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Microscopic evidence for imaginary charge density wave in a kagome metal
Authors:
S. Suetsugu,
F. Hori,
M. Shibata,
S. Kitagawa,
K. Ishida,
T. Asaba,
S. Nakazawa,
Q. Li,
H. -H. Wen,
T. Shibauchi,
H. Kontani,
Y. Matsuda
Abstract:
Dissipationless charge transport without any energy loss is one of the most fascinating phenomena in condensed matter physics. This extraordinary state manifests in two well-established systems: superconductors and quantum Hall systems. A proposed third category is associated with chiral loop current order, characterized by the spontaneous formation of microscopic electric current loops. The micro…
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Dissipationless charge transport without any energy loss is one of the most fascinating phenomena in condensed matter physics. This extraordinary state manifests in two well-established systems: superconductors and quantum Hall systems. A proposed third category is associated with chiral loop current order, characterized by the spontaneous formation of microscopic electric current loops. The microscopic origin of these currents stems from imaginary hopping terms, conceptualized as an imaginary charge density wave (iCDW). Despite extensive investigations, its existence remains highly controversial. Here we report site-selective spectroscopic evidence for a pure iCDW in the kagome nonmagnetic metal CsV$_3$Sb$_5$. Nuclear quadrupole resonance spectra at out-of-plane $^{121}$Sb site sensitive to in-plane currents reveal anomalous broadening below $T^*\approx$120 K, coinciding with the nematic transition well above the real charge density wave (CDW). Under magnetic fields, the spectra exhibit asymmetric lineshapes, demonstrating that this broadening purely originates from magnetic effects rather than from electric quadrupolar effects associated with CDW fluctuations. The observed lineshapes are quantitatively consistent with ~1 mT local fields induced by chiral loop currents, indicating spontaneous time-reversal symmetry breaking. This microscopic identification of the long-sought pure iCDW establishes a novel form of quantum order, potentially revolutionizing our understanding of exotic electronic states in quantum materials.
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Submitted 6 May, 2026;
originally announced May 2026.
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Microscopic Determination of the c-axis-Oriented Antiferromagnetic Structure in LaMnSi by $^{55}$Mn and $^{139}$La NMR
Authors:
Yusuke Sakai,
Fumiya Hori,
Hiroki Matsumura,
Shumpei Oguchi,
Shunsaku Kitagawa,
Kenji Ishida,
Hiroshi Tanida
Abstract:
We report a microscopic investigation of the magnetic structure and electronic properties of LaMnSi in its antiferromagnetic (AFM) state using nuclear magnetic resonance (NMR). Field-swept $^{55}$Mn- and $^{139}$La-NMR spectra, as well as zero-field 55Mn-NMR (ZFNMR) spectra, reveal that the Mn ordered moments are parallel to the tetragonal c axis, consistent with the C-type AFM structure and the r…
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We report a microscopic investigation of the magnetic structure and electronic properties of LaMnSi in its antiferromagnetic (AFM) state using nuclear magnetic resonance (NMR). Field-swept $^{55}$Mn- and $^{139}$La-NMR spectra, as well as zero-field 55Mn-NMR (ZFNMR) spectra, reveal that the Mn ordered moments are parallel to the tetragonal c axis, consistent with the C-type AFM structure and the realization of an odd-parity multipole order. The internal field at the Mn site is determined to be 19.64 T at 4.2 K, corresponding to a hyperfine coupling constant of Ahf = 6.0 T/uB. Nuclear spin-lattice relaxation rate 1/T1 exhibits a characteristic behavior of itinerant antiferromagnetism, showing metallic behavior at low temperatures and magnon-induced enhancement upon approaching the Neel temperature (TN = 295 K). These results show LaMnSi as an ideal compound to study 3d electron magnetism and odd-parity multipole order in the RT Si (R = rare-earth, T = transition metal) system, free of the complexities of 4f electrons.
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Submitted 28 January, 2026;
originally announced January 2026.
