these states lead to emergence of different classes of nodal rings with qualitatively different physical properties. Here we study optical response and the electromagnetic wave propagation in type I nodal ring semimetals, in which the low-energy quasiparticle dispersion is parabolic in momentum $k_x$ and $k_y$ and is linear in $k_z$. This leads to a highly anisotropic dielectric permittivity tensor in which the optical response is plasmonic in one spatial direction and dielectric in the other two directions. The resulting normal modes (polaritons) in the bulk material become hyperbolic over a broad frequency range, which is furthermore tunable by the doping level. The propagation, reflection, and polarization properties of the hyperbolic polaritons not only provide valuable information about the electronic structure of these fascinating materials in the most interesting region near the nodal rings but also pave the way to tunable hyperbolic materials with applications ranging from anomalous refraction and waveguiding to perfect absorption in ultrathin subwavelength films. △ Less

Submitted 22 March, 2023; originally announced March 2023.

Comments: 5 pages, 3 figures

Prethermalization and transient dynamics of the Multi-Channel Kondo systems under generic quantum quenches: Insights form Large-$N$ Schwinger-Keldysh approach

Authors: Iksu Jang, Po-Yao Chang

Abstract: Understanding out-of-equilibrium many-body quantum systems is crucial in contemporary physics. However, capturing the universal dynamics of such systems remains challenging despite advanced numerical methods. We investigate the multi-channel Kondo impurity (MCKI) model, hosting an over-screened Kondo state with non-Fermi liquid characteristics. Using the large-N Schwinger-Keldysh approach, we stud… ▽ More Understanding out-of-equilibrium many-body quantum systems is crucial in contemporary physics. However, capturing the universal dynamics of such systems remains challenging despite advanced numerical methods. We investigate the multi-channel Kondo impurity (MCKI) model, hosting an over-screened Kondo state with non-Fermi liquid characteristics. Using the large-N Schwinger-Keldysh approach, we study transient dynamics and long-time quasi-equilibrium properties following a sudden change in the Kondo coupling. We consider two initial states: over-screened Kondo and high-temperature Fermi liquid. In the over-screened Kondo state, we observe oscillations in spin-spin correlations and the Kondo order parameter, representing quantum revival of the entangled state. In the high-temperature Fermi liquid state, the absence of oscillations is attributed to the de-phasing mechanism. The system reaches quasi-equilibrium, manifested as incoherent thermalization between the impurity and conduction electrons. We observe a non-vanishing effective temperature difference between the impurity spin (Abrikosov fermion) and the composite boson formed with conduction electrons at the impurity site. This quasi-equilibrium is prethermalization, while complete thermalization occurs on an O(N) timescale with 1/N correction. We discuss the quantum cooling effect and quantum Boltzmann equations. Our study establishes a foundation for investigating large-N quantum field theory treatment of quantum many-body systems, revealing universal properties and a fresh perspective on prethermalization. △ Less

Submitted 29 June, 2023; v1 submitted 4 March, 2023; originally announced March 2023.

Comments: 23 pages, 20 figures, Changed into two-column format, typos are corrected, Some parts are rewritten

Pressure-induced magnetic properties of quasi-2D Cr2Si2Te6 and Mn3Si2Te6

Authors: Rubyann Olmos, Po-Hao Chang, Prakash Mishra, Rajendra R. Zope, Tunna Baruah, Cedomir Petrovic, Yu Liu, Srinivasa R. Singamaneni

Abstract: Recently, the pressure has been used as external stimuli to induce structural and magnetic phase transitions in many layered quantum materials whose layers are linked by van der Waals forces. Such materials with weakly held layers allow relatively easy manipulation of the superexchange mechanism and lead to novel magnetic behavior. Using the hydrostatic pressure as a disorderless means to manipula… ▽ More Recently, the pressure has been used as external stimuli to induce structural and magnetic phase transitions in many layered quantum materials whose layers are linked by van der Waals forces. Such materials with weakly held layers allow relatively easy manipulation of the superexchange mechanism and lead to novel magnetic behavior. Using the hydrostatic pressure as a disorderless means to manipulate the interlayer coupling, we applied pressure on two quasi-2D sister compounds, namely, Cr2Si2Te6 (CST) and Mn3Si2Te6 (MST), up to ~1 GPa. Magnetic property measurements with the application of pressure revealed that the ferromagnetic transition temperature decreases in CST while the opposite trend occurs for the ferrimagnet MST. In MST, the magnetization decreases with the increase in the pressure, and such trend is not clearly noticed in CST, within the pressure range studied. Theoretical calculations showed the overall pressure effect on layer separation, bond angle, and exchange coupling, strongly influencing the change in subsequent magnetic characteristics. Exchange coupling in Mn3Si2Te6 is strongly frustrated and the first nearest neighbor interaction is the most dominant of the components with the strongest pressure dependence. Whereas, in Cr2Si2Te6, the exchange coupling parameters exhibit very little dependence on the pressure. This combined experimental and theoretical work has the potential to expand to other relevant quantum materials. △ Less

Submitted 6 February, 2023; originally announced February 2023.

