Dynamic Spin Fluctuations in the Frustrated A-site Spinel CuAl2O4

We performed nuclear magnetic resonance (NMR) and muon spin relaxation ({\mu}SR) experiments to identify the magnetic ground state of the frustrated quantum A-site spinel, CuAl2O4. Our results verify that the ground state does not exhibit a long-range magnetic ordering, but a glass-like transition manifests at T*=2.3 K. However, the Gaussian shape and the weak longitudinal field dependence of {\mu}SR spectra below T* show that the ground state has dynamic spin fluctuations, distinct from those of conventional spin-glasses.Comment: 22 pages, 7 figure

Unconventional spin-phonon coupling via the Dzyaloshinskii???Moriya interaction

Spin-phonon coupling (SPC) plays a critical role in numerous intriguing phenomena of transition metal oxides (TMOs). In 3d and 4d TMOs, the coupling between spin and lattice degrees of freedom is known to originate from the exchange interaction. On the other hand, the origin of SPC in 5d TMOs remains to be elucidated. To address this issue, we measured the phonon spectra of the 5d pyrochlore iridate Y 2 Ir 2 O 7 using optical spectroscopy. Three infrared-active phonons soften below the N??el temperature of T N ??? 170 K, indicating the existence of strong SPC. Simulations using density functional theory showed that the coupling is closely related to the Ir???O???Ir bond angle. A tight-binding model analysis reveals that this SPC is mainly mediated by the Dzyaloshinskii???Moriya interaction rather than the usual exchange interaction. We suggest that such unconventional SPC may be realized in other 5d TMOs with non-collinear magnetic order

Unconventional Anomalous Hall Effect from Antiferromagnetic Domain Walls of Nd\u3csub\u3e2\u3c/sub\u3eIr\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e7\u3c/sub\u3e Thin Films

Ferroic domain walls (DWs) create different symmetries and ordered states compared with those in single-domain bulk materials. In particular, the DWs of an antiferromagnet with noncoplanar spin structure have a distinct symmetry that cannot be realized in those of their ferromagnet counterparts. In this paper, we show that an unconventional anomalous Hall effect (AHE) can arise from the DWs of a noncoplanar antiferromagnet, Nd2Ir2O7. Bulk Nd2Ir2O7 has a cubic symmetry; thus, its Hall signal should be zero without an applied magnetic field. The DWs generated in this material break the twofold rotational symmetry, which allows for finite anomalous Hall conductivity. A strong f−d exchange interaction between the Nd and Ir magnetic moments significantly influences antiferromagnetic (AFM) domain switching. Our epitaxial Nd2Ir2O7 thin film showed a large enhancement of the AHE signal when the AFM domains switched, indicating that the AHE is mainly due to DWs. Our paper highlights the symmetry-broken interface of AFM materials as a means of exploring topological effects and their relevant applications

Properties of spin 1/2 triangular lattice antiferromagnets: CuRE2Ge2O8 (RE=Y, La)

We found new two-dimensional (2D) quantum (S=1/2) antiferromagnetic systems: CuRE2Ge2O8 (RE=Y and La). According to our analysis of high-resolution X-ray and neutron diffraction experiments, the Cu-network of CuRE2Ge2O8 (RE=Y and La) exhibits a 2D triangular lattice linked via weak bonds along the perpendicular b-axis. Our bulk characterizations from 0.08 to 400 K show that they undergo a long-range order at 0.51(1) and 1.09(4) K for the Y and La systems, respectively. Interestingly, they also exhibit field induced phase transitions. For theoretical understanding, we carried out the density functional theory (DFT) band calculations to find that they are typical charge-transfer-type insulators with a gap of Eg = 2 eV. Taken together, our observations make CuRE2Ge2O8 (RE=Y and La) additional examples of low-dimensional quantum spin triangular antiferromagnets with the low-temperature magnetic ordering.Comment: 15 pages, 6 figures, and 1 tabl

Unconventional anomalous Hall effect from antiferromagnetic domain walls of N d2 i r2 O7 thin films