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Antiferromagnetic Order and Magnetic Frustration in the Honeycomb Heavy-Fermion System Ce(Pt$_{1-x}$Pd$_{x}$)$_6$Al$_3$: $^{27}$Al and $^{195}$Pt NMR Studies
Authors:
Shunsaku Kitagawa,
Fumiya Hori,
Kenji Ishida,
Ryohei Oishi,
Yasuyuki Shimura,
Takahiro Onimaru,
Toshiro Takabatake
Abstract:
Heavy-fermion systems with magnetic frustration offer a rich platform for investigating the interplay among Kondo screening, magnetic frustration, and quantum criticality. We report comprehensive $^{27}$Al and $^{195}$Pt nuclear magnetic resonance measurements on polycrystalline Ce(Pt$_{1-x}$Pd$_{x}$)$_6$Al$_3$ ($x = 0$, 0.1, 0.2, and 0.3). For $x = 0$, the Knight shift, linewidth, and nuclear spi…
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Heavy-fermion systems with magnetic frustration offer a rich platform for investigating the interplay among Kondo screening, magnetic frustration, and quantum criticality. We report comprehensive $^{27}$Al and $^{195}$Pt nuclear magnetic resonance measurements on polycrystalline Ce(Pt$_{1-x}$Pd$_{x}$)$_6$Al$_3$ ($x = 0$, 0.1, 0.2, and 0.3). For $x = 0$, the Knight shift, linewidth, and nuclear spin-lattice relaxation rate reveal a paramagnetic heavy-fermion ground state persisting down to 0.1~K, characterized by a coherence temperature $T_{\mathrm{coh}} \simeq 15$~K. Substituting Pd induces antiferromagnetic order at $T_{\mathrm{N}} \simeq 3.5$~K, while suppressing $T_{\mathrm{coh}}$. Comparison between $x = 0.1$ and $x = 0.3$ reveals a crossover from itinerant spin-density-wave antiferromagnetism to more localized-moment antiferromagnetism, indicating a shift toward the localized side of the Doniach phase diagram. These findings establish Ce(Pt$_{1-x}$Pd$_{x}$)$_6$Al$_3$ as a tunable platform to explore the competition between Kondo screening and magnetic frustration.
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Submitted 30 July, 2025;
originally announced July 2025.
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Magnetic-Field Dependence of Paramagnetic Properties Investigated by 63/65Cu-NMR on the Yb Zigzag-Chain Semiconductor YbCuS2
Authors:
Fumiya Hori,
Shunsaku Kitagawa,
Kenji Ishida,
Yudai Ohmagari,
Takahiro Onimaru
Abstract:
To investigate the paramagnetic properties of YbCuS2 under magnetic fields, we have performed the 63/65Cu-nuclear magnetic resonance (NMR) measurements. The NMR spectra can be reproduced by the simulations of the three-dimensional powder pattern and the additional two-dimensional powder pattern, indicating the partial sample orientation due to the anisotropy of the magnetic properties. These simul…
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To investigate the paramagnetic properties of YbCuS2 under magnetic fields, we have performed the 63/65Cu-nuclear magnetic resonance (NMR) measurements. The NMR spectra can be reproduced by the simulations of the three-dimensional powder pattern and the additional two-dimensional powder pattern, indicating the partial sample orientation due to the anisotropy of the magnetic properties. These simulations suggest that the ac plane is the easy plane in YbCuS2. The Knight shift K is proportional to the bulk magnetic susceptibility and field-independent. The broad maximum of the nuclear spin-lattice relaxation rate 1/T1 at Tmax ~ 50 K (50 K anomaly) observed at zero magnetic field is quickly suppressed by the magnetic fields. This indicates that the 50 K anomaly is field-dependent. Furthermore, an anomalous enhancement of 1/T1 at low temperatures was observed above 3 T. This field seemingly corresponds to the magnetic field at which a field-induced phase transition occurs below the antiferromagnetic transition temperature TN ~ 1 K. The changes in 1/T1 observed in the paramagnetic state suggest the presence of the complex quantum phenomena under magnetic fields in YbCuS2.
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Submitted 2 February, 2025;
originally announced February 2025.
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Lu/Se Substitution Effect on Magnetic Properties of Yb-Based Zigzag Chain Semiconductor YbCuS2
Authors:
Fumiya Hori,
Hiroyasu Matsudaira,
Shunsaku Kitagawa,
Kenji Ishida,
Souichiro Mizutani,
Hirotaka Shirai,
Takahiro Onimaru
Abstract:
We have investigated the changes in the magnetic properties on YbCuS2 by Lu or Se substitutions from a microscopic perspective. In general, it is expected that nonmagnetic Lu substitution dilutes the magnetic Yb3+ concentration, and that Se substitution induces the negative pressure in YbCuS2. The 63/65Cu-nuclear quadrupole resonance (NQR) measurements on polycrystalline Yb0.9Lu0.1CuS2 and YbCu(S0…
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We have investigated the changes in the magnetic properties on YbCuS2 by Lu or Se substitutions from a microscopic perspective. In general, it is expected that nonmagnetic Lu substitution dilutes the magnetic Yb3+ concentration, and that Se substitution induces the negative pressure in YbCuS2. The 63/65Cu-nuclear quadrupole resonance (NQR) measurements on polycrystalline Yb0.9Lu0.1CuS2 and YbCu(S0.9Se0.1)2 revealed that the Se substitution leads to larger lattice expansion compared to the Lu substitution. The antiferromagnetic transition temperature TN decreased from 0.95 K in the unsubstituted system to 0.75 K in both substituted systems. Furthermore, the T-linear behavior of the 63Cu-NQR spin-lattice relaxation rate 1/T1, suggesting the presence of the gapless quasiparticle excitations, was observed even in the substituted systems, and the value of 1/T1T increases. These results under the chemical substitutions are opposite to those under pressure as previously reported, and are consistent with the expectation. Our systematic study indicates a clear relationship between the magnetic ordered states and the quasiparticle excitations in YbCuS2.