Interaction-induced Metal to Topological Insulator Transition

Authors: Yu-Chin Tzeng, Po-Yao Chang, Min-Fong Yang

Abstract: By means of exact diagonalizations, the Bernevig-Hughes-Zhang model at quarter-filling in the limit of strong Hubbard on-site repulsion is investigated. We find that the non-interacting metallic state will be turned into a Chern insulator with saturated magnetization under strong correlations. That is, at such a metal-insulator transition, both the topological and the magnetic properties of the sy… ▽ More By means of exact diagonalizations, the Bernevig-Hughes-Zhang model at quarter-filling in the limit of strong Hubbard on-site repulsion is investigated. We find that the non-interacting metallic state will be turned into a Chern insulator with saturated magnetization under strong correlations. That is, at such a metal-insulator transition, both the topological and the magnetic properties of the system are changed due to spontaneous breaking of time reversal symmetry in the ground states. According to our findings, this topological phase transition seems to be of first order. Our results illustrate the interesting physics in topological Mott transitions and provide guidance to the search of more interaction-induced topological phases in similar systems. △ Less

Submitted 6 February, 2023; originally announced February 2023.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. B 107, 155106 (2023)

Relating non-Hermitian and Hermitian quantum systems at criticality

Authors: Chang-Tse Hsieh, Po-Yao Chang

Abstract: We demonstrate three types of transformations that establish connections between Hermitian and non-Hermitian quantum systems at criticality, which can be described by conformal field theories (CFTs). For the transformation preserving both the energy and the entanglement spectra, the corresponding central charges obtained from the logarithmic scaling of the entanglement entropy are identical for bo… ▽ More We demonstrate three types of transformations that establish connections between Hermitian and non-Hermitian quantum systems at criticality, which can be described by conformal field theories (CFTs). For the transformation preserving both the energy and the entanglement spectra, the corresponding central charges obtained from the logarithmic scaling of the entanglement entropy are identical for both Hermitian and non-Hermitian systems. The second transformation, while preserving the energy spectrum, does not perserve the entanglement spectrum. This leads to different entanglement entropy scalings and results in different central charges for the two types of systems. We demonstrate this transformation using the dilation method applied to the free fermion case. Through this method, we show that a non-Hermitian system with central charge $c = -4$ can be mapped to a Hermitian system with central charge $c = 2$. Lastly, we investigate the Galois conjugation in the Fibonacci model with the parameter $φ\to - 1/φ$, in which the transformation does not preserve both energy and entanglement spectra. We demonstrate the Fibonacci model and its Galois conjugation relate the tricritical Ising model/3-state Potts model and the Lee-Yang model with negative central charges from the scaling property of the entanglement entropy. △ Less

Submitted 30 July, 2023; v1 submitted 22 November, 2022; originally announced November 2022.

Comments: 19 pages, 5 figures, add the entanglement spectra comparisons, and an short introduction about the non-unitary CFTs. Comments are welcome

Self-consistent implementation of locally scaled self-interaction-correction method

Authors: Yoh Yamamoto, Tunna Baruah, Po-Hao Chang, Selim Romero, Rajendra R. Zope