Ferroic domain walls (DWs) create different symmetries and ordered states compared with those in single-domain bulk materials. In particular, the DWs of an antiferromagnet with noncoplanar spin structure have a distinct symmetry that cannot be realized in those of their ferromagnet counterparts. In this paper, we show that an unconventional anomalous Hall effect (AHE) can arise from the DWs of a noncoplanar antiferromagnet, Nd2Ir2O7. Bulk Nd2Ir2O7 has a cubic symmetry; thus, its Hall signal should be zero without an applied magnetic field. The DWs generated in this material break the twofold rotational symmetry, which allows for finite anomalous Hall conductivity. A strong f-d exchange interaction between the Nd and Ir magnetic moments significantly influences antiferromagnetic (AFM) domain switching. Our epitaxial Nd2Ir2O7 thin film showed a large enhancement of the AHE signal when the AFM domains switched, indicating that the AHE is mainly due to DWs. Our paper highlights the symmetry-broken interface of AFM materials as a means of exploring topological effects and their relevant applications. © 2018 American Physical Societ

새로운 구리2+ 산화물을 기반으로 한 양자 자성 연구

학위논문 (박사)-- 서울대학교 대학원 : 자연과학대학 물리·천문학부(물리학전공), 2019. 2. 박제근.응집물질물리학에서 양자 자성은 고전적으로는 예측될 수 없는 강한 양자역학적인 요동에 의해 유도되는 새로운 현상들에 초점이 맞춰진 흥미로운 분야이다. 특별히 산화수가 2+인 구리산화물은 스핀-S = 1/2의 강한 양자 요동과 큰 쿨롱 상호작용 때문에 양자 스핀 액체, 반강자성 삼각격자에서의 uud 상전이, 그리고 스피논 (spinon), 홀론 (holon), 오비톤 (orbiton)으로의 전자 붕괴와 같은 양자 자성을 연구하는데 있어 유망한 대상이 되어 왔다. 이 논문에서 강한 양자 요동과 전자 상관의 상호작용으로 인해 유도되는 새로운 현상들을 새 구리 산화물 시스템인 CuAl2O4와 CuR2Ge2O8 (R = Y, La)을 연구함으로써 발견하였다. 스핀-궤도가 얽혀진 유사 스핀 상태인 Jeff = 1/2는 Kitaev 양자 스핀 액체와 같은 발현 현상과 연관되어 있어 응집물질물리에서 흥미로운 주제가 되었다. 이 상태를 연구하기 위해서 4d나 5d 전이금속 화합물들이 많은 주목을 받아왔었던 반면 3d 시스템은 주목받지 못하였다. 이는 3d 이온의 스핀-궤도 결합이 루테늄 (4d)이나 이리듐 (5d) 이온보다 작기 때문이며 많은 3d 시스템이 궤도 각운동량의 소멸을 야기하는 뒤틀린 결정 구조를 갖기 때문이다. 하지만 이 연구를 통해서 구리 산화물 CuAl2O4에서 Jeff = 1/2 state가 나타남을 입증하였다. 엑스선 회절 및 구리 K-끝머리 엑스선 흡수에 의해 확인된 결정 구조의 입방 대칭성은 Jeff = 1/2의 발현을 지지한다. 또한 구리 L-끝머리 엑스선 흡수는 Jeff = 1/2 상태 발현의 직접적인 증거를 제공한다. 더욱이 이 시스템의 격자 반전은 쩔쩔맴 상태에 있는 Jeff = 1/2 파동함수의 강한 양자 요동과 더불어 자기 모멘트의 장거리 정렬을 막고 스핀-궤도가 얽힌 Jeff = 1/2에 대한 유리 상태를 유발시킨다. 다음 연구는 새로운 이차원 양자 반강자성 시스템인 CuR2Ge2O8 (R = Y와 La)에 초점을 맞추었다. 고분해능 엑스선 그리고 중성자 회절 실험 분석에 의하면, CuR2Ge2O8의 구리 그물망은 b축에 수직하게 약한 결합으로 연결되어 있는 이차원 삼각 격자를 나타낸다. 0.08에서부터 400 K까지의 덩치 성질은 CuR2Ge2O8가 각각 이트륨 시스템에서는 0.51(1) K 그리고 란타넘 시스템에서는 1.09(4) K에서 장거리 정렬함을 보인다. 또한 흥미롭게도 이들은 자기장에 의해 유도되는 특이한 상전이를 보여준다. 이론적인 이해를 위해 밀도 함수 이론 (DFT) 띠 구조 계산이 수행되었고 그 결과 띠틈이 약 2 eV인 전형적인 전하-전달류의 (charge-transfer type) 부도체임을 확인하였다. 종합해보면, 이러한 관측결과들 CuR2Ge2O8 (R = Y, La)가 낮은 온도에서 자기 정렬이 있는 이차원 양자 스핀 삼각 반강자성체의 추가적인 사례가 되도록 한다.In condensed matter physics, quantum magnetism is an intriguing field focused on novel phenomena, which could not be expected from classical formalisms but is driven by strong quantum mechanical fluctuations. Especially, cuprate systems, which have the oxidization of 2+, have been a promising playground to study quantum magnetism such as quantum spin liquid, a uud phase transitions in triangular lattice antiferromagnets, and the breakdown of electrons into spinons, holons, and orbitons because of strong quantum fluctuations from the spin-S = 1/2 and the large coulomb interaction. In this thesis, I succeeded to find novel phenomena relating to the interplay of the strong quantum fluctuations and electron correlation by studying several new cuprate systems: CuAl2O4 and CuR2Ge2O8 (R = Y, La). The main question of my thesis is