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Submitted 24 December, 2024;
originally announced December 2024.
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Pressure Evolution of Magnetic Structure and Quasiparticle Excitations in Anisotropic Frustrated Zigzag Chains
Authors:
Fumiya Hori,
Hiroyasu Matsudaira,
Shunsaku Kitagawa,
Kenji Ishida,
Hiroto Suzuki,
Takahiro Onimaru
Abstract:
Frustrated magnetic systems with anisotropic exchange interactions have been recognized as key platforms for discovering exotic quantum states and quasiparticles. In this study, we report the pressure evolution of magnetic structures and quasiparticle excitations in the frustrated semiconductor YbCuS2, characterized by Yb3+ zigzag chains with competing exchange interactions. At ambient pressure, Y…
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Frustrated magnetic systems with anisotropic exchange interactions have been recognized as key platforms for discovering exotic quantum states and quasiparticles. In this study, we report the pressure evolution of magnetic structures and quasiparticle excitations in the frustrated semiconductor YbCuS2, characterized by Yb3+ zigzag chains with competing exchange interactions. At ambient pressure, YbCuS2 exhibits a magnetic transition at TN ~ 0.95 K, forming an incommensurate helical magnetic order. Under hydrostatic pressure of 1.6 GPa, TN increases to 1.17 K, and the magnetic structure changes to a commensurate one, which can be regarded as an odd-parity magnetic multipole order. Remarkably, pressure suppresses the gapless quasiparticle excitations. These findings suggest that pressure alters the exchange interactions between the Yb ions, affecting both the magnetic ground state and the quasiparticle excitations. Our results highlight the pivotal role of anisotropic interactions in one-dimensionality to stabilize the complex quantum phases, offering insights into the interplay among frustration, dimensionality, multipoles and emergent quasiparticles.
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Submitted 23 December, 2024;
originally announced December 2024.
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Gapped Spin Excitation in Magnetic Ordered State on Yb-Based Zigzag Chain Compound YbAgSe2
Authors:
Fumiya Hori,
Shunsaku Kitagawa,
Kenji Ishida,
Souichiro Mizutani,
Yudai Ohmagari,
Takahiro Onimaru
Abstract:
We report the 77Se-nuclear magnetic resonance (NMR) results of trivalent Yb zigzag chain compound YbAgSe2, which is a sister compound of YbCuS2. The 77Se-NMR spectrum was reproduced by considering two different Se sites with negative Knight shifts and three-axis anisotropy. Above the Neel temperature TN, the Knight shift is proportional to the bulk magnetic susceptibility. Below TN, the extremely…
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We report the 77Se-nuclear magnetic resonance (NMR) results of trivalent Yb zigzag chain compound YbAgSe2, which is a sister compound of YbCuS2. The 77Se-NMR spectrum was reproduced by considering two different Se sites with negative Knight shifts and three-axis anisotropy. Above the Neel temperature TN, the Knight shift is proportional to the bulk magnetic susceptibility. Below TN, the extremely broad signal with weak intensity and the relatively sharp signal coexist, suggesting that one is strongly influenced by internal magnetic fields and the other remains relatively unaffected by these fields in the magnetic ordered state. The nuclear spin-lattice relaxation rate 1/T1 remains almost constant above TN and abruptly decreases below TN. In contrast to YbCuS2, a T-linear behavior of 1/T1 at low temperatures was not observed at least down to 1.0 K in YbAgSe2. Our results indicate that the gapless excitation is unique to YbCuS2, or is immediately suppressed in the magnetic fields.
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Submitted 14 November, 2024;
originally announced November 2024.