Abstract: Recently proposed local self-interaction correction (LSIC) method [Zope, R. R. et al., J. Chem. Phys. 151, 214108 (2019)] is a one-electron self-interaction-correction (SIC) method that uses an iso-orbital indicator to apply the SIC at each point in space by scaling the exchange-correlation and Coulomb energy densities. The LSIC method is exact for the one-electron densities, also recovers the uni… ▽ More Recently proposed local self-interaction correction (LSIC) method [Zope, R. R. et al., J. Chem. Phys. 151, 214108 (2019)] is a one-electron self-interaction-correction (SIC) method that uses an iso-orbital indicator to apply the SIC at each point in space by scaling the exchange-correlation and Coulomb energy densities. The LSIC method is exact for the one-electron densities, also recovers the uniform electron gas limit of the uncorrected density functional approximation, and reduces to the well-known Perdew-Zunger SIC (PZSIC) method as a special case. This article presents the self-consistent implementation of the LSIC method using the ratio of Weizsäcker and Kohn-Sham kinetic energy densities as an iso-orbital indicator. The atomic forces as well as the forces on the Fermi-Löwdin orbitals are also implemented for the LSIC energy functional. Results show that LSIC with the simplest local spin density functional predicts atomization energies of AE6 dataset better than some of the most widely used GGA functional (e.g. PBE) and barrier heights of BH6 database better than some of the most widely used hybrid functionals (e.g. PBE0 and B3LYP). The LSIC method [mean absolute error (MAE) of 0.008 Å] predicts bond lengths of a small set of molecules better than the PZSIC-LSDA (MAE 0.042 Å) and LSDA (0.011 Å). This work shows that accurate results can be obtained from the simplest density functional by removing the self-interaction-errors using an appropriately designed SIC method. △ Less

Submitted 7 November, 2022; originally announced November 2022.

Comments: 34 pages, 4 figures

2D Gapless Topological Superfluids Generated by Pairing Phases

Authors: Jiapei Zhuang, Ching-Yu Huang, Po-Yao Chang, Daw-Wei Wang

Abstract: We systematically investigate the ground state phase diagram and the finite temperature phase transitions for a Rydberg-dressed Fermi gas loaded in a bilayer optical lattice. When an effective finite-ranged attraction is induced, our self-consistent mean-field calculation shows that the gapped topological ( $p$-wave) superfluids in each layer are coupled together by the $s$-wave pairing in an inte… ▽ More We systematically investigate the ground state phase diagram and the finite temperature phase transitions for a Rydberg-dressed Fermi gas loaded in a bilayer optical lattice. When an effective finite-ranged attraction is induced, our self-consistent mean-field calculation shows that the gapped topological ( $p$-wave) superfluids in each layer are coupled together by the $s$-wave pairing in an intermediate inter-layer distance with a spontaneously modulated phases between these two order parameters. The obtained ground state is a gapless topological superfluid with quantized topological charges characterizing the gapless points, leading to a zero energy flat band at the edges. Finally, we calculate the finite temperature phase diagrams of this two-dimensional gapless superfluid and observe two distinct critical temperatures, demonstrating the fruitful many-body effects on a paired topological superfluids. △ Less

Submitted 30 July, 2022; originally announced August 2022.

Comments: 11 pages, 5 figures

General properties of fidelity in non-Hermitian quantum systems with PT symmetry

Authors: Yi-Ting Tu, Iksu Jang, Po-Yao Chang, Yu-Chin Tzeng

Abstract: The fidelity susceptibility is a tool for studying quantum phase transitions in the Hermitian condensed matter systems. Recently, it has been generalized with the biorthogonal basis for the non-Hermitian quantum systems. From the general perturbation description with the constraint of parity-time (PT) symmetry, we show that the fidelity $\mathcal{F}$ is always real for the PT-unbroken states. For… ▽ More The fidelity susceptibility is a tool for studying quantum phase transitions in the Hermitian condensed matter systems. Recently, it has been generalized with the biorthogonal basis for the non-Hermitian quantum systems. From the general perturbation description with the constraint of parity-time (PT) symmetry, we show that the fidelity $\mathcal{F}$ is always real for the PT-unbroken states. For the PT-broken states, the real part of the fidelity susceptibility $\mathrm{Re}[\mathcal{X}_F]$ is corresponding to considering both the PT partner states, and the negative infinity is explored by the perturbation theory when the parameter approaches the exceptional point (EP). Moreover, at the second-order EP, we prove that the real part of the fidelity between PT-unbroken and PT-broken states is $\mathrm{Re}\mathcal{F}=\frac{1}{2}$. Based on these general properties, we study the two-legged non-Hermitian Su-Schrieffer-Heeger (SSH) model and the non-Hermitian XXZ spin chain. We find that for both interacting and non-interacting systems, the real part of fidelity susceptibility density goes to negative infinity when the parameter approaches the EP, and verifies it is a second-order EP by $\mathrm{Re}\mathcal{F}=\frac{1}{2}$. △ Less

Submitted 21 March, 2023; v1 submitted 3 March, 2022; originally announced March 2022.