how to realize a spin-orbital entangled pseudo-spin state, Jeff = 1/2 with Cu2+. Although the spin-orbital entangled state been an intriguing topic in condensed matter physics due to related emergent phenomenon such as Kitaev quantum spin liquid, there have been less studies on 3d transition metal oxides because the spin-orbital coupling constant of 3d ion is smaller than that of Ru (4d) or Ir (5d) ions. In this work, I show that a cuprate system, CuAl2O4 hosts a Jeff = 1/2 state. Cubic symmetry of the crystal structure verified by x-ray diffraction and Cu K-edge x-ray absorption supports the Jeff = 1/2 of CuAl2O4. X-ray absorption on Cu L-edge also gives direct evidence for the emergence of this state. Moreover, the site inversion of the system and the strong quantum fluctuations of the frustrated Jeff = 1/2 wavefunctions leads to the absence of a long-range order with a possible glassy state from the spin-orbital entangled Jeff = 1/2 state. Following studies are focused on new two-dimensional (2D) quantum (S = 1/2) antiferromagnetic systems: CuR2Ge2O8 (R = Y and La). According to the analysis of high-resolution x-ray and neutron diffraction experiments, the Cu network of CuR2Ge2O8 (R = Y and La) exhibits a 2D triangular lattice linked via weak bonds along the perpendicular b axis. Bulk characterizations from 0.08 to 400 K show that they undergo long-range order at 0.51(1) and 1.09(4) K for the Y and La systems, respectively. Interestingly, they also exhibit unusual phase transitions induced by magnetic field. For theoretical understanding, the density functional theory (DFT) band calculations were carried out to find that they are typical charge-transfer-type insulators with a gap of Eg ≅ 2 eV. Taken together, these observations make CuR2Ge2O8 (R = Y and La) additional examples of two-dimensional quantum spin triangular antiferromagnets with the low-temperature magnetic ordering.Abstract List of Tables List of Figures 1 Introduction 1.1 Quantum spin systems 1.1.1 Large quantum fluctuation 1.1.2 Novel phenomena in quantum spin systems 1.2 Outline of thesis 2 Experimental Techniques 2.1 Sample synthesis 2.1.1 Flux method 2.1.2 Solid state reaction method 2.2 Crystal structure analysis 2.2.1 X-ray diffraction 2.2.2 Rietveld refinement 3 Emergence of Spin-Orbital Entangled Jeff = 1/2 state in CuAl2O4 3.1 Spin-orbital entangled Kramers doublet Jeff = 1/2 3.1.1 Nature of Jeff = 1/2 3.1.2 Novel phenomena in Jeff = 1/2 compounds 3.2 Jeff = 1/2 in a 3d compound 3.2.1 Conditions to induce Jeff = 1/2 in a 3d compound 3.2.2 Candidate materials: CuAl2O4 3.3 XAS simulation for CuAl2O4 3.4 XAS experiment on CuAl2O4 3.5 Discussion and summary 3.5.1 Magnetism of CuAl2O4 3.5.2 Summary 4 Properties of S = 1/2 Triangular-Lattice Antiferromagnets CuY2Ge2O8 and CuLa2Ge2O8 4.1 Quantum 2D triangular lattice 4.1.1 Enhancement of quantum fluctuation effects in 2D quantum triangular lattice 4.1.2 Quantum phase trnasitions in 2D triangular HAFM driven by quantum fluctuations 4.1.3 Candidate material for 2D triangular HAFM: CuR2Ge2O8 (R = Y, La) 63 4.2 Crystal structure of CuR2Ge2O8 (R = Y, La) 4.3 Bulk properties of CuR2Ge2O8 (R = Y, La) 4.3.1 Magnetic measurements 4.3.2 Heat capacity measurements 4.3.3 DFT calculation 4.4 Discussion and summary 4.4.1 Novel quasi 2D triangular lattice: CuR2Ge2O8 4.4.2 Summary 5 Outlook and Summary 5.1 Outlook 5.2 Summary Appendix: Electric Dipole Transitions Publication lists 국문 초록 (Abstract in Korean) 감사의 글 (Acknowledgement)Docto