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Superconducting-Spin Reorientation in Spin-Triplet Multiple Superconducting Phases of UTe2
Authors:
Katsuki Kinjo,
Hiroki Fujibayashi,
Hiroki Matsumura,
Fumiya Hori,
Shunsaku Kitagawa,
Kenji Ishida,
Yo Tokunaga,
Hironori Sakai,
Shinsaku Kambe,
Ai Nakamura,
Yusei Shimizu,
Yoshiya Homma,
Dexin Li,
Fuminori Honda,
Dai Aoki
Abstract:
Superconducting (SC) state has spin and orbital degrees of freedom, and spin-triplet superconductivity shows multiple SC phases due to the presence of these degrees of freedom. However, the observation of spin-direction rotation occurring inside the SC state (SC spin rotation) has hardly been reported. UTe2, a recently discovered topological superconductor, exhibits various SC phases under pressur…
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Superconducting (SC) state has spin and orbital degrees of freedom, and spin-triplet superconductivity shows multiple SC phases due to the presence of these degrees of freedom. However, the observation of spin-direction rotation occurring inside the SC state (SC spin rotation) has hardly been reported. UTe2, a recently discovered topological superconductor, exhibits various SC phases under pressure: SC state at ambient pressure (SC1), high-temperature SC state above 0.5 GPa (SC2), and low-temperature SC state above 0.5 GPa (SC3). We performed nuclear magnetic resonance and AC susceptibility measurements on single-crystal UTe2. The b-axis spin susceptibility remains unchanged in SC2, unlike in SC1, and decreases below the SC2-SC3 transition with spin modulation. These unique properties in SC3 arise from the coexistence of two SC order parameters. Our NMR results confirm the spin-triplet superconductivity with SC spin parallel to b in SC2, and unveil the remaining of spin degrees of freedom in superconducting UTe2.
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Submitted 28 July, 2023;
originally announced July 2023.
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Enhancement of charge-neutral fermionic excitation near spin-flop transition\\ in magnetic Kondo material YbIr$_3$Si$_7$
Authors:
Shunsaku Kitagawa,
Takumi Kobayashi,
Fumiya Hori,
Kenji Ishida,
Andriy H. Nevidomskyy,
Long Qian,
Emilia Morosan
Abstract:
The new Kondo material YbIr$_3$Si$_7$, similar to other Kondo insulators, has been reported to exhibit charge-neutral fermionic excitations through measurements of specific heat and thermal conductivity at low temperatures. We performed $^{29}$Si-NMR on YbIr$_3$Si$_7$ to investigate the magnetic response of charge-neutral fermions from a microscopic perspective. In low magnetic fields parallel to…
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The new Kondo material YbIr$_3$Si$_7$, similar to other Kondo insulators, has been reported to exhibit charge-neutral fermionic excitations through measurements of specific heat and thermal conductivity at low temperatures. We performed $^{29}$Si-NMR on YbIr$_3$Si$_7$ to investigate the magnetic response of charge-neutral fermions from a microscopic perspective. In low magnetic fields parallel to the $c$ axis, a single NMR peak in the paramagnetic state splits into three peaks below $T_{\rm N}$. In contrast, only a slight shift of the single NMR peak was observed in high magnetic fields. This spectral change as a function of the $c$-axis magnetic field is interpreted as spin-flop transition, at which the magnetic moments oriented along the $c$ axis (AF-I phase) are rotated to the $ab$ plane with ferromagnetic component along the $c$-axis (AF-II phase). In the vicinity of the spin-flop magnetic field $H_{\rm M}$, nuclear spin-lattice relaxation rate $1/T_1$ was found to be proportional to temperature at low temperatures, indicating the existence of charge-neutral fermions. Furthermore, a peak of $1/T_1$ vs. the $c$-axis magnetic field suggests that the charge-neutral fermions in YbIr$_3$Si$_7$ are closely related to its magnetic properties. Our findings shed light on the origin of charge-neutral fermions in insulators.
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Submitted 22 September, 2022;
originally announced September 2022.
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First-Order Antiferromagnetic Transition and Novel Gapless Excitation in a 4f Zigzag Chain Compound YbCuS2
Authors:
F. Hori,
K. Kinjo,
S. Kitagawa,
K. Ishida,
Yudai Ohmagari,
T. Onimaru
Abstract:
We report on the 63/65Cu-nuclear magnetic resonance and nuclear quadrupole resonance (NQR) studies of trivalent Yb zigzag chain compound YbCuS2. Sharp NQR signals were observed in the paramagnetic (PM) state. Below TO = 0.95 K, the multi peaks induced by the internal magnetic fields arising from the antferromagnetic (AFM) ordered moments appear and coexist with the PM signal down to 0.85 K, eviden…
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We report on the 63/65Cu-nuclear magnetic resonance and nuclear quadrupole resonance (NQR) studies of trivalent Yb zigzag chain compound YbCuS2. Sharp NQR signals were observed in the paramagnetic (PM) state. Below TO = 0.95 K, the multi peaks induced by the internal magnetic fields arising from the antferromagnetic (AFM) ordered moments appear and coexist with the PM signal down to 0.85 K, evidencing the first-order AFM phase transition at TO. In addition, the nuclear spin-lattice relaxation rate 1/T1 abruptly decreases below TO and shows the T-linear behavior below 0.5 K. The significant large 1/T1T value strongly suggests the presence of the novel gapless spin excitation in low temperature region.
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Submitted 19 January, 2022;
originally announced January 2022.