Comments: To appear in <Quantum> journal

Journal ref: Quantum 7, 960 (2023)

Two-dimensional Paired Topological Superfluids of Rydberg Fermi Gases

Authors: Ching-Yu Huang, Jiapei Zhuang, Po-Yao Chang, Daw-Wei Wang

Abstract: We systematically investigate the topological properties of spin polarized Rydberg-dressed fermionic atoms loaded in a bilayer optical lattice. Through tuning the Rydberg coupling strength and the inter-layer tunneling amplitude, we identify different types of topological superfluid states generated from the inter-layer pairing and relative gauge phase modulation of the couples 2D $p$-wave superfl… ▽ More We systematically investigate the topological properties of spin polarized Rydberg-dressed fermionic atoms loaded in a bilayer optical lattice. Through tuning the Rydberg coupling strength and the inter-layer tunneling amplitude, we identify different types of topological superfluid states generated from the inter-layer pairing and relative gauge phase modulation of the couples 2D $p$-wave superfluids. These phases includes gapped/gapless with/without time reversal symmetry. One of the most interesting states is a gapless paired topological superfluid with both the time-reversal symmetry and particle-hole symmetry. This state is equivalent to a topological Kondo lattice model with the spin-orbit coupling, an in-plane magnetic field, and an additional particle-hole symmetry. The flexibility of experimental manipulation in such Rydberg-dressed ferminoic systems therefore becomes a promising system for realizing interesting topological superfluids. △ Less

Submitted 28 December, 2021; originally announced December 2021.

Pressure dependent magnetic properties on bulk CrBr3 single crystals

Authors: Rubyann Olmos, Shamsul Alam, Po Hao Chang, Kinjal Gandha, Ikenna C. Nlebedim, Andrew Cole, Fazel Tafti, Rajendra Zope, Srinivasa R. Singamaneni

Abstract: The van der Waals class of materials offer an approach to two-dimensional magnetism as their spin fluctuations can be tuned upon exfoliation of layers. Moreover, it has recently been shown that spin-lattice coupling and long-range magnetic ordering can be modified with pressure in van der Waals materials. In this work, the magnetic properties of quasi two-dimensional CrBr3 are reported applying hy… ▽ More The van der Waals class of materials offer an approach to two-dimensional magnetism as their spin fluctuations can be tuned upon exfoliation of layers. Moreover, it has recently been shown that spin-lattice coupling and long-range magnetic ordering can be modified with pressure in van der Waals materials. In this work, the magnetic properties of quasi two-dimensional CrBr3 are reported applying hydrostatic pressure. The application of pressure (0 - 0.8 GPa) shows a 72 % decrease in saturation magnetization with small decrease in the Curie temperature from 33 to 29 K. Density functional theory calculations with pressure up to 1 GPa show a reduction in volume and interplanar distance as pressure increases. To further understand magnetic properties with applied pressure, the magnetocrystalline anisotropy energy (MAE) and exchange coupling parameter (J) are calculated. There is minimal decrease in MAE and the first nearest neighbor interaction (J1) (U = 2.7 eV and J = 0.7 eV), shows an increase in J1 with respect to pressure. Overall, CrBr3 displays ferromagnetic interlayer coupling and the calculated exchange coupling and MAE parameters match well with the observations from the experimental work. △ Less

Submitted 30 November, 2021; originally announced December 2021.

Proximity-Effect-Induced Anisotropic Superconductivity in Monolayer Ni-Pb Binary Alloy

Authors: Yen-Hui Lin, Chia-Hsiu Hsu, Iksu Jang, Chia-Ju Chen, Pok-Man Chiu, Deng-Sung Lin, Chien-Te Wu, Feng-Chuan Chuang, Po-Yao Chang, Pin-Jui Hsu