Structural investigation of the insulator-metal transition in NiS2-xSex compounds

We report on a combined measurement of high-resolution x-ray diffraction on powder and Raman scattering on single crystalline NiS2-xSex samples that exhibit the insulator-metal transition with Se doping. Via x rays, an abrupt change in the bond length between Ni and S (Se) ions was observed at the transition temperature, in sharp contrast to the almost constant bond length between chalcogen ions. Raman scattering, a complementary technique with the unique sensitivity to the vibrations of chalcogen bonds, revealed no anomalies in the phonon spectrum, consistent with the x-ray diffraction results. This indicates the important role of the interaction between Ni and S (Se) in the insulator-metal transition. The potential implication of this interpretation is discussed in terms of current theoretical models. © 2018 American Physical Societ

Frustrated antiferromagnetic honeycomb-tunnel-like lattice CuR2Ge2O8 (R=Pr, Nd, Sm, and Eu)

New frustrated antiferromagnetic compounds CuR2Ge2O8 (R=Pr, Nd, Sm, Eu) have been investigated using high-resolution x-ray diffraction, magnetic, and heat capacity measurements. These systems show different magnetic lattices depending on rare-earth element. The nonmagnetic Eu compound is a S=1/2 two-dimensional triangular antiferromagnetic lattice oriented in the ac plane with geometrical frustration. On the other hand, the Pr, Nd, and Sm compounds show a three-dimensional honeycomb-tunnel-like lattice made of R3+ running along the a axis with the characteristic behavior of frustrated antiferromagnets. © 2017 American Physical Society101sciescopu

Nodeless superconductivity in the noncentrosymmetric superconductor BiPd

We report low-temperature thermal conductivity measurements on high-quality single crystalline α-BiPd, a noncentrosymmetric superconductor with T c ≃ 3.8 K. In zero magnetic field, the residual linear term κ 0/T is absent, indicating a full gap and suggesting dominant singlet pairing. Consistent with this picture, κ 0(H)/T shows weak field dependence in low field, but grows relatively fast in fields above 0.03 T (≃1/5H c2). Implications for the material's disputed high-field behavior are discussed, and constraints are placed on the previously proposed scenarios. © 2016 IOP Publishing Ltd4