Abstract: Proximity effect facilitates the penetration of Cooper pairs that permits superconductivity in normal metal, offerring a promising approach to turn heterogeneous materials into superconducting and develop exceptional quantum phenomena. Here, we have systematically investigated proximity-induced anisotropic superconductivity in monolayer Ni-Pb binary alloy by combining scanning tunneling microscopy… ▽ More Proximity effect facilitates the penetration of Cooper pairs that permits superconductivity in normal metal, offerring a promising approach to turn heterogeneous materials into superconducting and develop exceptional quantum phenomena. Here, we have systematically investigated proximity-induced anisotropic superconductivity in monolayer Ni-Pb binary alloy by combining scanning tunneling microscopy/ spectroscopy(STM/STS) with theoretical calculations. By means of high temperature growth, the(3root3by3root3)R30o Ni-Pb surface alloy has been fabricated on the Pb(111), where the appearance of domain boundary as well as lattice transformation are further corroborated by the STM simulations. Given the high spatial and energy resolution, tunnelling conductance (dI/dU) spectra have resolved a reduced but anisotropic superconducting gap NiPb about 1.0 meV, in stark contrast to the isotropic Pb about 1.3 meV on the conventional Pb(111). In addition, the higher density of states at Fermi energy (D(EF)) of Ni-Pb surface alloy results in an enhancement of coherence peak height. According to the same Tc about 7.1 K with Pb(111) from the temperature dependent NiPb and a short decay length Ld about 3.55 nm from the spatially monotonic decrease of NiPb, both results are supportive for the proximity-induced superconductivity. Despite a lack of bulk counterpart, the atomic-thick Ni-Pb bimetallic compound opens a new pathway to engineer superconducting properties down to the low-dimensional limit, giving rise to the emergence of anisotropic superconductivity via proximity effect. △ Less

Submitted 21 September, 2021; originally announced September 2021.

Comments: 16 pages, 4 figures

Picosecond creation of switchable optomagnets with giant photoinduced Kerr rotations in polar antiferromagnetic (Fe$_{1-x}$Zn$_{x}$)$_{2}$Mo$_{3}$O$_{8}$

Authors: Y. M. Sheu, Y. M. Chang, C. P. Chang, Y. H. Li, K. R. Babu, G. Y. Guo, T. Kurumaji, Y. Tokura

Abstract: On-demand spin orientation with long polarized lifetime and easily detectable signal is an ultimate goal for spintronics. However, there still exists a trade-off between controllability and stability of spin polarization, awaiting a significant breakthrough. Here, we demonstrate switchable optomagnet effects in (Fe$_{1-x}$Zn$_{x}$)$_{2}$Mo$_{3}$O$_{8}$, from which we can obtain tunable magnetizati… ▽ More On-demand spin orientation with long polarized lifetime and easily detectable signal is an ultimate goal for spintronics. However, there still exists a trade-off between controllability and stability of spin polarization, awaiting a significant breakthrough. Here, we demonstrate switchable optomagnet effects in (Fe$_{1-x}$Zn$_{x}$)$_{2}$Mo$_{3}$O$_{8}$, from which we can obtain tunable magnetization, spanning from -40$\%$ to 40$\%$ of a saturated magnetization that is created from zero magnetization in the antiferromagnetic state without magnetic fields. It is accomplishable via utilizing circularly-polarized laser pulses to excite spin-flip transitions in polar antiferromagnets that have no spin canting, traditionally hard to control without very strong magnetic fields. The spin controllability in (Fe$_{1-x}$Zn$_{x}$)$_{2}$Mo$_{3}$O$_{8}$ originates from its polar structure that breaks the crystal inversion symmetry, allowing distinct on-site $d$-$d$ transitions for selective spin flip. By chemical doping, we exploit the phase competition between antiferromagnetic and ferrimagnetic states to enhance and stabilize the optomagnet effects, which result in long-lived photoinduced Kerr rotations. The present study, creating switchable giant optomagnet effects in polar antiferromagnets, sketches a new blueprint for the function of antiferromagnetic spintronics. △ Less

Submitted 8 September, 2021; originally announced September 2021.

Rényi entropies and negative central charges in non-Hermitian quantum systems

Authors: Yi-Ting Tu, Yu-Chin Tzeng, Po-Yao Chang

Abstract: Quantum entanglement is one essential element to characterize many-body quantum systems. However, the entanglement measures are mostly discussed in Hermitian systems. Here, we propose a natural extension of entanglement and Rényi entropies to non-Hermitian quantum systems. There have been other proposals for the computation of these quantities, which are distinct from what is proposed in the curre… ▽ More Quantum entanglement is one essential element to characterize many-body quantum systems. However, the entanglement measures are mostly discussed in Hermitian systems. Here, we propose a natural extension of entanglement and Rényi entropies to non-Hermitian quantum systems. There have been other proposals for the computation of these quantities, which are distinct from what is proposed in the current paper. We demonstrate the proposed entanglement quantities which are referred to as generic entanglement and Rényi entropies. These quantities capture the desired entanglement properties in non-Hermitian critical systems, where the low-energy properties are governed by the non-unitary conformal field theories (CFTs). We find excellent agreement between the numerical extrapolation of the negative central charges from the generic entanglement/Rényi entropy and the non-unitary CFT prediction. Furthermore, we apply the generic entanglement/Rényi entropy to symmetry-protected topological phases with non-Hermitian perturbations. We find the generic $n$-th Rényi entropy captures the expected entanglement property, whereas the traditional Rényi entropy can exhibit unnatural singularities due to its improper definition. △ Less

Submitted 18 May, 2022; v1 submitted 27 July, 2021; originally announced July 2021.

Comments: 17 pages, 6 figures, add the discussion about the XXZ model and the six-vertex model. Comments are welcome

Journal ref: SciPost Phys. 12, 194 (2022)

Non-Abelian fracton order from gauging a mixture of subsystem and global symmetries

Authors: Yi-Ting Tu, Po-Yao Chang

Abstract: We demonstrate a general gauging procedure of a pure matter theory on a lattice with a mixture of subsystem and global symmetries. This mixed symmetry can be either a semidirect product of a subsystem symmetry and a global symmetry, or a non-trivial extension of them. We demonstrate this gauging procedure on a cubic lattice in three dimensions with four examples:… ▽ More We demonstrate a general gauging procedure of a pure matter theory on a lattice with a mixture of subsystem and global symmetries. This mixed symmetry can be either a semidirect product of a subsystem symmetry and a global symmetry, or a non-trivial extension of them. We demonstrate this gauging procedure on a cubic lattice in three dimensions with four examples: $G=\mathbb{Z}_3^{\text{sub}} \rtimes \mathbb{Z}_2^{\text{glo}}$, $G=(\mathbb{Z}_2^{\text{sub}} \times \mathbb{Z}_2^{\text{sub}}) \rtimes \mathbb{Z}_2^{\text{glo}}$, $1\to \mathbb {Z}_2^\text {sub}\to G\to \mathbb {Z}_2^\text {glo}\to 1$, and $1\to \mathbb {Z}_2^\text {sub}\to G\to K_4^\text {glo}\to 1$. The former two cases and the last one produce the non-Abelian fracton orders. Our construction of the gauging procedure provides an identification of the electric charges of these fracton orders with irreducible representations of the symmetry. Furthermore, by constraining the local Hilbert space, the magnetic fluxes with different geometry (tube-like and plaquette-like) satisfy a subalgebra of the quantum double models (QDMs). This algebraic structure leads to an identification of the magnetic fluxes to the conjugacy classes of the symmetry. △ Less

Submitted 3 October, 2021; v1 submitted 15 March, 2021; originally announced March 2021.

Comments: 2 columns, 24 pages, 4 tables

Journal ref: Phys. Rev. Research 3, 043084 (2021)

Spirals and skyrmions in antiferromagnetic triangular lattices

Authors: Wuzhang Fang, Aldo Raeliarijaona, Po-Hao Chang, Alexey A. Kovalev, Kirill D. Belashchenko

Abstract: We study realizations of spirals and skyrmions in two-dimensional antiferromagnets with a triangular lattice on an inversion-symmetry-breaking substrate. As a possible material realization, we investigate the adsorption of transition-metal atoms (Cr, Mn, Fe, or Co) on a monolayer of MoS$_2$, WS$_2$, or WSe$_2$ and obtain the exchange, anisotropy, and Dzyaloshinskii-Moriya interaction parameters us… ▽ More We study realizations of spirals and skyrmions in two-dimensional antiferromagnets with a triangular lattice on an inversion-symmetry-breaking substrate. As a possible material realization, we investigate the adsorption of transition-metal atoms (Cr, Mn, Fe, or Co) on a monolayer of MoS$_2$, WS$_2$, or WSe$_2$ and obtain the exchange, anisotropy, and Dzyaloshinskii-Moriya interaction parameters using first-principles calculations. Using energy minimization and parallel-tempering Monte-Carlo simulations, we determine the magnetic phase diagrams for a wide range of interaction parameters. We find that skyrmion lattices can appear even with weak Dzyaloshinskii-Moriya interactions, but their stability is hindered by magnetic anisotropy. However, a weak easy plane magnetic anisotropy can be beneficial for stabilizing the skyrmion phase. Our results suggest that Cr$/$MoS$_2$, Fe$/$MoS$_2$, and Fe$/$WSe$_2$ interfaces can host spin spirals formed from the 120$^{\circ}$ antiferromagnetic states. Our results further suggests that for other interfaces, such as Fe$/$MoS$_2$, the Dzyaloshinskii-Moriya interaction is strong enough to drive the system into a three-sublattice skyrmion lattice in the presence of experimentally feasible external magnetic field. △ Less

Submitted 22 February, 2021; originally announced February 2021.

Comments: 11 pages, 10 figures

Journal ref: Phys. Rev. Materials 5, 054401 (2021)

Disorder-induced topology in quench dynamics

Authors: Hsiu-Chuan Hsu, Pok-Man Chiu, Po-Yao Chang

Abstract: We study the effect of strong disorder on topology and entanglement in quench dynamics. Although disorder-induced topological phases have been well studied in equilibrium, the disorder-induced topology in quench dynamics has not been explored. In this work, we predict a disorder-induced topology of post-quench states characterized by the quantized dynamical Chern number and the crossings in the en… ▽ More We study the effect of strong disorder on topology and entanglement in quench dynamics. Although disorder-induced topological phases have been well studied in equilibrium, the disorder-induced topology in quench dynamics has not been explored. In this work, we predict a disorder-induced topology of post-quench states characterized by the quantized dynamical Chern number and the crossings in the entanglement spectrum in $(1+1)$ dimensions. The dynamical Chern number undergoes transitions from zero to unity, and back to zero when increasing the disorder strength. The boundaries between different dynamical Chern numbers are determined by delocalized critical points in the post-quench Hamiltonian with the strong disorder. An experimental realization in quantum walks is discussed. △ Less

Submitted 13 September, 2021; v1 submitted 19 January, 2021; originally announced January 2021.

Comments: 10 pages, 8 figures

Journal ref: Phys. Rev. Research 3, 033242 (2021)

Voltage-controlled magnetic anisotropy in antiferromagnetic MgO-capped MnPt films

Authors: P. -H. Chang, W. Fang, T. Ozaki, K. D. Belashchenko

Abstract: The magnetic anisotropy in MgO-capped MnPt films and its voltage control are studied using first-principles calculations. Sharp variation of the magnetic anisotropy with film thickness, especially in the Pt-terminated film, suggests that it may be widely tuned by adjusting the film thickness. In thick films the linear voltage control coefficient is as large as 1.5 and $-0.6$ pJ/Vm for Pt-terminate… ▽ More The magnetic anisotropy in MgO-capped MnPt films and its voltage control are studied using first-principles calculations. Sharp variation of the magnetic anisotropy with film thickness, especially in the Pt-terminated film, suggests that it may be widely tuned by adjusting the film thickness. In thick films the linear voltage control coefficient is as large as 1.5 and $-0.6$ pJ/Vm for Pt-terminated and Mn-terminated interfaces, respectively. The combination of a widely tunable magnetic anisotropy energy and a large voltage-control coefficient suggest that MgO-capped MnPt films can serve as a versatile platform for magnetic memory and antiferromagnonic applications. △ Less

Submitted 21 April, 2021; v1 submitted 7 August, 2020; originally announced August 2020.

Comments: 6 pages, 5 figures. This version includes significant revisions after peer review and corrects an error in the values of the VCMA coefficient, which in the original version were mistakenly given with respect to the E-field in vacuum (rather than MgO)

Journal ref: Phys. Rev. Materials 5, 054406 (2021)

Interlocking nodal chains and their examples in carbon networks

Authors: Zhiwei Li, Yuee Xie1, Po-Yao Chang, Yuanping Chen

Abstract: Nodal chain is a typical topological phase in nodal line semimetals. Here, we propose a new topological phase -- interlocking nodal chains, in which two sets of nodal chains are interlocked each other. It includes one- (1D), two- (2D) and three-dimensional (3D) versions, which can be produced by a three-band model. The 2D and 3D interlocking nodal chains will evolve into some other phases, such as… ▽ More Nodal chain is a typical topological phase in nodal line semimetals. Here, we propose a new topological phase -- interlocking nodal chains, in which two sets of nodal chains are interlocked each other. It includes one- (1D), two- (2D) and three-dimensional (3D) versions, which can be produced by a three-band model. The 2D and 3D interlocking nodal chains will evolve into some other phases, such as double concentric isolated (or intersecting) nodal rings, coexisting nodal chain and isolated (or intersecting) nodal rings. These phases exhibit diverse surface states and Landau levels, which implies that there are rich electronic and magnetic properties associating with them. Moreover, the 2D interlocking nodal chains and related phase transitions can be realized in a series of carbon networks under strain. Without strain, topological phase in the carbon structures is double concentric isolated nodal rings. A larger tensile strain leads to the phase transiting to an interlocking nodal chain, while the middle phase is a coexisting phase of a nodal chain and isolated nodal rings. In addition, stability and synthesis of the carbon networks are discussed. △ Less

Submitted 10 June, 2020; originally announced June 2020.

Journal ref: Carbon 157, 563 (2020)

Orbital-enhanced Warping Effect in P\textsubscript{x},P\textsubscript{y}-derived Rashba Spin Splitting of Monatomic Bismuth Surface Alloy Surface Alloy

Authors: Guan-Yu Chen, Angus Huang, Yen-Hui Lin, Chia-Ju Chen, Deng-Sung Lin, Po-Yao Chang, Horng-Tay Jeng, Gustav Bihlmayer, Pin-Jui Hsu

Abstract: Spin-split Rashba bands have been exploited to efficiently control the spin degree of freedom of moving electrons, which possesses a great potential in frontier applications of designing spintronic devices and processing spin-based information. Given that intrinsic breaking of inversion symmetry and sizeable spin-orbit interaction, two-dimensional (2D) surface alloys formed by heavy metal elements… ▽ More Spin-split Rashba bands have been exploited to efficiently control the spin degree of freedom of moving electrons, which possesses a great potential in frontier applications of designing spintronic devices and processing spin-based information. Given that intrinsic breaking of inversion symmetry and sizeable spin-orbit interaction, two-dimensional (2D) surface alloys formed by heavy metal elements exhibit a pronounced Rashba-type spin splitting of the surface states. Here, we have revealed the essential role of atomic orbital symmetry in the hexagonally warped Rashba spin-split surface state of $\sqrt{3}\times\sqrt{3} R30^{\circ}$ BiCu$_{2}$ monatomic alloy by scanning tunneling spectroscopy (STS) and density functional theory (DFT). From $\mathrm{d}I/\mathrm{d}U$ spectra and calculated band structures, three hole-like Rashba-split bands hybridized from distinct orbital symmetries have been identified in the unoccupied energy region. Because of the hexagonally deformed Fermi surface, quasi-particle interference (QPI) mappings have resolved scattering channels opened from interband transitions of \textit{p$_{x},$p$_{y}$}($m_{j}=1/2$) band. In contrast to the \textit{s,p$_{z}$}-derived band, the hexagonal warping predominately is accompanied by substantial out-of-plane spin polarization $S_{z}$ up to 24\% in the dispersion of \textit{p$_{x}$,p$_{y}$}($m_{j}=1/2$) band with an in-plane orbital symmetry. △ Less

Submitted 8 June, 2020; originally announced June 2020.

Fermi surface topology and non-trivial Berry phase in the flat-band semimetal Pd$_3$Pb

Authors: Mojammel A. Khan, Po-Hao Chang, Nirmal Ghimire, Terence M. Bretz-Sullivan, Anand Bhattacharya, Jidong S. Jiang, John Singleton, John F. Mitchell

Abstract: A study of the Fermi surface of the putative topological semimetal Pd$_3$Pb has been carried out using Shubnikov-de Haas (SdH) oscillations measured in fields of up to 60 T. Pd$_3$Pb has garnered attention in the community due to a peculiar Fermi surface that has been proposed theoretically by Ahn, Pickett, and Lee, [Phys. Rev. B 98, 035130 (2018)] to host a dispersion-less band along $X-Γ$ as wel… ▽ More A study of the Fermi surface of the putative topological semimetal Pd$_3$Pb has been carried out using Shubnikov-de Haas (SdH) oscillations measured in fields of up to 60 T. Pd$_3$Pb has garnered attention in the community due to a peculiar Fermi surface that has been proposed theoretically by Ahn, Pickett, and Lee, [Phys. Rev. B 98, 035130 (2018)] to host a dispersion-less band along $X-Γ$ as well as multiple triply-degenerate band crossings that, under the influence of spin-orbit coupling, lead to ten four-fold degenerate Dirac points. Analysis of the SdH oscillation data verifies the calculated multi-sheet Fermi surface, revealing a $Γ$ centered spheroid that had not been resolved experimentally in prior studies. A comprehensive, angle-dependent analysis of the phase of the SdH oscillations convincingly demonstrates a non-trivial Berry phase for two bands along $Γ-R$, supporting the theoretical predictions, while simultaneously evidencing interference between extremal orbits that mimics a trivial Berry phase at intermediate angles. △ Less

Submitted 2 June, 2020; originally announced June 2020.

Comments: 9 pages, 8 figures (including supplemental document)

Journal ref: Phys. Rev. B 101, 245113 (2020)